School Science Lessons
32. Electric current, circuits, Ohm's law,  fuses, resistance, voltage
2012-01-28 SP
Please send comments to: J.Elfick@uq.edu.au
Table of contents
32.0.0 Electric circuits
32.5.0 Circuits, cells and resistors in series and parallel
39.5.0 Circuits, (Electronics)
32.4.0 Conductors of electricity
32.2.0 Current electricity
32.6.0 Fuse, fuse wire
32.3.0 Resistance
32.1.0 Voltage, EMF sources, Voltage produced by:
32.5.0 Circuits, cells and resistors in series and parallel
4.61 Cells in parallel
4.60 Cells in series
32.4.6 Cells in series and parallel
32.4.3 Charge density in circuits
33.6.0 Circuit analysis, house circuits
4.59 Circuit board
32.2.00 Circuit diagrams, electrical symbols
32.5.2.1 Circuit diagrams, Wiring system diagrams, (motor vehicle circuits)
32.4.4.3 Circuit elements in parallel
32.4.4.2 Circuit elements in series
39.5.0 Circuits (Electronics)
32.5.0 Circuits, series and parallel
32.4.0 d.c. circuits, circuit diagrams, Kirchhoff's laws, Ohm's law
32.4.6.4 Dry cells in an electric circuit, cells in series and parallel
6.37 Electric circuit (Primary)
32.4.4.0 Electric circuit, electric circuit board, water circuit board, water analogy circuit
4.57 Electric torch (flashlight)
32.4.6.2 Electric torch, flashlight (Primary)
32.4.4.7 IR drop in a wire, potential drop along wire
32.4.1 Kirchhoff's laws, Ohm's law
32.4.4.5 Lamps in parallel
32.4.4.6 Lamps in series and parallel
32.4.4.8 Potential drop with Wimshurst machine, static machine
32.4.4.4 Series and parallel (branching) circuits
32.4.2 Simple electric circuit
4.55 Simple switch
32.4.6.3 Six volt batteries in series and parallel (GIF)
32.4.5 Switch, switches in a circuit, tapping key
4.56 Switches in a circuit
39.5.0 Circuits, (Electronics)
39.5.0 Circuits, Bistable circuits (Electronics)
39.9.0 Circuits, Counting circuits (Electronics)
39.3.0 Circuits, Logic circuits (Electronics)
31.8.4 Circuits, RC circuits (Electronics)
38.1.16 Circuits, Triode circuits (Electronics)
32.4.0 Conductors of electricity
Order online: Energy Ball, electrical conductance, circuits
6.38 Electricity conductors (Primary)
32.4.0 Conductors of electricity
32.3.4.5 Conduction from hot wire
32.3.4.0 Conduction in gases, Jacob's ladder
32.3.4.1 Conduction of gaseous ions
32.2.1 Conductors and insulators
32.2.2 Conductors and non-conductors, conductivity of solids, conductance apparatus
4.58 Conductors and non-conductors of electricity
15.59.0 Conductors of electricity, electrical conductivity
15.59.1.1 Electrolysis, electrolytes, anode and cathode
15.59.2 Electrical conductivity of solids
15.59.3 Electrical conductivity of melted solids, fused solids
15.59.4 Electrical conductivity of liquids
32.2.5 High temperature and conductivity of sodium chloride and paraffin wax, liquid rheostat
32.2.4 Lead pencil conductor
4.19 Liquids that conduct electricity
4.18 Solids that conduct electricity
15.59.1 Substances that conduct electricity
32.2.3 Test materials for conductivity
32.2.0 Current electricity
32.2.8 Bite on aluminium foil
32.4.4.0 Circuit board, Electric circuit, electric circuit board, water circuit board, water analogy circuit
32.2.0 Current electricity, electric current
4.51.0 Current electricity experiments, UNESCO
32.2.7 Current electricity from frog's leg
6.3.1.4 Electric current, ampere
4.66 Electric current detector
4.65 Electric light bulb (incandescent filament lamp)
33.7.0 Instruments to detect electric current
32.2.6 "Taste" current electricity
32.6.0 Fuse, fuse wire
32.4.7.0 Fuse, fuse wires, make and use a fuse
4.63 Fuse, Make a fuse
4.64 Fuse, Use a fuse
32.4.7.1 Power surge circuit breaker
32.3.0 Resistance
32.3.0 Resistance, resistivity, specific resistance, resistors
32.3.2.3 Carbon and tungsten light bulbs
32.3.4.2 Discharge by ions in a tube, recombination of ions
4.65 Electric light bulb (incandescent filament lamp)
32.3.2.7 Electrical conductivity of molten glass at high temperature
32.3.4.9 Electrohydrodynamics
32.3.2.1 Heat and cool resistors
32.4.6.9 Heat and light from electricity, make a model electric lamp
32.4.6.7 Internal resistance of a cell
32.3.4.4 Ionization by radioactivity, conduction in air by ions, smoke alarms
32.3.4.8 Ionization by X-rays
32.3.2.5 Liquid nitrogen / liquid air experiments
32.3.3.0 Liquids that conduct electricity
32.3.1.4 "Megger"
32.3.3.2 Migration of ions, speed of ions
32.3.1.3 Ohmic and non-ohmic resistors (nonohmic resistors)
32.3.2.2 Put the light out with heat
32.3.1.2 Resistance model
32.3.2 0 Resistivity with metre wire bridge, resistivity and temperature
32.3.1 Resistor assortment, resistor colour code
38.2.03 Resistors colour code, (Electronics)
32.4.4.1 Resistors in series and parallel
32.3.1.1 Resistors, resistances, switches
32.3.3.1 Saltwater string, electrolytic conduction
32.3.4.3 Separate ions from flame
32.3.2.4 Temperature of incandescent lamps with silicon solar cells
32.3.4.6 Thermionic effect in air
32.3.4.7 Thermionic effect in air, thermionic emission
32.3.2.6 Thermistors, effect of heat on a thermistor
32.1.0 Voltage, EMF sources, Voltage produced by:
4.1.0 Voltage, EMF sources, Voltage produced by:
4.1.5.0 Voltage produced by chemical action, batteries
4.1.5.1 Voltmeter
4.1.5.2 Voltameter, coulometer
4.1.6 Voltage produced by electromagnetism
4.1.1 Voltage produced by friction
4.1.3 Voltage produced by heat, thermocouple
4.1.4 Voltage produced by light, photoelectric voltage, photoelectric cell
4.1.2 Voltage produced by mechanical stress to crystals, piezoelectricity
4.1.2.1 Rochelle salt crystals, potassium sodium tartrate-4-water
4.1.0 Voltage, EMF sources
SI derived units. Quantity: Electric potential difference, Name of unit: volt, Symbol: V, Equivalent: W A-1
The volt, symbol V (Alessandro Volta 1745 - 1827) is the SI unit of electric potential (potential difference, e.m.f.). The value expressed in volts is called the voltage, defined as the difference of potential between two points on a conductor carrying one ampere of current when the power dissipated is one watt. So 1 volt = 1 joule per coulomb, 1 J / C.
The force that causes free electrons to move in a conductor may be called voltage, electromotive force, EMF, e.m.f., difference in potential, potential difference or even "electrical pressure". If connected by a conductor, electrons will flow from a negatively charged body to a positively charged body until the two charges are equal and the potential difference no longer exists. When a cell does work W to drive a charge Q around a circuit, the cell has an electromotive force E, E = W / Q = P / I (where W = watt, the SI unit of power, charge = Q, P = power and I = current). So a source of potential difference, e.g. a cell, has electromotive force, EMF in this document, but EMF is not a "force" although it does cause charges to move around the circuit. Thus, EMF is really energy per unit charge. Potential difference is different from EMF because in current electricity potential difference always refers to energy loss in a circuit, e.g. conversion to heat and light in an incandescent bulb. The unit of potential difference is the volt, V. An electric current can flow in a conductor only if a potential difference, V, exists across it. A potential difference of 1 volt = 1 joule per coulomb.
4.1.1 Voltage produced by friction
Voltage is produced by rubbing two materials together. The least used of the six methods is friction. Its main application is in Van de Graaff generators, used by some laboratories to produce high voltages. Friction electricity (static electricity) is usually a nuisance. For example a flying aircraft may accumulate electric charges from the friction between its skin and the passing air. These charges may interfere with radio communication. Sliding across dry seat covers or walking across dry carpets, and then contacting other objects may give a mild electric shock.
4.1.2 Voltage produced by mechanical stress to crystals, piezoelectricity
See diagram 4.1.1.2: Voltage produced by pressure
Natural crystals are rare, e.g. diamond. They manufacture most crystals used in industry. When a crystal of quartz or Rochelle salt (Seignette salt) is compressed, some electrons move through the crystal. This movement creates an electric potential difference between the two opposite faces of the crystal. If an external wire is connected while the pressure and EMF are present, electrons will flow until the charges are equalized. When the force is removed, the crystal is decompressed, and immediately causes an electric force in the opposite direction. The crystal can convert mechanical force to electrical force. Although the power capacity of a crystal is extremely small, they are useful because of their extreme sensitivity to changes of mechanical force or changes in temperature. The regular piezoelectric signal from a quartz crystal in an electronic watch allow the watch to keep almost perfect time.
