School Science Lessons
Electrochemistry, electrolysis, electrochemical cells, batteries, piezoelectricity
2008-09-15
Please send comments to: J.Elfick@uq.edu.au
Table of contents
33.1.0 Electrolysis, electrolytes, anode and cathode
33.3.0 Cells and batteries, electrochemical cells
33.4.0 Dry cells, Leclanche cell, flashlight battery
33.5.0 Lead accumulator cell, car battery
33.6.0 Thermoelectricity
33.7.0 Piezoelectricity
3.59.0 Conductors of electricity
15.00 Electrolytes
15.1.0 Electroplating
15.2.0 Oxidation and reduction, redox reactions
15.3.0 Rusting, corrosion
15.4.0 Electrical conductivity of a substance
15.5.0 Electrolysis
15.6.0 Electrochemical cells
15.7.0 Electrode potential of metals
32.5.0 Power and energy
32.6.0 Circuit analysis, house circuits
32.7.0 Instruments to detect electric current
33.3.0 Cells and batteries, electrochemical cells
3.84 Electrical energy from a simple cell, displacement of copper by zinc
3.84.1 Electrochemical cell, voltaic cell, galvanic cell
3.84.2 Test a simple cell with different metals
3.84.3 Test a simple electric cell with copper and zinc in dilute sulfuric acid
3.84.4 Simple galvanic cell, zinc in hydrochloric acid
3.84.5 A voltaic cell with a salt bridge
3.85 Daniell cell
3.86 Electrode potentials of metals
3.87 Lead accumulator cell
3.88 Dry cells, Leclanche cell
3.89 Movement of copper and chromate ions
3.90 Movement of ions between microscope slides, Cu2+ ions, CO2+ ions
15.6.13 Magnesium / copper battery
15.6.14 Nickel / cadmium battery, NiCad battery
15.7.0 Investigate an electrode potential order among the metals
33.3.1 Simple electric cell
33.3.2 Voltaic cell, Daniell cell, with salt bridge
33.3.3 Coin cells
33.3.4 Lemon cell
33.3.5 Simple chemical rectifier
33.3.6 Put chocolate wrapper cell in the mouth
33.3.7 Noisy potato cell
33.3.8 Hydrogen / oxygen fuel cell
33.3.9 Ionic migration
3.89 Movement of copper and chromate ions
33.3.10 Ionic friction
33.3.11 EMF dependence on electrode material
33.3.12 Contact potential difference
33.3.13 Crowsfoot or gravity cell
33.4.0 Dry cells, Leclanche cell, flashlight battery
3.88 Dry cells, Leclanche cell
33.4.2 Examine a dry cell battery
33.4.2.1 Dry cell in an electric circuit
33.4.3 Bring a dead battery to life
33.4.4 Dry cell terminals
33.5.0 Lead accumulator cell, car battery
2.87 Lead accumulator cell
33.5.1 Make a lead accumulator cell
33.5.2 Simple battery, lead acid simple battery
33.5.3 Melt nail with a storage battery, lead-salt cell
33.5.4 Internal resistance of batteries, weak and good battery
33.6.0 Thermoelectricity, thermocouple
33.6.1 Thermocouple
33.6.2 Seebeck effect and Peltier effects
33.6.3 Copper-iron junctions ring
33.6.4 Thermoelectric compass
33.6.5 Thermocouple coil magnet
33.6.6 Thermoelectric effect in a wire
33.6.7 Thompson effect
33.6.8 Thermoelectric magnet
33.6.9 Thermocouple magnet
33.6.10 Thermoelectric heat pump
33.6.11 Pyroelectric crystals, domains of electric polarization
33.7.0 Piezoelectricity
33.7.1 Piezoelectric model
33.7.2 Rochelle salt experiments
33.7.3 Piezoelectric sparker
33.7.4 Stress vs voltage
32.5.0 Power and energy
32.5.01 Heat from current through a conductor
32.5.1 Light from electrical energy
32.5.2 Heat from electrical energy
32.5.3 Make an electric heater from steel wool
32.5.4 Make a model electric light and a switch
32.5.5 Make a model electric jug
32.5.6 Measure the voltage and current to a heating coil in a calorimeter.
32.5.7 KWH meter and loads, heating with current
32.5.8 Heating wires in series
32.5.9 Hot dog / pickle cooker
32.5.10 Current through a torch globe
32.5.11 Compare the power of incandescent torch globes
32.5.12 Compare light from incandescent lamps
32.5.13 Measure light from a lamp
32.6.0 Circuit analysis, house circuits
32.6.1 Continuity of current
32.6.2 Superposition of currents
32.6.3 Standard reciprocity circuit with a potentiometer
32.6.4 Wheatstone bridge, bridge circuits, slide wire, metre wire bridge
32.6.5 Wheatstone bridge with a human galvanometer, Wheatstone bridge with light bulbs
32.6.6 Light bulb board, 12 V
32.6.7 Equivalent resistance, series and parallel
32.6.8 a.c.chopstick fan
32.6.9 Electrical circuits in a room +
32.7.0 Instruments to detect electric current
32.7.1 Simple instrument to show electric current, current detector
32.7.2 Galvanometer
32.7.2.1 Sensitivity and resistance of a galvanometer
32.7.2.2 Convert a galvanometer to a voltmeter
32.7.2.3 Convert a galvanometer to an ammeter
32.7.2.4 Convert a galvanometer to an ammeter, hot wire ammeter, heat a wire red-hot with electricity, hot wire current meter +
32.7.2.5 Measure reduction factor k of a tangent galvanometer
32.7.3 Ammeter
32.7.4 Voltmeter
32.7.4.1 Connect a voltmeter
32.7.4.2 Voltmeter as cell counter
32.7.4.3 Calibrate a voltmeter
32.7.4.4 Potential difference and electromotive force
32.7.4.5 Loading by a voltmeter
33.1.0 Electrolysis, electrolytes, anode and cathode
Nelson cell, laws of electrolysis, m / Q = constant the electrochemical equivalent (e.c.e.), electroplating
Electrodes for electrolysis experiments include C, Cu Zn, Pb, Fe and Al.
Use a car battery, large dry cells or a 2-12 volt transformer rectifier as a source of current. Use two copper wires as electrodes for the electrolysis of dilute sodium sulfate solution. Bubbles of gas (hydrogen gas) rise from one electrode. The other electrode is attacked. If instead of copper, the electrode is a short length of platinum wire or platinum foil, bubbles of oxygen will be produced. Water is decomposed into hydrogen gas and oxygen. Hydrogen gas and oxygen are obtained similarly by electrolysis of dilute solutions of many common substances.
The Hoffman electrolysis apparatus can be used as a Coulomb meter or to show Faraday's laws.
33.3.0 Cells and batteries
A simple electric cell, primary cell, secondary cell, Daniell cell, Leclanche cell, lead cell accumulator
A battery supplies direct current from two or more connected electrolytic cells. A dry cell battery can be discharged once only. A floating battery can be simultaneously discharged and charged. A rechargeable battery can be discharged and later recharged.
Anode and cathode
In both voltaic cells (electrochemical cells) and electrolytic cells (electrolysis), cations (positive ions) move towards the cathode and anions (negative ions) move towards the anode. In both cells, electrons carry current through the external circuit and ions carry current through the solutions. An electrolytic cell uses electrons supplied by an external source. The anode is the positive electrode of an electrolytic cell through which electrons leave and conventional current enters. The cathode is the negative electrode of an electrolytic cell through which electrons enter and conventional current leaves. In a voltaic cell (electrochemical cell) the reaction occurs spontaneously. The anode becomes positive attracting electrons from the external circuit.
Electrochemical cells (voltaic cell, galvanic cell) are the devices to exchange chemical energy into electricity. The current formed by a chemical cell is from chemical reactions inside the cell. A primary cell is a voltaic cell in which the chemical reaction that produces the emf cannot be reversed properly so the cell cannot be recharged by electrical means, e.g. voltaic cell (galvanic cell), Daniell cell, Leclanche cell, Weston cell (cadmium cell), mercury cell (HgO cathode). These cells are light, small and easy to replace. A secondary cell, accumulator, is a voltaic cell that can be recharged after discharged. However they are large and heavy and contain a lot of dangerous liquid electrolyte, e.g. lead cell accumulator (car battery, storage battery), nickel iron accumulator (Edison cell, NIFE cell), Ni-Cd storage batteries with potassium hydroxide electrolyte can be loaded again up to 1000 times. In a circuit where ions conduct electricity, the positive and negative ions move at the same time as an ion electric current to positive or negative poles acted on by the electric field force.
Volta's EMF idea. The distinction between EMF and electrostatic potential difference. Contact potentials between metals.
33.3.1 Simple electric cell
See diagram 33.3.1: Simple electric cell
1. Copper and zinc in dilute sulfuric acid produce electricity. Put a piece of zinc metal and a piece of copper metal into a large beaker 3 / 4 full of dilute sulfuric acid. Connect a galvanometer between the zinc metal and the copper metal. Observe the deflection of the galvanometer needle, many hydrogen gas bubbles on the copper surface, but few bubbles on the zinc surface. Zinc atoms transfer their electrons to the copper so zinc atoms become zinc ions into the solution. The copper transfers the electrons to hydrogen ions to form hydrogen gas by the contact surface of the copper with sulfuric acid. The copper acts as catalyst.
2. Use a beaker containing dilute (5%) sulfuric acid, copper and zinc electrodes and a galvanometer to show current flow. Note which electrode the bubbles gather on and how this affects the reading on the galvanometer? Stir the liquid so as to dislodge the bubbles and read the galvanometer again.
3. Put some strong aqueous copper (II) sulfate solution in a beaker. Connect copper foil to the positive terminal of a voltmeter and a zinc rod or foil to the other terminal. Dip the two metals briefly into the copper (II) sulfate solution. Note the readings on the voltmeter. The voltage falls to zero after a short time because copper deposited on the zinc and caused the reaction to stop. Note what happens at the copper rod and at the zinc rod. Determine the direction of electron flow.
4. Pour concentrated copper (II) sulfate solution into a beaker. Insert a sheet copper and sheet zinc into the solution. Measure the voltage drop by connecting the copper to the +ve terminal of a voltmeter and the zinc to the -ve terminal. Record the reading. Observe that the sheet zinc dissolves and hydrogen gas bubbles form on the surface of the copper. Observe that the voltage will decrease to zero with increasing of the copper on the sheet zinc.
5. Repeat the experiment with magnesium ribbon, an iron nail or lead foil instead of the zinc. Record the voltage each time. The larger the difference in activity between two metals, the larger the voltage. When copper deposits on the zinc electrode, it prevents more zinc from entering the solution. This causes the voltage fall to zero after a short time and the cell becomes "dead". You can add some subsidiary devices to prevent this happening.
6. Use a beaker containing dilute sulfuric acid; pieces of copper and zinc as electrodes; a switch; conducting wire and a galvanometer to show current flow. Note on which electrode the bubbles gather. Note the effect of their formation on the galvanometer. Stir the liquid to dislodge the bubbles and note what happens. Instead of zinc and copper strips use two zinc strips and, later, two copper strips. Note any current flow in either case.
33.3.2 Voltaic cell (Galvanic cell), Daniell cell, salt bridge
See diagram 33.3.2: Voltaic cells
1. The Daniell cell has emf about 1.1 volts, and low internal resistance. It is a primary cell whose emf is constant for a considerable period of time and maintains a steady, small current. However, since the copper (II) sulfate solution slowly diffuses through the clay porous pot to attack the zinc rod, the cell must be emptied and washed after use. The Daniell cell uses a battery jar, clay porous pot, zinc electrode cylinder, copper electrode rod, 10% copper (II) sulfate solution, 10% zinc sulfate solution.
Put sheet zinc in a beaker containing zinc sulfate solution or dilute sulfuric acid solution and put a sheet copper in a beaker containing saturated copper (II) sulfate solution. Connect the copper to the positive terminal of a voltmeter and the zinc to the negative terminal of the voltmeter. Observe the voltmeter reading is zero. Make a simple salt bridge by soaking filter paper in a concentrated solution of an electrolyte, e.g. sodium chloride or potassium nitrate. Fix the filter paper to dip into the zinc sulfate and copper (II) sulfate solutions. Observe the voltmeter and record the reading. The voltmeter shows that current is flowing. Read the voltmeter. Disconnect the voltmeter and substitute a 1.5 volt bulb, ammeter, and conducting wire. Record the observations. Examine the electrodes after two minutes. Observe that the zinc corrodes and new copper has deposited on the copper electrode. Observe whether the copper (II) sulfate solution loses some of its blue colour. To prolong the life of the salt bridge, make a permanent salt bridge from a glass U-tube filled with a 1 M aqueous potassium nitrate solution. You can mix the solution with agar gel to keep it in the U-tube. Put cotton wool plugs at each end of the U-tube.
2. Pour concentrated copper (II) sulfate solution into a clay porous pot in a large beaker. The solutions should be at the same level. Bend a sheet copper into a cylinder shape and put it in the beaker to surround the porous pot. Put sheet zinc into the porous pot. Connect the copper to the positive terminal of a voltmeter. Connect the zinc to the negative terminal of the voltmeter. Record the reading and observe whether there are changes in colour of the sheet copper, the sheet zinc and the solution. Disconnect the voltmeter and substitute: 1.5 volt bulb, ammeter; conduction wire.