Piezoelectricity is the phenomenon in certain crystals anc ceramics when application of mechanical stress causes electric charge with the voltage proportional to the stress. It occurs in crystals of cane sugar, quartz, Rochelle salt (potassium sodium tartrate-4-water), topaz, tourmaline,
Demonstrate piezoelectricity by dusting the cooling or warming crystal with a dust of red lead and sulfur that has passed through a silk or nylon screen. A simple bellows can be made from a plastic nasal spray or deodorant bottle in that the aperture has been enlarged to allow a sizeable spray to be emitted. Place in the bottle a mixture of about 2 parts red lead to 1 part sulfur. Put a small piece of silk or nylon stocking over the mouth of the bottle. Tighten this with a rubber band. The dust particles receive electric charges as they pass through the screen formed by the stocking. They settle on the end of the crystal that attracts them. The red lead gets a positive charge and goes to the negative end of the crystal. The sulfur gets a negative charge and settles on the positive end of the crystal.
Barbecue piezo-igniters
These fire-lighters contain quartz crystals that have piezoelectric properties and so develop electric potential under stress. When two crystals are struck together separation of charge in the crystal lattice can produce a very high voltage.
Mosquito bite clicker
This handy gadget can relieve the pain caused by bites of mosquitoes, sand flies and midges, and also the stings of small jellyfish, by injecting a small electric current into the affected skin. It is powered by piezoelectricity and not batteries, so it is small, light weight and easy to use.
Piezoelectric sheets are made from ceramic lead-zirconate-titanate (PZT)
1. Make a ball and spring model of the piezoelectric effect.
2. Attach a commercial piezoelectric sparker to Braun electroscope. Mount a sphere on the end of a piezoelectric gas lighter. Use a piezoelectric gun to discharge a set of charged nylon strings. Attach one end of a piezoelectric crystal to a needle point in the piezoelectric pistol.
The best generators of piezoelectric electric are very thin zinc oxide wires that can generate 200 milliwatts per cubic centimetres when squeezed, e.g. by the foot. So piezoelectric crystals may be incorporated into footwear to produce people-powered energy.
4.1.2.1 Rochelle salt crystals, piezoelectric, hysteresis
See: Rochelle salt
1. Show ferroelectricity hysteresis, Curie-point and the direct piezoelectric effect with a  Rochelle salt, (potassium sodium tartrate-4-water). Connect Rochelle salt to a neon lamp or electrostatic voltmeter. Make sheets of polycrystalline Rochelle salt that show piezoelectric effects.
2. Measure the voltage of a Rochelle salt crystal under various stresses produced by a mass on a lever arm. Excite a Rochelle salt crystal with an audio voltage and couple it to a sounding board. Connect an audio oscillator to a large Rochelle salt crystal and the sound can be distinctly heard. Apply an audio oscillator to a Rochelle salt and amplify with a wood sounding board.
4.1.3 Voltage produced by heat, thermocouple
Voltage is produced by heating the joint (junction) where two unlike metals are joined.
See diagram 4.1.1.3: Voltage produced by heat, thermocouple
When a length of most metals, e.g. copper, is heated at one end, some electrons move away from the hot end towards the cooler end. However, in some metals, e.g. iron, some electrons move towards the hot end. If the metals are connected, the electrons can cross from the iron to the copper at the hot junction, and from the copper through the current meter to the iron at the cold junction. This device is called a thermocouple. Use it to measure temperature, and as heat sensing devices in automatic temperature control equipment. Thermocouples can be subjected to greater temperatures than thermometers using mercury or alcohol.
4.1.4 Voltage produced by light, photoelectric voltage, photoelectric cell
Voltage is produced by light striking photosensitive (light sensitive) substances. When light strikes the surface of a substance, e.g. compounds of silver oxide or copper oxide, it may dislodge electrons from the atoms at the surface, the substance becomes positively charged, and an electric force is created
1. See diagram 4.1.1.4a: Voltage produced by light, photoelectric cell
This photoelectric cell has a curved light-sensitive surface focussed on the central anode. When light from the direction shown strikes the sensitive surface, it emits electrons towards the anode. The more intense the light, the greater is the number of electrons emitted. When you connect a wire between the filament and the back or dark side, the accumulated electrons will flow to the dark side. These electrons will eventually pass through the metal of the reflector and replace the electrons leaving the light-sensitive surface. Thus light energy is converted to a flow of electrons, and a usable current develops.
2. See diagram 4.1.1.4b: Photoelectric cell construction
This photoelectric cell is constructed in layers. A base plate of pure copper is coated with light-sensitive copper oxide. An extremely thin additional layer of metal is put over the copper oxide to allow penetration of light to the copper oxide and accumulate the electrons emitted by the copper oxide. An externally connected wire completes the electron path, the same as in the reflector type cell. The photocell's voltage is used as needed by connecting the external wires to another device, which amplifies (enlarges) it to a usable level. A photocell's power capacity is very small. However it reacts to light intensity variations in an extremely short time. This characteristic makes the photocell very useful in detecting or accurately controlling many processes or operations. For instance, the photoelectric cell, or some form of the photoelectric principle, is used in television cameras, automatic manufacturing process controls, door openers, burglar alarms, and so forth.
4.1.5.0 Voltage produced by chemical action, batteries
See diagram 3.2.84: Copper and zinc foil in a voltmeter
Voltage is produced by chemical reaction in a battery cell. Electrons may be removed from atoms and set in motion by energy derived from forms of energy, e.g. friction, pressure, heat, or light. These physical actions do not alter the molecules of the substances being acted upon. Molecules are not added, taken away, or split. Only electrons are lost or added. If the molecules of a substance combine with atoms of another substance, or give up atoms of its own, the action is chemical in nature. When atoms are added to or taken from the molecules of a substance, the chemical change will cause the substance to take an electric charge. The process of producing a voltage by chemical action is used in batteries.
4.1.5.1 Voltmeter
A voltmeter measures electric potential in volts. A voltmeter measures "electron pressure", potential or electromotive force (E.M.F.), the ability of a cell to move electrons around a circuit, potential difference, with SI unit the volt.
4.1.5.2 Voltameter, coulometer
See diagram 15.5.4: Hofmann voltameter
A voltameter is an electric cell used to to measure the amount of electricity that has moved through the cell by measuring the products of the electrolytic process. So a voltameter (coulometer) measures current (quantity of electricity) by measuring the amount of metal deposited or gas liberated from an electrolyte in given time caused by the passage of the current, with SI unit the coulomb. However a Hofmann voltameter, used to demonstrate the decomposition of water into hydrogen gas and oxygen gas, has platinum electrodes that are too inert to allow deposition of any chemical on them. Voltameters were used to measure current before the invention of the ammeter. Voltammetry is used to measure the concentration of different ions in solution by plotting the relation of current and voltage in a micro electrode.
4.1.6 Voltage produced by electromagnetism
Voltage is produced in a conductor when the conductor moves through a magnetic field, or, a magnetic field moves through the conductor in a way that cuts the magnetic lines of force of the field.
See diagram: 4.1.23: Voltage produced by magnetism, generator | See diagram: 4.1.1.6: Right hand motor rule for electron flow
Generators produce electricity by electromagnetic induction.
Voltage can be produced by magnetism by the following:
1. a conductor, in which the voltage will be produced.,
2. a magnetic field in the conductor's vicinity and,
3. relative motion between the field and the conductor.
So 1. 2. and 3. must all be present. The conductor must be moved to cut across the magnetic lines of force, or, the field must be moved so that the lines of force are cut by the conductor. When a conductor moves across a magnetic field to cut the lines of force, electrons within the conductor are moved in one direction or another so an electromotive force, EMF, or voltage, is created.
In diagram 4.1.23 note the following:
1. the magnetic field existing between the poles of the C-shape magnet,
2. the copper wire conductor 3. the relative motion as the wire is moved across the magnetic field.
In diagram 4.1.23 (A) the copper wire conductor is moving towards you because of the magnetically induced electromotive force, EMF, acting on the electrons in the copper. The right hand end of the conductor becomes negative and the left hand end becomes positive.
In diagram 4.1.23 (B) the conductor is not moving, so there is no longer an induced EMF and no difference in potential between the two ends of the copper wire. In diagram 4.1.23 (C) the conductor is moving away from you creating an induced EMF in the reversed direction.
In diagram 4.1.23(D) it shows a path for electron flow between the ends of the conductor. Electrons can leave the negative end and flow to the positive end and continue as long as the EMF exists. The induced EMF can also be created by holding the conductor stationary and moving the magnetic field.
4.52.1 Lemon-powered clock
Use a digital clock without a plug, powered by two AA batteries, two galvanized nails, e.g. 16d (3.5 inches), two lengths of bare copper wire, and three crocodile clips. Cut a large lemon in halves to form half lemon A, and half lemon B. Push the galvanized nails into one end of each half lemon. Push the ends of the lengths of bare copper wire into the other ends of each half lemon. Remove the batteries from the clock and note the positive and negative terminals. Clip a length of copper wire to the positive terminal of the clock. Use a crocodile clip to connect the wire from half lemon A to the positive terminal of the clock. Use a crocodile clip to connect the galvanized nail in wire half lemon B to the negative terminal of the clock. Use a crocodile clip to join the galvanized nail in half lemon A to the wire in half lemon B. Citric acid in the lemon juice electrolyte dissolves some zinc on the galvanized nail so the nail loses electrons and become positive. The copper wire gains some of the electrons and become negative. So a circuit is formed through the clock and the clock can go again.
6.37 Electric circuit
See diagram: 32.2.1: Simple electric circuit
Be able to set up an electric circuit to light a light bulb.
Use torch light batteries, wires, sticky tape, bulbs, bulb holders.
Electricity can only be generated if there is a complete circuit. Revise electric circuits before the lesson.
1. Give each group a battery, wires and a light bulb. You must need some sticky tape to hold the wires on the bulb contacts. Do not show the children what to do. Light the bulb. Which group can do it first? Can you tell the other groups how you did it? This is one way of encouraging the scientific attitude.
2. Look at the torch battery. What do you see at each end? [The top has a bump marked on it and is marked "+", the bottom is marked "-". Each end is made of metal.] How did you make the bulb light? [One wire touches one end of the battery. The other wire touches the other end of the battery.]
3. Look at the light bulb. What do you see inside the glass bulb? [A thin twisted wire called the filament.] Is there a metal part? [Yes, below the glass part.] How did they make the bulb light up? [One wire touches the bottom of the metal part, the other wire touches the side of the metal part.]
4. Electricity travels along a path. If the path is blocked, then electricity cannot keep flowing. The whole path along which the electricity travels is called a circuit. A broken path is called an open circuit.
Extra Activity:
1. Break open an old torch battery with the back of an axe. Be careful!
Can you see the 1. cardboard cover, 2. zinc case,. 3. black chemical which is a sticky powder, 4. a black carbon rod down the centre of the battery?
32.2.0 Current electricity, electric current
See 6.3.1.4: Electric current, ampere
Electric current, heat and light from electricity, direct current and alternating current, effects of a current: heat and light, magnetic, chemical, Q (coulombs) = I (amperes) X t (seconds), current, nature of electric current and d.c. / a.c. EMF sources
Current electricity, electric current, ampere or amp
Current, I, of electricity exists when an electric charge is transported. The directional movement of charges through a wire is called current, I, and it has the SI unit ampere or amp, symbol A. The ampere is defined as the current in two parallel conductors one metre apart in a vacuum with a force between them of 2 X 10-7 newton per metre of conductor. The direction of the current is the same as that of the movement of charges. The size of the current, current intensity, equals the charge flowing through the cross-section of a conductor in unit time. If q = charge in coulombs and t = time in seconds, and I = current in amperes, I = q / t. Assume that the direction is in the direction of the flow of positive charge, so flow of electrons to the right means flow of current to the left. When charge flows through a conductor, the rate of flow of charge through any section of the conductor is called the electric current. 1 amp = 1 coulomb per second. In a copper plating tank, 1 amp carries 3.29 10-7 kg of copper across every second.
The flow of electrons through a conductor is called electric current and is measured in amperes, with the symbol I amp. One ampere represents the flow of 6.28 x 1018 electrons per second past a fixed point in a conductor. The unit quantity of electricity when one ampere of current flows for one second is called the coulomb, symbol Q. So, I (ampere) = Q (coulomb) / t (second), amps = coulombs per second. A "current of 1 amp" means 1 coulomb of electricity, charge, moves past each point in the circuit per second. 1 amp is the current flowing in two parallel wires one metre apart to produce a force of 2 x 10-7 newton on each metre of wire.
32.2.1 Conductors and insulators
Use the ends of two conducting wires in the circuit, or use two 4 mm plugs, to act as probes. Test the material by noting whether the light bulb lights. Also note whether the brightness is the same 1. for different materials 2. for different distances between the probes on the same material. Select common materials, e.g. string, live plant, plastic ruler, pencil, rubber, fork, knife, pipe, paper, soil, brick, bread, clothes, screwdriver, deionized water, tap water, milk. After testing a liquid, wash and dry the probes.
1. Test a clean dry matchstick. The light bulb does not light. The matchstick is an insulator. Soak matchsticks in water, vinegar and sodium chloride solution. When the matchstick is soaked, the light bulb will light so this piece of match will become to a conductor.
2. Test a burnt matchstick. If the light bulb is not light, shorten the distance between the probes or increase the voltage of the circuit. The light bulb lights.
32.2.2 Conductors and non-conductors, conductivity of solids, conductance apparatus
See diagram: 4.155: Testing for conductivity
1. Use a dry cell, switch, lamp, wire, two crocodile clips, battery box and lamp socket to connect in series a simple d.c. circuit. The lamp will show if there is electric current flowing through the circuit. Note if the lamp lights. Do not let the two crocodile clips touch. Connect two ends of a wool thread, 50 cm long, folded repeatedly and twisted together. Put the wool thread connected by crocodile clips into water. Put the wool thread connected by crocodile clips into thick salt water.
2. Use a simple electric circuit to test whether different substance s conduct electricity, e.g. paper, rubber eraser, plastic, key, coin, cloth, string, chalk, glass, pin, nail file, insulated wire, bare wire, finger, water. Test these in a circuit across an open knife switch. Materials that carry are called electricity conductors. Materials that do not carry electricity are called non-conductors or insulators. The copper core of bell wire is a conductor. Its covering is an insulator.
3. Use six volts direct current, a low power bulb and electrodes made of carbon or steel, and mounted in a cork to keep them at a constant distance apart. Use the carbon centres of old six volt dry cells as electrodes. Test the conductivity of solids by making a good contact between the surface of the solid and the two electrodes. The surface of the solid must first be cleaned. All metals conduct electricity. Carbon conducts electricity. Test whether non-metallic solids conduct electricity, e.g. plastics, naphthalene (moth balls), wax, sugar, sodium chloride and sulfur.
32.2.3 Test materials for conductivity
Connect a torch globe and two torch cells with metal wire, leaving a break AB in the wire. Connect A to B with the material to be tested. If the material is a conductor, the lamp will glow. If the material is an insulator, it will not glow.
Test the following substances: 1. metals, e.g. iron, brass, aluminium, copper, 2. plastics, e.g. PVC, 3. sulfur, 4. rubber, 5. wood, 6. graphite, 7. glass, 8. cork, 9. textile fibres.
32.2.4 Lead pencil conductor
See diagram: 4.152: Lead pencil conductor
Connect a flashlight bulb with a battery by means of a pair of scissors and a pencil. The bulb lights up. Current then flows through the graphite in the "lead" shaft of the pencil to the positive pole of the battery.
32.2.5 High temperature and conductivity of sodium chloride and paraffin wax, liquid rheostat
1. Place a small amount of salt in the bottom of the crucible. Support two stiff copper wires so that they reach the crucible and make electrical contact with the salt. Close the switch. The solid sodium chloride does not conduct electricity. Remove the electrodes from the salt and heat the crucible strongly until the salt melts. Replace the electrodes and adjust the rheostat to current of 1 amp. Remove the burner and let the salt cool. The current rapidly falls to zero. Repeat the experiment using paraffin wax. The paraffin wax fails to pass a current when melted.
2. To make a liquid rheostat attach leads to the carbon rods from two dry cells. Dip the ends of these carbon electrodes in a dilute sodium chloride solution. Put a switch, a torch globe and 1.5 volt battery in the circuit. Close the switch and adjust the distance between the carbon electrodes or add more sodium chloride until the torch globe glows. Changing the distance between the carbon electrodes changes the strength of the current in the circuit just like a rheostat. Instead of using carbon electrodes, attach leads to two metal milk bottle tops floating in the sodium chloride solution in a Pyrex dish or earthenware dish. Do not use a metal dish.
32.2.6 Taste current electricity
Touch two wires from a 1.5 volt battery with the tip of your tongue. Do not let the wires touch each other. You should feel or "taste" something. The electric current has set up a current in the nerve cells of your tongue and these are carried to the brain causing the sensation you feel. This is an old method of testing whether there was still any "juice" in the battery. Do not try it with a car battery or mains!
32.2.7 Current electricity from frog's leg
Put one wire on the body of a dead frog and with the other, probe the frog near the pit of its stomach. You should be able to make the legs twitch. The electric current from the wires set up electric currents in the nerves of the frog and these currents run along the nerves to the muscles causing them to contract and move just as if the current was coming along the nerve of a live frog.
This experiment seems to have nothing to do with physics. However, Luigi Galvani (1737-1798) was one day cutting the legs off dead frog to make soup. To dry the legs he hung them on an iron fence using copper hooks. He noticed that the dead frog legs started shaking when the toes of the legs touched the iron fence. He concluded that "animal electricity" was in the muscles of the frog legs. Later Alessandro Volta (1745-1827) repeated the observation as an experiment and concluded that the electricity came from the copper and the iron. He experimented with other metals and dipped pieces of copper and zinc into a container of salt solution. To get more electricity he made a pile of these containers and so invented the voltaic pile, a battery. Although Galvani was wrong about the frogs' legs we still use the terms galvanized, galvanometer and even galvanic. But Volta is better remembered as the inventor of the volt.