3. Introduce a porous pot to prevent copper deposited on the zinc. The Daniell cell uses a porous pot and a salt bridge. Put 0.5 M aqueous zinc sulfate in the porous pot. Put a strong solution of aqueous copper (II) sulfate in the beaker surrounding the porous pot and fill to the same level as that of the zinc sulfate solution. Make a cylinder shape with copper foil and place it in the beaker to surround the porous pot. Connect the copper to the positive terminal of a voltmeter. Connect a zinc rod to the negative terminal and lower the zinc rod into the zinc sulfate solution. Note the reading on the voltmeter. Insert a 1.5 volt bulb in place of the voltmeter. Note whether it glows. Insert an ammeter into the circuit to measure the current flowing. Try to vary the current by moving the copper nearer to the zinc, or by changing the surface area of the copper foil.
4. If a porous pot is not available, use a salt bridge between the two solutions. Make a salt bridge by filling a glass U-tube with 1 M aqueous potassium nitrate solution and agar gel. Note the voltage, the current and whether the light bulb glows.
5. Make a voltaic cell with copper and zinc electrodes in a sulfuric acid solution. Short a few voltaic cells in series through a loop of iron or nichrome wire.
Use a cardboard model voltaic cell circuit to illustrate potential difference and electromotive force in a voltaic cell circuit.
To show voltaic cell polarization, heat the copper cathode in a Bunsen burner flame to oxidize the surface.
33.3.3 Coin cells
1. Take two coins made of different metals. Clean them well with steel wool or fine sand paper. Fold some paper into a pad so that it is larger than the coins. Soak the paper in salt water. Place one coin on top of the pad and the other underneath. Hold them between your thumb and finger. Connect both leads of a sensitive galvanometer or multimeter to the coins and note the deflection.
2. Soak absorbent cotton in salt water. Obtain two coins made of different metals. Place the cotton between the coins. The thickness of the cotton should be more than 2 mm. Connect both leads of a sensitive galvanometer to the coins at the same time. Observe the current flowing at the galvanometer. Clean the coins with steel wool or fine sandpaper beforehand and make sure that no salt water remains on the leads.
3. Put aluminium foil with a copper coin on it in water for a day. The water appears cloudy and the aluminium foil is perforated where the coin was lying on it. The water becomes cloudy due to dissolved aluminium.
4. Make a pile of copper coins alternating with pieces of sheet zinc. Between each pair of metals insert newspaper soaked in sodium chloride solution. Wind thin, insulated copper wire 50 times around a plotting compass. Press each end of the wire against each end of the pile. A current causes a deflection of the compass needle.
5. Make a simple cell with two coins. Use two coins made of different metals. Clean them well with steel wool or fine sand paper. Fold some paper hand towel or absorbent paper into a pad so that it is larger than the coins. Soak the absorbent paper in salt water. Put one coin on top of the pad and the other underneath. Hold them between the thumb and finger. Connect both leads of a sensitive galvanometer to the coins and watch the deflection.
6. Use two coins made of different metals. Clean them well with steel wool or fine sand paper. Fold some paper into a pad so that it is larger than the coins. Soak the paper in salt water. Place one coin on top of the pad and the other underneath. Hold them between your thumb and finger. Connect both leads of a sensitive galvanometer or multimeter to the coins and note the deflection.
33.3.4 Lemon cell
See diagram 33.3.4: Lemon cell
1. Connect a wire to a piece of zinc. You can use zinc cut from the can of a used dry cell. Connect another wire to a piece of copper. Roll a lemon on the table with your hand to break up some tissue inside. Push the zinc and copper strips through the skin of the lemon so that they do not touch. Connect both leads of a sensitive galvanometer or multimeter to wires and note the deflection. Repeat the experiment using a potato. Observe whether the distance between the metal strips affects the galvanometer reading.
2. Gently press or roll a lemon on the table to squash the tissue inside. Connect one terminal of a galvanometer to a piece of zinc and connect the other terminal to a piece of copper. Push the two pieces of metal through the skin of the lemon. The metals must not touch. Observe the deflection of the galvanometer needle to see whether current flows. The lemon juice acts as an electrolyte. Does the distance between the metals affect the deflection of the galvanometer needle? Repeat the experiment with a potato. There is almost no deflection.
3. Use a lemon, orange or any piece of fruit, preferably a juicy one, a copper wire and a coated iron wire such as galvanized packing case binders, a centre zero galvanometer out of the storeroom, a voltmeter, 0 to 10 volts. Put the different kinds of wire in turn into the lemon, the acidified water, add a few drops of dilute sulfuric acid, and acidified hydrogen gas peroxide solution, 5 mL of hydrogen peroxide plus a few drops of little dilute sulfuric acid. Connect the wires to a galvanometer in each case and see what the pointer does when the wires are in each electrolyte or solution such as fruit juice, or acidified peroxide. Leave the wires in the acid and see that the zinc dissolves off the iron beneath it. The copper cathode seems unchanged but is covered with bubbles of hydrogen gas. The peroxide removes these bubbles. Try to light a 11 / 2 volt globe. Remember that a series connection means copper of one cell to zinc of the next cell and so on. 4. Connect one terminal of a galvanometer to a piece of zinc and connect the other terminal to a piece of copper. Use the hand to roll a lemon on the table to squash the tissue inside. Push the two pieces of metal through the skin of the lemon. The metals must not touch. Note any deflection of the galvanometer needle. The lemon juice acts as an electrolyte. Does the distance between the metals affect the deflection of the galvanometer needle? Repeat the experiment with a potato. There is almost no deflection.
4. Make two slits in the skin of a lemon and push a "copper" coin, or a piece of copper, and a zinc washer. Attach wires make a circuit. A chemical reaction takes place between the metals and the acid in the lemon juice, causes the current to flow. The lemon is acting as a battery which lights an LE4. Long lead of LED goes towards the copper coin.
5. To make a lemon battery / voltaic cell, "Lemon screamer lasagna cell" stick copper and galvanized steel electrodes into a lemon and attach a voltmeter. Attach zinc and copper strips to a galvanometer and stick them into fruits and vegetables.
33.3.5 Simple chemical rectifier
Use a glass container filled with saturated solution of borax and electrodes of aluminium and lead. Use a low volt direct current power source. You can use this half wave rectifier as a battery charger. Connect four such cells to form a bridge rectifier. Check "+" and "-" terminals with a voltmeter
33.3.6 Chocolate wrapper cell in the mouth
When the aluminium foil from a chocolate wrapper ("silver paper") touches an amalgam filling mainly tin, two metals are in contact in the saliva electrolyte. A current is generated, the aluminium tending to dissolve and the current of electrons sends an unpleasant pulse along the nerves. Also a metallic taste may be due to dissolved aluminium ions, Al3+.
33.3.7 Noisy potato cell
Push 2 cm of copper wire and zinc wire one into a raw potato. Hold an earphone connected to the wires to hear a crackling sound caused by a weak electric current.
33.3.8 Hydrogen / oxygen fuel cell
The fuel cell allows the reaction between hydrogen and oxygen to generate electricity. The equipment is sold with a 2 V motor to generate power. The reaction requires potassium hydroxide pellets and palladium (II) chloride.
33.3.9 Ionic migration
Migration of coloured ions in an electrolyte can be shown with a flat chamber the size of a microscope slide and coloured ions, e.g. MnO4-, Cu2+, Cr202.