32.2.8 Bite on aluminium foil
If you bite on a piece of aluminium cooking foil or a lump of foil from a packet of chocolate you may feel a sudden pain, especially if you bite on the foil with your molars that have a large amalgam tooth filling. The pain is caused by current flowing between the foil and the metal amalgam through the saliva containing salts to stimulate the nerve ending in the tooth. If you have perfect teeth with no tooth filling the experiment does not work!
32.3.0 Resistance, symbol R, unit ohm (Greek ω, omega) resistivity, rho, specific resistance, resistors
The resistance of an object, R, e.g. a wire, measures the potential difference in volts, V, needed for one ampere, A, of electric current to flow through it. R = V / I. So 1 ohm = 1 V / A, 1 volt per amp
Different materials offer different resistance to the flow of electric current through them and convert electrical energy to heat energy. Copper, silver and aluminium are examples of good conductors that offer very little resistance. Glass, wood, and paper are examples of poor conductors, insulators, which offer high resistance to current flow. The material of the wires in an electric circuit is chosen to keep the electrical resistance as low as possible so that current can flow easily through the conductors.
In an electric circuit, the larger the diameter of the wires, the lower will be their electrical resistance to the flow of current through them. For alternating current resistance is a component of impedance. The electrical resistance of the conductors depends upon 1. the length of the wires, 2. the diameter of the wires, 3. the material of the wires, e.g. copper, aluminium, 4. Temperature.
For most conductors, e.g. copper, aluminium, iron, resistance increases with temperature. However the resistance of carbon decreases as temperature increases and for some alloys of metals, e.g. manganin and constantan, resistance hardly changes with temperature.
1. Effect of length and thickness on the resistance of a wire
2. Specific resistance, resistance wire, manganin wire, nichrome wire
3. A fixed resistance is usually a coil of insulated resistance wire in a container. Mark the value or the resistance on it unless it is an "unknown" for testing students.
4. A The rheostat consists of a long solenoid of resistance wire that can be " tapped " at any part by a sliding contact. When connected as shown, the current enters at A, flows along the copper or brass bar (negligible resistance) to s, then via the sliding contact to the solenoid, through that part of the solenoid shown in heavy line, and out at A. The maximum resistance of the rheostat and the maximum current that may safely be passed through it is usually stamped on the instrument, e.g. 5 ohms, 2 amps.
32.3.1 Resistor assortment, resistor colour code
See diagram 32.3.1: Resistor colour code
Carbon resistors and adjustable carbon composition resistors are commonly used in electronics because they are compact and cheap but they are not accurate especially at high power levels. Wire wound resistors and adjustable wire wound resistors may be very accurate except at very high power ratings.
Examine assortment of different resistors.
32.3.1.1 Resistors, resistances, switches
See diagram 32.3.1.1
See 22.7.6: Thermocouple, thermistor, constantan, optical pyrometer
1. A resistance box contains coils of wire of known resistance connected in series by thick brass blocks. You must remove plugs to obtain the required resistance in the circuit.
3. The relative resistance of the following conductors of the same length and cross-section, with silver as a standard of "1", are arranged in an order of ascending resistance as follows: Silver 1.0, Copper 1.08, Gold 1.4, Aluminium 1.8, Platinum 7.0, Lead 13.5. The resistance in an electrical circuit is expressed by the symbol R and is measured in ohms. One ohm is the resistance of a circuit element that permits a steady current of 1 ampere (1 coulomb per second) to flow when a steady EMF of 1 volt is applied to the circuit.
4. Resistor boxes are used to show Ohm's law. A resistance board is set up as a simple Wheatstone bridge to find the resistance.
5. Rheostats are used as protective resistors or voltage dividers. Coils are rated by number of windings and resistance. Manufactured circuit parts containing definite amounts of resistance are called resistors.
6. Resistance wire diameters are measured by Standard Wire Gauge, SWG (UK, Australia) or Brown and Sharpe (B. and S.) (American Wire Gauge). SWG 50 is smallest gauge and SWG 70 the largest. Cable sizes are shown as follows: 14 / 36 = 14 strands of 36 SWG wire to carry 2 amps for internal lighting in a motor car, or 61 / 20 = 61 strands of 20 SWG wires to carry 150 amps suitable for 6 volt starter motors in a car.
1. Connect one meter lengths of various wires in series and measure the voltage across each. Measure voltages on a commercial board with seven one meter lengths of various wires is series so all carry the same current. Place 6V across a set of wires of different lengths and / or diameters and measure the currents.
2. Make a simple switch. Fasten the end of a piece of wire to a pencil with two rubber bands. A second wire makes a connection.
3. Examine switches in a circuit. Put a knife switch in a circuit with a cell and a lamp and turn the light on and off by operating the switch. Replace the lamp with a bell or buzzer and operate the switch. Replace the knife switch with a push button switch. Take apart some common switches such as the common household, tumbler switch, rocker switch. See how they are constructed.
4. To recognize the operating principle of usual electric switches, find all kinds of electric switches used in daily life, such as pull switch, reading lamp switch, suspension wire switch, cassette switch. Teachers also should find some electric keys such as single knife switch, double knives switch, single throw switch, multithrow switch, some switches of apparatus and range switch and so on. Disassemble some switches which can be disassembled. Observe the composition of those switches and the connection among down leads and inner components of switches, and how switches switch on and switch off rapidly. In the on-off operation after turning on the switch, you must pay attention to preventing the spring from bouncing out; if it bounces out, you must install it to its original place quickly. Discuss and summarize what kind of significance the knife number and the multithrow of switches have. Reassemble the disassembled switches. Connect different switches into the circuit composed of cells and bulbs. Prepare more conducting wires and bulbs for the multiknife or multithrow experiment. At the end of the experiment observe the values of allow voltage and allow current, and explicate their significance. The values of allowed voltage and allowed current are labelled on the outsides of switches.
32.3.1.2 Resistance model
Roll a ball down a board with randomly spaced nails. Roll ball bearings simultaneously down two ramps one with pegs and one without.
32.3.1.3 Ohmic and non-ohmic resistors (nonohmic resistors)
Resistors which obey Ohm's Law are called ohmic resistors, so if you draw a graph of voltage against current the curve is linear, the slope is constant and gives the resistance. If the resistance of a resistor changes as the voltage increases, the resistor is a non-ohmic resistor, so if you draw a graph of voltage against current the curve is not linear, so the slope of the curve at a particular voltage shows the resistance only at that voltage. A light bulb filament is a non-ohmic resistor.
32.3.1.4 "Megger"
Trade name for an instrument used by electricians to measure electrical insulation resistance.
32.3.2.0 Resistivity with metre wire bridge, Resistivity and temperature
See diagram 32.2.60: Metre wire bridge
Copper wire, 18 SWG, bare, 1.22 mm diameter, 0.0418 Ohm / m
Nichrome wire, 32 SWG, bare, 0.274 mm diameter, 18.4. Ohm / m
Resistivity symbol is ρ (Greek: rho) and the unit is ohm metres. If resistance, R, of a wire length, L, and cross-section area, a m2, R = rho x (L / a). The resistance wire Constantan (Eureka wire) has a high volume resistivity and almost negligible temperature coefficient. Resistivity depends on the material but resistance depends on the nature of the material, its length and its cross-section area. Resistivity in ohm metres of conductors = 10-8 to 10-6, semiconductors = 10-6 to 10-1, insulators 107 to 1023. Resistivity is the reciprocal of conductivity. In semiconductors, the higher the level of doping, the lower the resistivity.
An A C battery, 3 V flashlight bulb, and a copper wire coil make a hand held temperature coefficient of resistivity apparatus. Resistance changes with temperature. If resistance changes with temperature, a wire with resistance R0 at temperature T0, then resistance R at temperature T = R0+ aR0(T - T0), where "a" = temperature coefficient of resistance.
1. To observe resistivity with metre wire bridge set up as in diagram 32.2.60 except substitute for R1 a material, e.g. a wire, of known length and cross-section area. Measure the resistance of length I cm of the specimen taking care that on interchanging the specimen R, and known resistance R2 the same length I cm of the specimen is exposed between the terminals in each case (tie small knots in the wire near each end and ensure that these knots just emerge from the terminals in each case). Measure the length I cm of the specimen under test with a metre rule. Use a micrometer screw gauge to measure its diameter d cm. at four different places. As in 32.2.60 for determining the resistance R, of the specimen, the resistance of a wire, is proportional to its length l, and inversely proportional to its cross-section area A, so resistance R is proportional to length l / area A. So R = rho X (l / A), where rho is a constant called the resistivity of the material. So rho = (RA) / l ohm cm.
2. To measure temperature coefficient of resistance of material with a metre wire bridge, insert the thermometer through the cork and wind the specimen in a coil round the stem of the thermometer keeping the turns separate. Tie the coil to the thermometer with thread. Connect the ends of the coiled specimen to copper wire leads. Record this temperature t1oC after heating for ten minutes when the temperature of the coil becomes constant. and measure the balance point AB and DC. Heat the beaker plus contents for 10 minutes, record the temperature of the coil, t2oC, and measure the new balance point AB2 and B2C. Electrical resistance of a material varies with temperature. For metals, over small ranges of temperature, the variation is regular. If resistance of a metal wire is R0 at 0oC. and Rt, at temperature toC, Rt = R0(1 + at) where "a" is a constant called the temperature coefficient of resistance. So Rt1 = R0(1 + at1), and Rt2 = R0(1 + at2). So Rt1 / Rt2 = (1 + at1) / (1 + at2). Rt1 = R2(AB1) / (B1C) ohms. Rt2 = R2(AB2 / (B2C) ohms.
32.3.2.1 Heat and cool resistors
1. To observe change in the resistance of a conductor if the temperature changes, use a 6 volt lamp and adjust the variable resistance so that you get voltmeter and ammeter readings for a range of filament temperatures, the globe changing from cool to red-hot to white hot. Tabulate your results and draw a graph plotting potential difference against current. The resistance increases as the temperature increases. Investigate the effect of a Bunsen flame or dry ice on the resistance of a piece of jug element.
2. To observe temperature change and resistance use 8 coils of one metre SWG 32 enamelled copper wire. Connect long leads to the loosely wound coil of copper wire. Adjust the rheostat to 5 amp in the coil. Open the switch. After 1 minute close the switch and read the ammeter and voltmeter several times during the next half minute. During this time the coil heats and the current changes rapidly. Repeat with the coil of copper suspended in water in the container. Keep stirring the water.
3. Increase the current in a long U-shape of iron wire until it glows then insert half of the U into a beaker of water. Heat and cool resistance coils with a test light bulb in series. Put two coils of different material but the same resistance in a Wheatstone bridge and either is heat or cool. Heat a coil of iron wire in series with a battery and a lamp and the lamp will dim. Heat a coil of forty turns of iron wire in a flame while connected in series with a light bulb circuit.
32.3.2.2 Put the light out with heat
Wind a coil of iron wire on a porcelain core in series with a lamp and battery then heat until the lamp goes out. Connect a coil of nickel wire to a battery and galvanometer then heat in a flame.
32.3.2.3 Carbon and tungsten light bulbs
Measure current and resistance at various voltages for a carbon and tungsten bulb for positive and negative resistance coefficients. Plot current vs voltage for carbon and tungsten lamps.
32.3.2.4 Temperature of incandescent lamps with silicon solar cells
Use two silicon solar cells with interference filters to measure the light at different wavelengths to determine the temperature of the filament.
32.3.2.5 Liquid nitrogen / liquid air experiments
A lamp glows brighter when a series resistance coil is immersed in liquid nitrogen / liquid air. A copper coil in series with a battery and lamp is immersed in liquid nitrogen.
32.3.2.6 Thermistors effect of heat on a thermistor, effect of low heat changes on conductivity with a thermistor
See diagram 4.1.8
The effect of low heat changes on conductivity with a thermistor.
1. A thermistor, thermal resistor, is a semiconductor made of Co, Ni, Mn oxides and copper powder. Its resistance is very sensitive to temperature. When the thermistor is cold, a current of 25 mA will not be detected by the ammeter. Heat the thermistor very carefully with a Bunsen burner and the current rises. Stop heating when the current reaches 0.3 amps.
2. To show that materials change their conductivity, set up a simple series circuit with a voltage supply, the rheostat, the copper coil and the ammeter. Adjust the rheostat or the battery connectors until 0.8 amp flows. A very low voltage is needed. Warm the coil very gently in a low Bunsen burner flame and read the ammeter. Also, immerse the coil in a mixture of ice and salt. Replace the copper coil by a coil of high resistance wire and repeat the experiment with a greater EMF. Use a gently warmed thermistor to replace the copper coil. Use a block of salt in a crucible, into which two stout pieces of copper wire dip. Then gently heat. Two stout pieces of copper wire are embedded into the paraffin wax so that they do not touch. These pieces of wire are then connected by the circuit in place of the high resistance coil. Heat the tube and observe the current. The paraffin wax fails to pass a current even when melted. Use a glass rod. Wind three turns of stout copper wire round 8 cm of glass rod connected into the circuit. Heat the rod gently. It does not conduct electricity.
3. Use commercial thermistors and display the differential negative resistance of a fast thermistor on a transistor curve tracer. Show the resistance of a thermistor placed in an ice water bath.
32.3.2.7 Electrical conductivity of molten glass at high temperature
See diagram 4.1.4: Heat a glass rod
1. Glass can be a conductor. Heat a glass rod until it becomes very hot and begins to soften. Test the hot, soft part with the conductivity apparatus. When molten, glass is a good conductor of electricity.
2. Wrap each end of two copper wires tightly on a glass tube so that the distance between the wrappings is less than one cm. Connect the other end of the two copper wires to the lamp and storage battery to form a series circuit. The glass tube between the wires becomes a part of the circuit. Observe if the lamp is lights. Heat the glass tube with an alcohol burner. As the lamp is lights, turn off the burner immediately. Make two closely fitting coils four turns of bare copper wire, e.g. SWG 14, on the soft soda glass rod at points 5 cm apart. Bend the lengths of wire at right angles to the rod and terminate them in two well insulated 4 mm plugs. Put a tray of sand under the glass rod. Support this assembly so that the electrodes are insulated from the iron retort stands. Connect the two electrodes into a series circuit consisting of a rheostat 05 a.c. ammeter and the 240 volt a.c. mains supply. Retort stands and the Bunsen burner should be connected to earth. Close switch. No current flows. Heat the glass rod. Watch closely the contacts between the copper coils and the glass. When the glass starts to become self-luminous, it will conduct electricity. Remove the flame and watch the rod slowly redden, and melt. Open the switch.
3. Heat a capillary tube in a Bunsen burner until it is hot enough to sustain a current that maintains a bright glow. Heat a glass tube with a flame until it is hot enough to sustain conduction then vary the current by changing the ballast resistance. Heat a Nerst glower with a flame until the resistance is low enough to sustain electrical heating, negative temperature coefficient of resistance. The glower is a solid-body radiator that is made up of a filament of rare earth oxides. Heating the filament by continuous ohmic heating results in conduction. The glower operates best in wavelengths from 2 to 14 µm.
32.3.3.0 Liquids that conduct electricity, conduction in solutions, conduction through electrolytes, conductivity of solutions
Classify substances into the following groups:
1. Substances that conduct electricity in the solid state,
2. Substances that conduct in the liquid state,
3. Substances that conduct electricity when dissolved in water.
1. Relationship between volts and amps for electrolytes
Connect the copper voltmeter in a series circuit. Find the voltage / current relationship by connecting 1, then 2, 3, 4, 5, 6 cells across the voltmeter. Draw a graph of voltage against current. The technique of changing the number of cells without introducing a rheostat is essential to avoid difficulties with polarization. The 12 volt batteries must allow tapping off intermediate voltages.
1.1 Use the same procedure for the gas voltmeter. With the copper voltmeter a rheostat could be used. Use 4 mm sockets.
2. Test liquids obtained by melting substances. Melt the following substances, but heat very gently because they may ignite: sulfur, wax, naphthalene (moth balls), polyethylene material, tin, lead and, if available, a low melting point salt such as lead bromide, mp 488oC, or potassium iodide, mp 682oC. Test the conductivity of the melt by dipping in the electrodes and waiting a few moments for the electrodes to reach the same temperature. This ensures that the electrodes are in contact with the liquid and not the solidified melt. Scrape and clean the electrodes between each test.
3. Test ethanol or methylated spirit, acetone, vinegar, sugar solution, copper (II) sulfate solution, sodium chloride solution, and other substances dissolved in water. Clean and dry the electrodes between each test.
4. Test pure deionized water for conductivity. Put the electrodes into a beaker of deionized water. Students find that the bulb does not light up and therefore pure water does not conduct. Slowly stir small crystals of common salt into the water. Note what happens to the bulb as the salt dissolves.
5. Tests for conductivity tap water, distilled water and deionized water.
6. Dip two metal electrodes in series with a light bulb in various solutions of electrolytes. Immerse two copper plates in series with a lamp in deionized water, then add barium hydroxide, then add sulfuric acid. Put two copper plates in series with a lamp in deionized water and add salt or acid. Dip two electrodes in series with a 110 V lamp into deionized water, salt water, sugar solution, vinegar solution and tap water.
32.3.3.1 Saltwater string, electrolytic conduction on chamois
Suspend a chamois between ring stands to show no conduction with a battery resistor meter then soak in deionized water repeat, then sprinkle on salt and repeat again.
32.3.3.2 Migration of ions, speed of ions
Show KMnO4 migrating with current towards the positive electrode in KNO3. Permanganate ions migrate in an electric field. Dip two platinum electrodes into an ammoniated copper (II) sulfate solution containing some phenolphthalein. Blue moves from the anode of in a potassium chloride gel when 120 volts is applied.
32.3.4.0 Conduction in gases, Jacob's ladder
Voltage / current relationship for a gas
See diagram 32.3.4.0: Conduction in neon lamp
Set up a series circuit consisting of the supply, the neon lamp and the 100 mA meter. Connect the voltmeter across the lamp and a 100 mA meter. Apply increasing voltages from 0 to 240 volts and record both the current and the voltage. The striking potential for a neon lamp is about 170 volts. The glow will be extinguished when they reduce the voltage to about 150 volts. To prevent excessive currents, neon lamps have ballast resistors of about 2 000 ohms in the bases. An arc rises between rabbit ear electrodes attached to a 15 KV transformer.