Dissolve some sodium sulfate in sufficient water to half fill a U-tube. Add drops of universal indicator that is red in acidic solutions and blue purple in alkaline solutions. The colour of the indicator should be green showing that the solution is neutral. To an equal volume of water add 1 gram agar agar gel for each 100 mL of water. Warm until the gel dissolves and then mix the two solutions. Pour this solution into the U-tube until the arms are about half full. When the gel has set, pour dilute sulfuric acid into one arm and dilute sodium hydroxide into the other. Insert platinum or carbon electrodes into the solutions. Connect the electrode in contact with sulfuric acid to the positive terminal of a battery. Connect the electrode in contact with the sodium hydroxide solution to the negative terminal of the battery. Allow the current to pass for some time and observe the colour changes produced in each arm. The violet colour in the gel below the sodium hydroxide solution is because of the movement of hydroxide ions into it under the influence of the electric field. The red colour in the gel below the sulfuric acid solution is because of the movement of hydrogen ions into it. So there is evidence for a two way flow of ions.
33.3.10 Ionic friction
The apparatus contains a floater with 2 flags which start rotating due to ionic movement. A direct voltage between the central bar electrode and the ring electrode cause two radial and contrary ion streams in the absence of a magnetic field. The vertically orientated field of the inserted ring magnet cause a deviation towards the right or the left depending on the polarity of the ions This cause a tangential flow component in the same direction for both types of ions which is transferred to the water molecules and lets the floater rotate.(Lorentz force)
33.3.11 EMF dependence on electrode material
Use two stands each holding several strips of different metals which can be paired and dipped into a dilute acid bath. Dip combinations of copper, lead, zinc and iron into a dilute sulfuric acid solution.
33.3.12 Contact potential difference
Show the contact potential difference between copper and zinc with a condensing electroscope.
33.3.13 Crowsfoot or gravity cell
Use a zinc-zinc sulfate / copper-copper (II) sulfate battery.
33.4.0 Dry cells, Leclanche cell, flashlight battery
Dry cells, Capacity of dry cells (torch / flashlight "battery") ammonium chloride electrolyte
Capacity of dry cells
The total charge output from the cell is called the capacity of the cell. You measure capacity of a cell or accumulator in the number of ampere hours of charge it can deliver. The capacity of the cell and its work principle depend on the volume. The more the capacity the cell has, the greater work current appliance can be used and the longer the times it uses. Use different dry cells. Connect a dry cell with a light bulb to form a circuit. Observe the normal brightness of the bulb and record the time. Then insert in parallel other bulbs to the bulb on the circuit in turn and observe the variation of the brightness of the bulbs. When the bulbs no longer light, stop the experiment and record the number of the bulbs connected to the circuit and time after first connection in the circuit. If the bulbs still give light, wait until they all no longer light and record the time from the first connection in the circuit. Repeat the experiment with some different size dry cells. Note the numbers of the bulbs increased until they go out and stop doing the experiment. Compare the difference of the discharge time between the dry cells. Use a millivoltmeter to measure the voltage of dry cells before and after discharge. The voltage of new cells is about 1.5 V. The voltage of "no charge" cells is about 0.75 V, the discharge stop voltage.
33.4.2 Examine a dry cell torch battery
See diagram 33.4.2: Electric torch battery
1. Remove the outer covering from an old dry cell. Use a saw to cut the cell in half and observe its structure. Note the carbon (+ ve pole) in the centre. The zinc container is the negative (- ve pole). The material between the two poles is the electrolyte. Note how the zinc has been eaten away by the chemical.
2. 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.
33.4.2.1 Dry cell in an electric circuit
See diagram: 32.4.2.1: Simple electric circuit | See: 32.2.00: Electric circuit symbols
Connect an electric bulb, e.g. 2.4 V, 0.5 A, and lampholder, 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. Use circuit diagrams to represent the electrical components in a circuit
33.4.3 Bring a dead battery to life
Warm a used 1.5 v torch battery. The bulb may light again. The zinc container of a battery cell becomes corroded by the ammonium chloride solution as a paste. This creates an excess of electrons in the zinc and an electron loss in the carbon rod. The carbon rod is coated with manganese dioxide to prevent the build-up of hydrogen gas that would stop the reaction. The bulb's incandescent filament gives out light when enough electrons flow through it. When the chemical reactions in the battery slow and flow of the electrons are not enough to make the filament glow. However, warming the battery accelerates the chemical reaction so that the filament can briefly give out light again. No chemical reaction can occur. When the zinc has corroded entirely and turned into white powder, zinc chloride.
33.4.4 Dry cell terminals
Charge an electroscope with batteries in series. Connect several dry cells in series to a condensing electroscope, remove the capacitance and test polarity with charged rods
33.5.1 Make a lead accumulator cell
The Lead cell accumulator has emf about 2 volts and very low internal resistance. It is a secondary cell. The terminals are usually marked + (red) and - (black). Since the internal resistance is very low, great care must be taken to avoid "short circuiting " the cell, i.e. there must always be a resistance of at least 1 ohm in the external circuit connecting the terminals.
Use a 250 mL beaker or jar with a cover to prevent drying by evaporation when the cell is not in use. You need 2 sheets of 40 x 10 cm thin lead foil and 2 lead strips 2 x 14 cm as terminals. These lead pieces require thorough cleaning by means of wire wool. Fold the long sheets of lead tightly to the shorter strips so that they make good electrical contact. The projecting ends will serve as terminals. A blotting paper B lead c terminals A sandwich is made of alternating strips of lead foil and blotting paper. When the sandwich is ready it is rolled up quite tightly, secured round the outside with one or two elastic bands, and placed with terminals at the top, in the cup or jar. Mark one terminal positive, and the other negative. The roll is covered with a solution of sodium sulfate made by dissolving 40 g of anhydrous sodium sulfate crystals in 200 mL water. The cell is now ready to charge with electricity. This can be done with a 6 volt battery charger, or with any low voltage direct current supply giving up to 10 amps. Connect positive on the charger to positive on the cell. After only a few minutes charging, the cell will light a 1.5 volt bulb. Provided that the cell is always connected to the charger in the same way, as described above, the more times it is charged and discharged, the more efficient it becomes. There will be enough current to make a small 1 volt electric motor spin round. The cell will remain serviceable for several months if the cover is put on when not in use.
33.5.2 Simple battery, lead acid simple battery
Charge a simple lead acid battery with two electrodes, lead plates, in a sulfuric acid solution for a short time and then discharge through a doorbell. Charge two lead plates in 30% sulfuric acid and discharge through a flashlight bulb.
33.5.3 Melt nail with a storage battery, lead-salt cell
Instead of acid use a saturated salt solution of sodium bicarbonate and magnesium sulfate.
33.5.4 Internal resistance of batteries, weak and good battery
Measure similar no load voltage on identical looking batteries and then apply a load to each and show the difference in voltage between a good and weak battery.
33.6.1 Thermocouple
Connect to a galvanometer two iron-copper junctions one in ice and the other in a flame. Attach a voltmeter to the iron wires of two copper-iron junctions while they are differentially heated. Place a twisted wire thermocouple in a flame and observe the current.
A commercial thermoelectric generator is made from 150 constantan / nickel molybdenum thermocouples in series.
33.6.2 Seebeck effect and Peltier effects
To show the thermoelectric effect of copper-iron junctions, send current through a copper-iron-copper circuit for several seconds and immediately disconnect and switch to a galvanometer. For a thermoelectric cooler, use a Peltier device to cool a drop of water. Make an antimony-bismuth junction and an apparatus to show heating and cooling due to the Peltier effect.
33.6.3 Copper-iron junctions ring
Use a Bunsen burner to simultaneously heat sixty copper-iron junctions in series and arrayed in a ring to produce 90 mA current
33.6.4 Thermoelectric compass
Join bars of copper and iron to form a case for a compass needle. The needle will indicate the direction of the current as one or the other junction is heated.
33.6.5 Thermocouple coil magnet
Heat a thermocouple loop and the current produces a magnetic field that can be detected by a compass needle.
33.6.6 Thermoelectric effect in a wire
A piece of soft iron wire connected to a galvanometer has little thermoelectric effect until the wire is kinked.
33.6.7 Thompson effect
A flame moved along a long wire will push ahead current.
33.6.8 Thermoelectric magnet
Heat one side of a heavy copper loop closed by an unknown metal to generate thermoelectricity for an electromagnet. A ring of copper shorted by iron forms a thermocouple that powers an electromagnet when one end is in water and the other is heated in a flame. Bend one end of a heavy copper bar into a loop and closed with a copper-nickel alloy, heat one end and cool the other end.
33.6.9 Thermocouple magnet
Use a Bunsen burner to heat one side of a thermocouple magnet supporting over 10 Kg. Heat and cool opposite sides of a large thermocouple then suspend a large weight from an electromagnet powered by the thermocouple current.
33.6.10 Thermoelectric heat pump
Mount aluminium blocks with digital thermometers on either side of a Peltier device. Run the current both ways.
33.6.11 Pyroelectric crystals, domains of electric polarization
Show the temperature effect on the polarization of pyroelectric crystals by heating tiny BaTiO3 crystals on a microscope slide until the domains disappear.
33.7.0 Piezoelectricity
Piezoelectric sheets using ceramic lead-zirconate-titanate (PZT)
33.7.1 Piezoelectric model
Make a ball and spring model of the piezoelectric effect.
33.7.2 Rochelle salt experiments
Show ferroelectricity hysteresis, Curie-point and the direct piezoelectric effect with a Rochelle salt. Connect Rochelle salt to a neon lamp or electrostatic voltmeter. Make sheets of polycrystalline Rochelle salt that show piezoelectric effects
33.7.3 Piezoelectric sparker
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.
33.7.4 Stress vs. voltage
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.
32.5.0 Power and energy, electrical equivalent of heat, electrical energy
Power and Energy, watt W = 1 joule per second, watts = volts X amps, filament lamps, fluorescent lamps, radiant electric fires, three heat switch, fuses, W (joules = Q (coulombs) X V (volts)
Electrical energy, W, consumed by an electrical appliance is equal to the work done to move charge through the appliance. If potential difference is v volts and quantity of electricity passed = Q coulombs, the work done = QV joules. Charge (Q) = current (I) x time (t) so work done = QV = VIt joules.
Electric power, P, is the rate at which electrical energy, supplied by batteries, thermocouples, photoelectric cells (photo-cells), generators, is converted to another form of energy. The unit of power is joules per second or watt, W. Power = work done / time taken, = Volts x Amperes, VI = joule / coulomb x coulomb / second = joule / second = watt. So a 100 watt light globe, an incandescent lamp, consumes 100 joules of electrical energy per second. In this example "consumes" means converts electrical energy to heat energy and light energy. The amount of electric energy used by an electrical appliance, is equal to the work done to move charge through that appliance. The longer the appliance operates, the more electrical energy is used. All the electrical energy supplied to ohmic resistors is converted into heat. The I-V graph for an ohmic resistor is a straight line graph. Ohm's Law states that the ratio of the potential difference across the conductor to the current flowing through it is constant, volt / amp = ohm, V / I = R. Power = VI watt, IR watt, VAR. watt. Some circuit elements, e.g. vacuum diode, do not have uniform I-V graphs and do not have a constant value for resistance.
Electric current, I, ampere, A, potential difference, volt, V
Power = work done / time taken, P = W / t
Charge transferred = current time, Q = It
Power, P in watts = VI, volts x amps
Power = current2 x resistance
Power systems, UK, 50 Hz 240 volts, RMS.
32.5.01 Heat from current through a conductor is proportional to: 1. time, and 2. current2
See diagram 32.2.63
When heat losses are small due to efficient lagging, the temperature rise of water in the calorimeter is proportional to the heat given out by the coil. Pour water in the calorimeter to cover the heating coil, resistance 2 ohms. Adjust the rheostat so current of 3 amps flows through the coil. Record the initial temperature of the water. Close switch S and record the time. Stir the water continuously and record the temperature after each minute for 10 minutes. Plot a graph of temperature (Y axis) against 1. time of passage of current (x axis) and 2. the square of the current. Close switch S and adjust the rheostat for a current of 3 amps. Open the switch S, stir the water and note its initial temperature. Close the switch and note the time. Stir continuously until the temperature reaches 10oC, open switch S, record the time and record the highest temperature reached by the water in the calorimeter. Repeat the procedure but adjust the rheostat for a current of 4 amps. Repeat the procedure but adjust the rheostat for a current of 5 amps. Plot a graph of temperature rise (y axis) against the square of the current (x axis).
Repeat the experiment with another heating coil R, of resistance 3 ohms. Adjust the rheostat for a current of 3 amps. Note the initial temperature of the water. Close switch S, record the time and stir well. When the temperature has risen by 15oC, open switch S, record the time, continue stirring and record the highest steady temperature. Replace the heating coil with another of known resistance R, e.g. 5 ohms.
Repeat the above procedure with the same current, after adjusting the rheostat Rh, for the same time with the same volume of water at the same initial temperature. Record the initial and final temperatures.
32.5.1 Light from electrical energy
See diagram 32.5.4
Connect a 2.5 volt torch globe to a single 1½ volt torch cell using a fine metal wire. Connect the cap of the globe to the cap of the cell. Connect the side of the globe to the bottom of the cell. What happens when the connections are broken? Repeat the experiment using two cells in series, i.e. the cap of one connected to the base of the other. Also, do the experiment with three cells. Can you detect a difference from the use of the second and third cells? Cover the globe with a scrap of clear plastic to prevent flying glass. Carefully break the glass so as not to damage the wire filament and connect as before to one cell. What do you observe? What purposes does the glass serve?