32.3.4.1 Conduction of gaseous ions
A nearby flame will discharge an electroscope. Insert a flame connected to a high voltage source between charged parallel plates. Use compressed air to blow ions from a flame through the area between charged parallel plates onto a mesh hooked to an electrometer. Connect electrodes at the bottom, middle and top of a tube to an electrometer while a Bunsen flame burns at the bottom.
32.3.4.2 Discharge by ions in a tube, recombination of ions
Draw ions from a flame past a series of charged plates attached to a Zeleny electroscope.
32.3.4.3 Separate ions from flame
Shadow project a flame between two charged metal plates to observe separation of gas into two streams of oppositely charged ions.
32.3.4.4 Ionization by radioactivity, conduction in air by ions, smoke alarms
1. All ionization smoke alarms use an extremely small amount of a radioactive element in their ionization chambers, e.g. 37 Bq of Americium 241 in compliance with U.S. NRC safety criteria in 10CFR 32.27.
2. Charge an electroscope with a radioactive source. Bring various sources of ionization near parallel wires attached to a 100 V battery and a Zeleny electroscope. Increase the voltage across a plate close to a wire mesh with a radioactive source nearby and observe the current with a Zeleny electroscope. Use an electrometer to measure the current between parallel plates as a flame is burned between them or an alpha source is held nearby. In a Cerberus smoke detector combustion products decrease conductivity in a chamber with an alpha source.
32.3.4.5 Conduction from hot wire
Hold a constantan wire near a charged electroscope to cause discharge when it is heated red-hot.
32.3.4.6 Thermionic effect in air
A Zeleny electroscope indicates electron emission from a wire when it is heated.
32.3.4.7 Thermionic effect in air, thermionic emission
Use a commercial tube neon tube to apply 90 V forward and reverse and monitor the current. A neon lamp lights at about 80 V and shuts off at about 60 V.
32.3.4.8 Ionization by X-rays
Charge an electroscope with X-rays. Pass an X-ray beam through a simple ionization chamber.
32.3.4.9 Electrohydrodynamics
Practical examples of electrohydrodynamics are ink jet printing and electrically driven convection.
32.4.0 d.c. circuits, circuit diagrams, Kirchhoff's laws, Ohm's law
See 32.2.00: Circuit diagrams, electrical symbols
An electric circuit is a complete conducting path around which the current can flow. The EMF is the source of work per unit charge and is used up by the potential difference in the circuit to turn an electric powered device. Circuit diagrams use a system of conventional signs. To connect a circuit first arrange the apparatus in the pattern shown in the circuit diagram. Bare the ends of the connecting wires. Connect the components with suitable lengths of wire. Check that all connections are tight. With large currents use thick connecting wires.
32.4.06 Effect of change of resistance on an electric motor
See diagram 32.4.06: Change of resistance
Connect a little electric motor to a large 1.5 volt dry cell. Use a rheostat to make your motor start slowly, come up to full speed and then slow down. As the copper wire is moved nearer to C, you make the electron current to flow through more of the jug element and meet more resistance. Thus the voltage of the battery cannot push electrons around the circuit as rapidly as before and the motor slows.
32.4.1 Kirchhoff's laws, Ohm's law
1. Volts / amps relationship for electrolytes, voltage / current relationship for gases
See diagram 32.2.54: Resistors in series and parallel
1. Kirchhoff's laws (Gustav Kirchhoff 1824 - 87)
Law 1 (Junction law): At any junction point in an electrical circuit, the sum of all currents entering the junction = the sum of all currents leaving the junction. I = I1 + I2 + I3, where I = total current and I1, I2, I3 = separate currents.
Law 2 (Loop law): For any closed loop in an electrical circuit, the sum of the voltages = zero.
V = V1 + V2 + V3, where V = total voltage and V1, V2, V3 = separate voltages.
2. Ohm's law (George Simon Ohm 1789 - 1854)
The electric current in a conductor is proportional to the potential difference
V = IR, volts = ampere X ohm. Ohm's law, volts / amps relationship for electrolytes, voltage / current relationship for gases, Ohmic conductors, Ohm's law and Kirchoff's laws in simple circuits
V = IR, P = VI, W= VIt, connection of simple circuits and use of appropriate meters to measure current, EMF, and potential difference around the circuit, verification of Ohm's law with a simple series circuit or voltage divider network; plotting of I / V characteristic curve.
Ohm's law defines the equation for resistance, V = IR where V = potential difference (pd) between the ends of a resistor, I = current through the resistor, R = resistance of the resistor.
2. Use three dry cells or 6 volt batteries or from a 12 volt battery. By adjusting the rheostat a series of corresponding values of current and potential difference across the high resistance can be obtained. Use both arithmetic and a graph to find the ratio potential difference / current.
3. Measure current and voltage in a simple circuit. Change the voltage or resistance. Connect an ammeter, voltmeter, rheostat and battery pack to show Ohm's law. Place 2 V, 4 V, and 6 V across a resistor and measure the current then graph the results.
4. To observe resistance of a conductor using an ammeter and voltmeter, apply a potential difference to an electrical conductor and some current flows through it. Ohm's Law states that, provided the conductor does not get hot, the current is proportional to the applied potential difference, so the ratio (PD applied to the conductor) / (current through the conductor) is a constant called the resistance, R of the conductor. Connect the circuit as shown in the above diagram. Close switch S. Adjust the rheostat Rh so that a small current passes through the conductor of unknown resistance R ohms. Record the current I amps and the potential difference V volts between the ends of R. Adjust the rheostat Rh to get of five pairs of readings of current I amps and potential difference V volts. Calculate R = V / I for each pair of readings.
32.4.2 Simple electric circuit
See also 32.2.00: Electric circuit symbols
Connect an electric bulb, e.g. 2.4V, 0.5A, and lamp holder, to the +ve and -ve terminals of a dry cell or lead cell accumulator or low voltage power supply. Notice the filament made of tungsten carbide. Passage of the electric current through the tungsten carbide wire causes it to become very hot and give off light. Reverse the connections to the source of electricity and the lamp still operates although the electricity is flowing in the opposite direction. Draw a diagram to show the path of the current through the bulb and around to the other end of the cell. This is a simple electric circuit. Circuit diagrams are used to represent the electrical components in a circuit.
32.4.3 Charge density in circuits
Use an electroscope to probe the charge density along a large resistance attached to a 5 KV supply.
32.4.4.0 Electric circuit, electric circuit board, water circuit board, water analogy circuit
See 32.2.00: Circuit diagrams, electrical symbols
1. A water analogy illustrates voltage drops across a d.c. circuit.
2. Use a piece of heavy cardboard 30 x 30 cm as a base. Fixed clips on it for holding the cells, and sprung metal strips for providing connections between cells. Screw brass curtain rod holders for circuit making into the base. Make spring connectors of varying lengths from curtain wire with hooks inserted at each end. Put light bulb holders into circuits by using curtain wire connectors or heavy No. 16 uninsulated copper wire. Make other connections with lengths of uninsulated copper wire attached to crocodile clips.
32.4.4.1 Resistors in series and parallel
See diagram 4.157 Resistors in series | See diagram 4.158: Resistors in parallel | See diagram 32.2.2.1: Resistors in series and parallel
For resistors in series R. = R1 + R2 For resistors in parallel 1 / R = 1 / R1 + 1 / R2
Resistors in series: Connect two resistors in series, e.g. R1, 2 ohms and R2, 4 ohms, with combined resistance R, 6 ohms. Adjust the rheostat Rh to get of five pairs of readings of current I amps and potential difference V volts. Calculate R = V / I for each pair of readings.
Resistors in parallel: (The ammeter should read about 6 amps.) Adjust the rheostat Rh to get of five pairs of readings of current I amps and potential difference V volts. Calculate R = V / I for each pair of readings.
32.4.4.2 Circuit elements in series
Circuit elements in series have the same current flowing through them. The total potential difference across them is the sum of the separate potential differences. Total potential difference of a circuit = V1+ V2+ V3. Total resistance = R1+ R2+ R3
32.4.4.3 Circuit elements in parallel
Circuit elements in parallel have a common potential difference across them, and the total current through them is the sum of the separate currents. Total current = I1+ I2+ I3. Total resistance = 1 / R1+ 1 / R2+ 1 / R3, so the total resistance will always be less than the smallest resistance in parallel. Kirchhoff's laws state that the total current entering a junction in a circuit must equal the total current leaving it and the sum of the potential drops around a circuit must be equal to the total EMF.
32.4.4.4 Series and parallel (branching) circuits
See diagram 32.4.4.4: Branching circuits
In a series circuit, the current is the same in all parts of the circuit. In a branching or parallel circuit, the total current = sum of currents in the branches.
1. For a series circuit, adjust the current to 0.4 amps with the rheostat. Can you include a fourth ammeter between two of the cells in the battery?
2. With a 12 volt battery and two 12 volt watt lamps, A1 reads 0.5 amps. Adjust R3 so that A3 reads 0.3 amps. Adjust R2 so that A2 reads 0.2 amps. Now A4 reads 0.5 amps (same as A1), A5 reads 0.7 amps (A2 + A4), and A6 reads 1.0 amps (A3 + A5).