32.5.2 Heat from electrical energy
See diagram 32.5.4
Connect a piece of jug element wire, about 5 cm long, to a pair of torch cells in parallel, i.e. each connected in the same way to the element wire. Observe any effect on the jug element wire. Compare with the effect of one cell and of three cells, connected both in series and in parallel. A jug element should not be switched on unless covered by water.
32.5.3 Make an electric heater from steel wool
See diagram 32.5.3: Steel wool heater
Connect a bare copper wire to the outer case or outer terminal of an ordinary torch cell by means of solder, sticky tape. That wire is the negative wire because electrons leave the cell and travel along it. Similarly fasten another bare copper wire to the brass end of the centre terminal or the inner terminal of the cell where there is a deficiency of electrons and twist a piece of bare copper wire on this clip. That wire is the positive wire. Electrons return to the cell along it. The steel wool becomes hot and not the copper leads connecting it to the battery because copper is a better conductor than steel and the steel in steel wool is much thinner than the copper in the leads.
32.5.4 Make a model electric light and a switch
See diagram 32.5.4: Light switches
Use a bored cork as a lamp holder or use a simple torch globe holder that takes a screw-in globe. The positive wire touches the screw part of your torch globe and the negative wire touches the little solder blob at the end of the globe. Squeeze the switch wires together and light up your lamp.
32.5.5 Make a model electric jug, immersion heater
See diagram 32.5.5: Immersion heater | See diagram 32.5.5a: Electric jug
1. Use electric jug element wire and attach it to a heavy duty dry cell. Use a 6 volt storage battery. Include a switch in your circuit. The wire in the jug element is called nichrome wire because it has nickel and chromium in it. Hang this electric jug element in a cup of water and switch on the current. The water gets hot because much of the heat produced by the current in the wire is transferred to it.
2. Use a wasted electric heater wire. Cut pieces of the wire so you can parallel connect them to make a new heater. The number of the wires depends on the electric current provided by source power. The working current of each wire can be calculated according to original working volt and power. Put the wires in a U-tube. Dip the U-tube into a cup of water and turn on the power. The water in the jug absorbs the heat from the element and thus keeps the temperature down below the melting point of the metal in the element.
32.5.6 Measure the voltage and current to a heating coil in a calorimeter
Use an electrocalorimeter to determine the power delivered by temperature change in water and compare to that computed from voltage current and time.
32.5.7 KWH meter and loads, heating with current
Measure the power consumed by an assortment of household appliances. Pass large currents through No. 18 nichrome wire and measure the volts and amps.
32.5.8 Heating wires in series
Solder together several lengths of different wires of the same length in series and hang a piece of paper from each wire with soft wax so that as current is passed through the wire the
pieces of paper falls off at different times.
32.5.9 Hot dog / pickle cooker
Hook nails to 110V and place them on and then in a hot dog sausage. Apply 110 V through a hot dog and cook it.
32.5.10 Current through a torch globe
See diagram 36.1
Place the ammeter in series with the globe so that any charge that passes through the globe must also pass through the ammeter. Switch on the current and note the reading on the ammeter. One ampere = one coulomb per second. Two torch globes connected in series to one battery each give a duller light than one globe attached to the battery because two similar torch globes connected in series would have twice the resistance of a single globe, less current would flow through the globes and the light emitted would be duller. However, as the current from the battery is reduced, it would last longer.
32.5.11 Compare the power of incandescent torch globes
Use different globes, headlamps, tail-light globes and even torch globes, provided you have a suitable socket for them. Short lengths of wire with small bulldog clips soldered at each end are useful leads for electrical connections.
32.5.12 Compare light from incandescent lamps
Show that the two lamps using the same current emit different amounts of light. Fit the two lamp holders with insulated 4 mm terminals. In two circuits one circuit contains a mains lamp taking about 0.4 amp and is connected to a 240 volt power supply and the other circuit contains a motor car side lamp or tail lamp taking about 0.5 amp from a 12 volt a.c. supply. Connect the two lamp bases in series with the ammeter and connect the circuit to the 240 volt main supply. A low voltage lamp and a high voltage lamp take the same current. Similarly with a small electric motor and a large electric motor that take the same current, e.g. 1.6 amps, the large motor may turn a generator and light 3 lamps in series while the small motor may not even turn the generator.
32.5.13 Measure light from a lamp
See diagram 32.5.13: Exposure meter
If the lamp is lit from the a.c. terminals of the variable voltage supply and not the d.c. then a.c. meters will be required. Place the exposure meter 15 cm from the lamp. Record readings of the light meter reading for various currents through the lamp. Change the input power from 10 to 30 watts, corresponding to a change of 7-14 volts. Draw a graph of light meter readings against power input. The graph will be a straight line, not passing through the origin. If a mains lamp is used, put the exposure meter further away from the lamp. For a 100 watt lamp, a variac provides a suitable supply used with an a.c. meter giving I amp full-scale deflection or better 500 mA full-scale deflection.
32.6.0 Circuit analysis, house circuits
Kirchhoff's voltage law, house circuits, circuits in parallel, switches, fuses, two-way staircase switch, ring main circuit, fused plugs, earthing, 3-pin plug, electricity meter, kilowatt hour, power ratings
Measure the voltages around a three resistor and battery series circuit.
32.6.1 Continuity of current
Insert an ammeter into any branch of a circuit to show currents in and out of a node.
32.6.2 Superposition of currents
Measure the current from one battery, the current from a second battery in another position and the combination in a circuit.
32.6.3 Standard reciprocity circuit with a potentiometer
Use a slide wire potentiometer with a battery and demonstration galvanometer. Use a slide wire potentiometer with a standard cell. Contrast the slide wire rheostat when used as a rheostat, or potential divider with rheostat and six volt battery.
32.6.4 Wheatstone bridge, bridge circuits, slide wire, metre wire bridge
See diagram 32.2.60
Stretch two nichrome wires across the bench and connect sliding clips to a galvanometer to find equal potential points.
A bridge circuit usually contains 4 resistors, a source of direct or alternating current and a galvanometer as a null point detector. If resistors A and B are connected in series in one arm, resistors C and D are connected in series in the other arm. Connect the galvanometer from between A and B to between C and D, when the bridge is balanced, i.e. the galvanometer shows no current flowing, then A / C = B / 4.
Examples of bridge circuits include the following:
Measure a resistance with a Wheatstone bridge, metre wire bridge, post office box
Measure a capacitance or frequency with a Wien bridge
Measure inductance with a Maxwell bridge.
Measure the value of an unknown resistance with a metre wire bridge. When switch S is closed and the resistances are such that no current flows through the galvanometer G, the bridge is balanced, R1 / R2 = R3 / R4. A 100 cm length of uniform resistance wire a.c. is attached to brass strips of negligible resistance. The resistance of a uniform wire is proportional to its length, so if B is the balance point, R1 / R2 = AB / BC, R1 = R2 X (BC /AB). 1. Use the sliding contact to find B on the wire where no current flows through the galvanometer when the switch is closed.
Remove the shunt to make the galvanometer more sensitive and find the balance point B more accurately. Measure AB and B3.
Replace the shunt and interchange R1 and R2 and measure AB and BC again.
32.6.5 Wheatstone bridge with a human galvanometer, Wheatstone bridge with light bulbs
1. Stretch a loop of clothesline previously soaked in salt solution in a parallelogram and hook the ends to a 110 V line then touch two points of the same potential without electric shock.
2. Use a Wheatstone bridge configuration with 4 light bulbs for resistors using 110 ac. Use four 60 W lamps in a diamond bridge with a 10 W lamp as the indicator then switch in an additional 6 V lamp when the circuit is balanced. Use three 110 V lamps and a rheostat to make up the diamond of a Wheatstone bridge and use a small lamp to serve as an indicator. Use series and parallel light bulbs in a light bulb board with switches to allows configuration of several combinations. Use three similar wattage lamps in series, three in parallel. Connect a series and parallel circuit with three bulbs and six switches in 14 ways! Use three 110 V lamps wired in series and three wired in parallel.
32.6.6 Light bulb board, 12 V
Use a board with 12V bulbs and a car battery to allow combinations of up to three series or
three parallel loads. Measure the current flowing through a wire resistor with 6 V applied and then series and parallel combinations.
32.6.7 Equivalent resistance, series and parallel
Replace a series of resistors in a circuit by a single resistor. Use the formula for obtaining integral values of resistors in parallel to obtain an integral equivalent resistance. Replace parallel resistors by a single resistor in a circuit. Use a Wheatstone bridge resistance circuit to reduce resistor combinations to an equivalent resistance. Use a neon flasher circuit to show the combination rules for series and parallel combinations of resistance and capacitance by timing light flashes. Use a circuit board laid out so meters can be plugged in and readings taken for demonstrations of series and parallel circuits and Kirchhoff's laws.
32.6.8 a.c. Chopstick fan
Wave a white chopstick quickly forwards and backwards in neon light. A Chinese fan with light and dark ribs appears. Neon tubes contain a gas, which flashes on and off 60 (in US) times a second because of rapid reversals in alternating current. The moving rod is thrown alternatively into light and darkness in rapid sequence, so that it seems to move by jerks in a semicircle. The light from a television set will produce the same effect. Normally, the eye is too slow to notice these breaks in illumination clearly. In a regular electric light bulb, the metal filament continues glowing between the peaks in current.
32.6.9 Electrical circuits in a room
Investigate that the positions of every electrical appliance in the house you live in and how their circuit connected. This activity will help you to learn the lighting circuit and its application. If some day you want to decorate or fit up your house, it will help you too. Observe the actual position of every electrical appliance and draw an actual distribution curve. Again begin at the place that the wires goes in, observe how the circuit is connected. Draw a circuit of the room to show from which wire the electrical appliances in the room use the electric current and how the switch on the wall control the electrical appliances. Do not touch the dangerous parts like a switch, plug. [Some teachers fail to see the point of this exercise because if students cannot trace circuits they are not be able to find how circuits are wired. The exercise only allows students to see what elements are included in the circuit not how they are connected. However, there are no safety problems with the activity.]
32.7.0 Instruments to detect electric current
Electrical measuring instruments include voltmeters 5 / 15 V and 0.3 to 300 V, ammeters 1 / 5 A and 1 mA to 3 A, with overload protection through fuses and diodes. Multi-range meters are moving coil instruments to measure direct and alternating currents and voltages that can be used for all current ranges up to 10 1. Work and power meters show the relationship between voltages and current intensity, time, power and energy and find the efficiency during energy transformations. Special measuring instruments include light intensity measuring instrument or lux meter and liquid conductivity meter.
32.7.1 Simple instrument to show electric current, current detector
See diagram 2.163
Wrap 50 to 60 turns of bell wire to form a coil around a jar 8 cm in diameter. Remove the coil from the jar and bind it with short pieces of wire or insulating tape. Mount the coil on a piece of cardboard. Attach a 16 mm plotting compass to a cork and fix it inside the vertical coil. Rotate the coil until it is in line with the compass needle. Connect a battery to the coil and observe the deflexion of the compass needle. Reverse the connections, and observe the deflexion of the compass needle again. Make a more sensitive instrument by putting a compass in the tray of a match box then winding the coil wire over the box.
32.7.2 Galvanometer
See diagram: 32.3.01 | See diagram 32.0.1.1.6: Right hand motor rule
1. Meters for measuring voltage or current are made from moving coil galvanometers. To keep the maximum force acting on the moving coil, the magnetic field is drawn inwards by a "soft" iron core, making the field appear radial. The moving coil turns against springs, which carry the galvanometer current in and out of the coil, and return the coil to zero. The moving coil turns a pointer across a scale, so that the scale reading is proportional to the current through the coil.
2. The galvanometer is an instrument that detects an electric current. Many types exist but usually use the moving coil galvanometer. The current to be detected passes through a coil inside the instrument is in a magnetic field. This causes the coil and the attached pointer to be deflected, the direction and size of the deflection depending upon the direction and size of the current. There is a risk of sending a larger current through the galvanometer than is safe for the instrument. While this risk persists, for example when trying to find a balance point in a potentiometer experiment, the greater part of this current is made to "bypass" the galvanometer through a "protective shunt" as in the diagram. A short length of fine, cotton covered copper wire serves as a convenient shunt. This moving coil meter works on the same principle as a simple d.c. electric motor and is called the D'Arsonval movement after its inventor. It consists of a stationary magnet and a moving coil. When current flows through the coil the resultant magnetic field reacts with the magnetic field from the permanent magnet and causes the coil to rotate. The greater the current the greater the rotation.
3. Mount a coil vertically on a phosphor bronze suspension that conducts current between the circuit under the test and the coil. The phosphor bronze suspension also provides the restoring force when it twists balanced against the driving force of the coil's magnetic field. In some galvanometers a coil spring below the moving coil, with an attached pointer, controls how far the coil turns and measures the current. The direction of movement follows the right hand motor rule for electron flow, where first finger points towards from North to South, the second finger points in the direction of electron flow in the conductor, the thumb points to the direction of motion of the conductor.
32.7.2.1 Sensitivity and resistance of a galvanometer
Determination of galvanometric constants. Use external resistors to measure the resistance and sensitivity of a galvanometer. Connect series resistance to a galvanometer to make a voltmeter with low sensitivity and measure several dry batteries in series with both the voltmeter and an electroscope.
32.7.2.2 Convert a galvanometer to a voltmeter
Knowing the resistance and sensitivity of a galvanometer add a series resistance and then measure a voltage. Use a galvanometer with shunt and series resistors.
32.7.2.3 Convert a galvanometer to an ammeter
Knowing the resistance and sensitivity of a galvanometer add a shunt resistance and then measure a current.