32.4.4.5 Lamps in parallel
See diagram 32.4.4.5: Lamps in parallel
The lamp holder bases and the single pole switches should be fitted with 4 mm insulated terminals. Connect the ammeter, the four lamp holders and switches to the mains supply and note the current as you switch on more lamps. This shows that the rate of obtaining the output energy in joules / second is proportional to the rate at which coulombs pass, coulombs / second, as shown by the readings on the ammeter.
32.4.4.6 Lamps in series and parallel
See diagram 36.1.0: Household lamps in series and parallel
1. Connect one, two and three identical bulbs in series. Record the brightness of the bulbs. When you connect bulbs in series, the total voltage is divided between them, e.g. if three bulbs are connected in series to a 3 volt battery, each bulb receives 1 volt. Connect one, two and three bulbs in parallel. Record the brightness of the bulbs. When lamps are connected in parallel, each bulb receives the full voltage of the supply.
2. Make up two boards containing three 60 W household lamps, one board wired in series and the other board wired in parallel. When plugged into the mains the series wired lamps will be dimmer than the parallel wired lamps. If you substitute a 15-W lamp for one lamp in the series board, the other two lamps are dimmed. If you substitute a 15 W lamp for one lamp in the parallel board, the other two lamps are not dimmed. So parallel wiring is used in household electrical circuits.
32.4.4.7 IR drop in a wire, potential drop along a wire
See diagram 32.4.4.7: Potential drop along a wire
1. To observe change in voltage as a current flows through a wire, use a straightened electric jug element. Attach one metre of it to a board. Observe any voltage drop between any two points in the circuit by pressing the bared ends of the voltmeter connecting wires to the points. The potential difference between two points along a uniform conductor is proportional to the distance between the points.
2. To measure the fall in potential along a wire carrying current note that the shorter the length of the wire the smaller the fall in potential. If a wire has uniform cross-section, the potential difference V between two places along the wire should be proportional to the distance between them. If potential falls uniformly along the wire, a graph of distance potential against distance should be a straight line. Adjust the rheostat so that when the sliding contact B is near C and the switch is closed, the voltmeter V shows full scale deflection, e.g. 3 V. Close switch S and make contact the resistance wire a.c. so that AB = 10 cm and record the potential difference V volts between A and B. Repeat for AB = up to 100 cm. Plot a graph of AB cm. (x axis) against V volts (y axis).
3. Clip wires from the terminals of flashlight lamps at various points along a stretched wire carrying 2 to 5 amps. Use a voltmeter and ammeter to measure current and voltage on several samples of wire of the same length or use a slide clip to vary length. Measure the voltage at six points on a long resistance wire.
32.4.4.8 Potential drop with Wimshurst machine, static machine
Attach a 3 m long wood bar at one end to one terminal of a static machine, with the other end grounded or insulated, then attach electroscopes along the bar to show flow of charge and potential drop. Attach two ends of a dry stick to a static machine then measure with an electrostatic voltmeter and micro-ammeter.
32.4.5 Switch, switches in a circuit, tapping key
See diagram 4.152: Simple switch
1. Make a simple switch. Fasten the end of a piece of wire to a pencil with two rubber bands. A second wire makes a connection.
2. Insert switches in a circuit. Put a knife switch in a circuit with a dry cell and a light bulb. Turn the light on and off by operating the switch. Replace the light bulb with a bell or buzzer and operate the switch. Replace the knife switch with a push button switch. Examine the construction of different switches, e.g. household tumbler switch, rocker switch. Use them in a circuit.
3. Collect materials to be tested for electrical conductivity, and to suggest answers to this question. Try paper, eraser, plastic button, key, coins, cloth, string, chalk, glass, nail, nail file, insulated wire, bare wire. Test these in a circuit across an open knife switch, or in a tester made as shown in the diagram. Materials which carry electricity are called conductors. Materials which do not carry electricity are non-conductors (insulators). The copper of a wire is a conductor; its covering is an insulator.
32.4.6 Cells in series and parallel
See diagram 32.4.6.0: Cells in series | See diagram 32.4.6.1: Cells in parallel | See diagram 32.2.1.1: Cells in series and parallel
1. Cells connected in the same direction in series each add their own voltage, e.m.f., to the total voltage. However each cell has an internal resistance, r. So if connecting three cells of voltage V1, V2 and V3, if current through each of the cells is I amps, then total voltage = [V1 +V2 + V3 - (Ir1 + Ir2 + Ir3)].
Connect two dry cells or lead cell accumulators so that the negative terminal of one is in contact with the positive terminal of the other. Connect them in series. Put a light bulb in the circuit. Close the circuit with one cell, two cells, three cells in series. Record the changes in the brightness of the light bulb. The brightness of the light depends on the number of cells connected in series. When you connect cells in series, the total voltage is the sum of the individual voltages of the cells. If you use 1.5 V cells, then two cells give 3 V, and three cells give 4.5 V, four cells give 6 V.
2. Total EMF of cells in series is the sum of each EMF = EMF1 + EMF2 + EMF3, e.g. 3 X 1.5 volt torch batteries correctly positioned in series produce 4.5 volts. A group of similar cells is called a battery.
Cells in series: If the EMF and internal resistance of each cell are e volts and r ohms respectively, and there are n cells in series, EMF of battery = ne volts and internal resistance of battery = nr ohms.
Cells in parallel: If the EMF and internal resistance of each cell are e volts and r ohms respectively, and there are n cells in parallel, EMF of battery = e volts and internal resistance of battery = r / n ohms.
3. Connect two dry cells or lead cell accumulators so that the negative terminal of one is in contact with the positive terminal of the other. They are connected in series. Put a bulb in the circuit. Close the circuit with one cell, two cells, three cells, in series. Record the changes in the brightness of the lamp. The brightness of the light depends on the number of cells connected in series. When you connect cells in series, the total voltage is the sum of the individual voltages of the cells. If you use 1.5 V cells, two cells give 3 volts, and three cells give 4.5 volts, four cells give 6 volts. The current will change.
4. When three identical cells are connected in parallel the total voltage is as if for one cell. However the total resistance for one cell is 1/3 r. So total voltage = V -3I(1/3 r). So motor car batteries may be connected in parallel to provide the extra current needed to start the engine.
Connect two or three fresh dry cells or lead cell accumulators so that you join their positive terminals and they join their negative terminals. They are connected in parallel. Set up a circuit on a circuit cardboard with three cells in parallel. Disconnect one or two of the cells. The circuit is not broken and the brightness of the light does not change. The voltage drop in the circuit is the same if you use one, two or three cells. The total current is unchanged. If four cells in the circuit, the total current is 0.125 X 4 = 0.5 A.
5. Total EMF of identical cells in parallel is the same as for one cell, e.g. 3 X 1.5 volt torch batteries in parallel produce 1.5 volts. However, the effect of internal resistance is reduced because total resistance = r / 3. Total EMF = [EMF1- 3I(r / 3)].
6. Connect two or three fresh dry cells or lead cell accumulators so that their positive terminals are joined and their negative terminals are joined. They are connected in parallel. Set up a circuit on a circuit cardboard with three cells in parallel. Disconnect one or two of the cells. The circuit is not broken and the brightness of the light does not change. The voltage drop in the circuit is the same if one, two or three cells are used. The total current is unchanged. If four cells in the circuit, the total current is 0.125 x 4 = 0.5 amps.
32.4.6.2 Electric torch, flashlight
See diagram: 4.154 Workings of a flashlight | See diagram: 4.154.1: Electric torch
A Glass screen in front protects the light bulb, B Small incandescent light bulb (lamp), C Reflector, D Electric switch, E Batteries, F Cover that can be gripped in the hand and containing part of the electric circuit, G Spring to keep batteries tightly together, H Screw opening at the end for battery replacement.
1. Be able to trace the circuit in an electric torch. Use a torch with metal sides and a torch battery opened with the back of an axe. Take a torch to pieces and put it together again. Show the class the torch, turn it on and off with the sliding switch. Dismantle the torch. Show the class the different parts: switch, metal case. Use the arrows to show them how these parts are part of a circuit. Show the children the opened battery. The electricity comes from the zinc case when some zinc dissolves in the black chemical. In the very old batteries so much zinc dissolves that holes in the zinc case let some chemical leak out. The carbon rod does not dissolve. Why should the two batteries be in the same direction in the circuit? [Otherwise they would push electric currents against each other.] Take out the batteries in a radio. Are they all in the same direction in the circuit? [Yes.] The electrical strength of a battery is measured in volts. How many volts in one battery? [1. 5 volts.] If the batteries are put end to end in the circuit, how many batteries do you need for a total of six volts? [Four.]
2. Take apart an electric torch to see the following different parts. Note the directions of insertion of batteries. The batteries must be in series. Note the rating on the side of the light bulb, e.g. 4.4 V, 0.5 A. Larger light bulbs are rated in volts, V and watts, W, e.g. in Australia, 240 V 40 W. Note the lamp type, fitting, e.g. screw or bayonet.