32.7.2.4 Convert a galvanometer to an ammeter, hot wire ammeter
See diagram 32.7.2.4
In a hot wire metre you pass the current through a platinum alloy hot wire. When current passes through the wire, the wire expands due to the heat effect of the current. The expansion is taken up by the spring metal strip. The spring metal strip is much like a spring in mechanical watch that it always maintains a tendency of stretch that pulls a silk thread tightly. Wind the silk thread around a small pulley attached to the pointer. When the silk strip moves, it pulls the pulley resulting in the deflection of the pointer. Tie the other end of the silk strip to a phosphor bronze wire attached to the hot wire. The silk strip could not be connected directly to the wire, as it would burn when large current passed through the wire. The phosphor bronze wire is insulated from heat. So the hot wire expands as the current passes through it and loosens the phosphor bronze wire, stretch the spring metal strip through silk's transfer. Finally, silk, pulley and pointer move in turn. In view of energy, first the electric energy transforms into heat energy. Then heat energy transforms into kinetic energy of pointer and potential energy.
32.7.2.5 Measure reduction factor k of a tangent galvanometer
See diagram 32.2.66
The tangent galvanometer measures current flowing through a vertical circular coil of known number of turns of insulated wire. The magnetic effect of this current at right angles to the plane of the coil is measured by an aluminium pointer attached to a turning bar magnet. The galvanometer is made horizontal with adjustable legs and a spirit level. If the strength of the magnetic field at the centre of the coil is H oersted, and the horizontal component of the earth's magnetic field at that point is H1 oersted, and theta (Greek) is the angle of deflection of the pivoted magnet, H = H1 tan theta. H, is a constant at a particular place so H is proportional to tan theta. H is proportional to I, so I is proportional to tan theta or I = k tan theta, The symbol k represents the reduction factor constant of the tangent galvanometer. Using two turns of the coil. Place the tangent galvanometer away from any magnetic fields from other devices in the circuit. Rotate the tangent galvanometer until the plane of the coil is in the magnetic meridian as shown by the pivoted magnet, and one end of the aluminium pointer is over the 0o mark. Check that the tangent galvanometer is horizontal. Close switch S1. Adjust the rheostat Rh until the galvanometer deflection is 30o. Record the readings of both ends of the pointer (theta1o and theta2o). Reverse the current with the reversing switch S2 and again read both ends of the pointer (theta3o and theta4o). Record the current I amps. Repeat to give more deflections between 30o and 60o, each time reversing the current through the galvanometer and recording the current I amps.
32.7.3 Ammeter
See diagram: 32.3.02: Ammeter
The ammeter is an instrument that measures an electric current. The resistance of an ammeter must be very small so that when it is placed in a circuit it will not diminish the current it is intended to measure. The ammeter is always placed in series in the circuit, and to make the pointer deflect the right way, its positive terminal must be connected to the positive side of the circuit. An ammeter is a galvanometer with a low value resistor placed in parallel across its terminals so that the largest current in the circuit causes full-scale deflection and no more. Ammeters include moving coil ammeter, moving iron ammeter, thermocouple, hot wire ammeter.
32.7.4.0 Voltmeter
See diagram: 32.3.03
The voltmeter is an instrument that measures the potential difference between two points in a circuit. The resistance of a voltmeter must be very high so that when it is placed across part of a circuit it does not divert an appreciable amount of current from the main circuit. The voltmeter is always placed in parallel with (i.e. across) the resistance to measure the potential difference between its ends. When, connecting, make sure that you connect the positive terminal to the positive side of the circuit. A voltmeter is a sensitive galvanometer with a high value resistor placed in series so that the largest voltage in the circuit causes full-scale deflection and no more, i.e. a shunted galvanometer. Connect a voltmeter across a resistor to measure resistance and power consumption. Connect an ammeter in series in the circuit to measure the current flowing through the ohmic resistor. The voltmeter counts how many joules each coulomb delivers as it travels through a lamp or motor. A voltmeter measure the energy transferred from electrical energy to heat or mechanical energy. Volts = joules per coulomb, or volts = joules of energy transferred from electrical form of energy to other forms of energy in that part of the circuit for every coulomb passing through it.
Resistance value of a resistor, R, = V / I, ohm = volt / amps.
Power consumed by resistor, P = V X I = volt X amps
Resistance, R, of a material increases with length, decreases with cross-section area, and
depends on the resistivity quality of the material.
R = resistivity X length / area, e.g. resistivity copper = 1.7 X 10-8 ohm metre, rubber = 1013 ohm metre.
32.7.4.1 Connect a voltmeter
See diagram 32.3.03.1
Connect the whole circuit first without a voltmeter then add the voltmeter, e.g. across a lamp.
Set up a simple series circuit of 12 V battery, lamp and ammeter. Connect a voltmeter in parallel with the lamp. Add an electric heating element or small motor to the circuit. Connect the voltmeter may be connected across the lamp then across the heating element.
Connect a series circuit of two similar lamps in series and repeat the experiment. The circuit should always be switched off during changes in wiring. 3. Voltmeter connections: Wire the lamp fitted to the lamp base into a series circuit with the ammeter. Switch on 12 volt battery. Switch off and connect the voltmeter parallel with the lamp.
32.7.4.2 Voltmeter as cell counter
See diagram 32.3.03.2a
Connect a voltmeter to one cell of the 12 volt battery. Then connect to two, three, four, five and six cells. The 12 volt car batteries used must allow you to tap off intermediate voltages. Use 4 mm sockets for connection. Connect a series circuit of seven dry cells, two rheostats and ammeter, and record the current. Use two rheostats to keep a low current. Allow the current to flow for a very short time only. Reverse one cell so that five cells are effective to drive current through the circuit. Record the current.
Repeat this procedure by reversing two cells, leaving three cells effective. Record the current.
Reverse 3 cells leaving one cell effective. Record the current. Tabulate the currents and the numbers of effective cells. Change the current, either by reversing cells, or by adding a rheostat to the circuit. Record the corresponding values of the ammeter and voltmeter.
32.7.4.3 Calibrate a voltmeter
See diagram 32.3.03.3
Use plastic drinking cups with a low heat capacity. Put 200 g water in a container. Put an immersion heater in the circuit. Record the initial temperature. Close the switch and note the time. Use the heater as a stirrer. Allow the current to flow for two minutes. Record ammeter and voltmeter readings. At the end of two minutes open the circuit, stir the water again and note the maximum temperature.
32.7.4.4 Potential difference and electromotive force
See diagram 32.3.03.4
Potential difference can be thought of as "electrical pressure difference" between the ends of a part of a circuit, where energy transfer occurs, e.g. electrical energy to heat.
1. Connect the voltmeter first across lamp 1, then across the ammeter, then across lamp 2, then across the three together (between P and Q), and finally connect across the battery. Note the potential difference in each case.
2. Repeat with a series of dry cells. Prepare this battery using accumulators joined with about 50 cm of SWG 26 high resistance wire.
32.7.4.5 Loading by a voltmeter
Measure the voltage across a high resistance circuit with high and low impedance.