3. The flashlight is an electrical device which makes use of a switch, insulators and conductors, dry cells and a bulb. Examine various kinds of flashlights and take them apart. Connect the bulb to the dry cell without using the flashlight case. Reassemble the flashlight. Find the circuit in a flashlight and to determine where the circuit is completed and broken. In metal flashlights, the case is part of the circuit. In a two-cell flashlight, the cells must be inserted so that the bottom of one cell touches the top of the other to provide the proper electrical circuit. Place the cells in various positions to discover which way works best.
3. Observe its interior structure and the position of each component (bulb, switch, and cell), its circuit and how the switch operates. Secondly, install cells, operate the switch and observe if the bulb works normally. Note the installation of the cells' polarity and the electrical source in series. Draw the circuit diagram of the electric torch. Start from one battery connection or terminal and trace the conducting path to the other terminal. Make sure that you include the switch and element of the globe. Using the following standard symbols as used for radio and other electrical circuits, draw the circuit for the torch. Take apart an electric torch, e.g. electric torch, 2.4V, 0.5A, to see the different parts. Draw a circuit diagram. Note the directions of insertion of batteries.
32.4.6.4 Dry cells in an electric circuit, cells in series and parallel
See diagram 32.4.6.4: Dry cells in series and parallel
1. To observe the effect on current of increasing potential difference use an ammeter to record the electric current flowing when 1, 2 and 3 of the 1.5 volt dry cells are connected in series in the circuit. The greater the rate at which the electrons pass, the further the needle moves in the ammeter. Increasing the potential difference increases the current that flows through the wire.
2. To observe the current through an electric jug element when voltage drop changes, stretch out and cut off about six inches of the jug element; screw it firmly across the terminals of the voltmeter. Connect your ammeter, switch and four dry cells, all in series. Record the voltage when 4, 3, 2, 1 of the 1.5 volt dry cells are connected in series.
32.4.6.5 Battery, source of EMF
A battery is a source of electrical energy with electromotive force, EMF, measured in volts, equal to the potential difference between its terminals, assuming no loss of internal energy in the battery. A current whose direction does not change with time is called direct current. The current whose current intensity is invariable in the circuit is called constant current. The end of the resistor where current enters is the high potential end. Current flows through a resistor from high potential to low potential. The positive terminal of a battery is always the high potential terminal assuming the internal resistance is small. In the external circuit of the electrical source, the constant current flows from the high potential to the low potential. In the internal circuit of the electrical source, the current flows from the low potential to the high potential.
32.4.6.6 Electromotive force, EMF, measure EMF of cells
See diagram 29.03: Open right hand rule (Left hand rule) | | See diagram 32.2.58: Two accumulator cells and another cell
1. Electromotive force, EMF, measured in volts, provides a potential difference across a conductor and causes an electric current to flow through the conductor. Sources of EMF include batteries, generators, photocells and thermocouples. When a potential difference across a conductor produces an electric field that pushes on charges which force them to move and cause current flow, the direction of the electric field is from higher potential to lower potential. Show direction of current as the direction of the electric field in the conductor. By convention current goes from higher potential to lower potential. In liquids and gases that conduct electricity, positive charges move in the direction of the electric field and negative charges move in the opposite direction to the electric field. In metals and vacuum tubes only electrons (negative charges) move, and they move in the opposite direction to the electric field. Although the current starts moving around a circuit almost instantaneously, the charges move slowly, e.g. electrons in a current of five amps through a copper wire move at about 0.5 mm per second, yet in the vacuum of a cathode ray tube the electrons can move at 3 X 107 metres per second.
32.4.6.6.1 EMF and internal resistance of a cell with an ammeter and a voltmeter
See diagram 32.2.56: EMF of a cell
1. The EMF, E volts, of a cell is the potential difference between its terminals, when the circuit is open. The resistance of the voltmeter is high so little current passes through it. When the switch is closed, the voltmeter reads V volts, i.e. less than E volts. V is the potential difference needed to cause the current I amps to flow through the resistance external to the cell, mainly the rheostat Rh. (E - V) volts = potential difference required for the current I amps to flow through the internal resistance r of the cell. So I = (E - V) / r or r = (E - V) / I. With the switch S open, record the reading E volts on the voltmeter across the cell C, e.g. Daniell cell. Close the switch and adjust the rheostat to give a small current I amps and V volts on the voltmeter. Adjust the rheostat Rh to get five pairs of current I amps, and potential difference V volts. Calculate R = V / I for each pair of readings of readings. Calculate the internal resistance of the cell r = (E -V) / I.
32.4.6.6.2 EMF of two cells with a potentiometer
See diagram 32.2.56: Potentiometer
1. A potentiometer is a length of resistance wire AC of uniform cross-section with a terminal at each end, and a graduated scale. When a current flows through the resistance wire there is a steady fall in potential from A to C. So the difference in potential between two places on the resistance wire is proportional to the distance between them.
See diagram 32.2.58: Two accumulator cells and another cell
2. Use two accumulator cells, and a Leclanche cell, carbon electrode positive (dry cell) at L. Put a resistor as a protective shunt across a sensitive centre zero galvanometer G. Close switch S. Touch the potentiometer wire with the sliding contact near A then touch near C to check that the galvanometer G deflections are be in opposite directions. If not, adjust the rheostat Rh to increase the current through the circuit. Move the sliding contact to a point B1 on the resistance wire where the galvanometer shows no deflection. Disconnect the shunt across the galvanometer to make it more sensitive and measure the distance AB1 more accurately.
3. Use two accumulator cells, and a Daniell cell (copper electrode positive) at D. Replace the shunt across the galvanometer. Move the sliding contact to a point B2 on the resistance wire where the galvanometer shows no deflection. Measure AB2. When the galvanometer shows no deflection, no current is supplied by the cell at C2, that circuit is an open circuit and the potential difference between A and B is equal to the EMF of the cell. The EMF E1 of the Leclanche cell is proportional to AB1. The EMF E2 of the Daniell cell is proportional to AB2. So (EMF E1) / (EMF E2) = AB1 / AB2.
32.4.6.7 Internal resistance of a cell
The terminal potential difference, voltage, of a cell when it causes current I to flow is related to its electromotive force, EMF, and its internal resistance r, so the potential difference across each cell in series = (EMF - Ir). Total EMF = (EMF1 + EMF2 + EMF3) - (Ir1 + Ir2 + Ir3)
Terminal voltage (terminal potential difference): When a battery is producing current, i.e. discharging, terminal voltage V = (EMF) - (voltage drop in internal resistance), V = EMF -Ir
When a battery is receiving current, charging, terminal voltage V = (EMF) + (voltage drop in internal resistance), V = EMF + r.
1. See diagram 32.2.58: Two accumulator cells and another cell
To measure the internal resistance of a cell with a potentiometer, put a resistor as a protective shunt across a galvanometer G. Close switch S1. With switch S2 open, measure the balance point B1 on the potentiometer wire AC 1. with the protective shunt 2. without the protective shunt. Record AB1. Put a resistor as a protective shunt across a galvanometer G. Close switch S1.
With R = 5 ohms and S2 closed, measure the new balance point AB2. Record AB2. With R = 4 ohms and with S2 closed, measure the balance point AB3.
2. Repeat with R = 3 ohms.
Repeat with R = 2 ohms.
E = I(R + r), E is the EMF of the cell D and r the internal resistance of cell D. V = IR, where V is the PD between the terminals of D when it is sending current through R. So E / V = ® + r) / R, r = (E - V) / V X R. AB1 is proportional to E and AB2 is proportional to V, so r = (AB1 - AB2) / AB2 x R ohms.
32.4.6.8 Power wasted inside a battery
The three accumulators with negligible internal resistance are enclosed in a suitable box. Connect the terminals to two external terminals on the box. The high resistance is coiled and put in series with the accumulators inside the box to provide the "internal resistance". Record the readings of the ammeter and the voltmeter.
32.4.6.9 Heat and light from electricity, make a model electric lamp
See diagram 32.4.6.9: Model electric lamp
1. Push the ends of two pieces of copper wire, 16 swg, through a cork in a small bottle. Connect the ends of the copper wire inside the bottle with a stand of steel wool. Connect this model electric lamp model in a circuit with one or more dry cells, or lead cell accumulators, and a switch. Close the switch until the fine wire filament begins to glow. At first the heated iron wire produces light but soon the iron combines with the oxygen of the air inside the bottle and burns. Examine a manufactured lamp bulb. It contains no oxygen. It has a tungsten carbide wire filament that may be heated to a high temperature so that it glows without burning.
2. Investigate electric appliances at school and home. Note electric appliances that can produce light and heat, heat but no light, light but little heat such as fluorescence. To show that rising temperature causes objects to emit light, use equipment is similar to that in Diagram 32.2.3. The difference is that the two copper rods should penetrate through the cork. Twist the filament around the copper rods under the nether surface of the cork, and the down lead should connect with the copper sticks out of the jar. The filament is made of a thin thread of an electric iron. It should be shaped into the beeline hanged camber at the first time, the twist at the second time, and the length of the filament should be equal. Twist the thread of the electric iron around the copper end of a ball point pen's core for three to five times, and remain a little part at the end of the filament to connect with the bare copper posts. If it is difficult to connect the filament with the bare copper rods, make the ends of the rods and the filament into cap shape and hitch the filament on the rods. Connect the circuit and switch on the electric key. Then observe the heat, the light intensity and the light time of the two different kinds of "filaments" in the jar.