School Science Lessons
2015-10-29 SP L
Please send comments to: J.Elfick@uq.edu.au
33.0 Electric current, ammeter,
galvanometer, voltmeter, power and energy, house circuits, incandescence,
Table of contents
33.7.0 Electric current, ammeter, galvanometer,
33.5.0 Electric power and energy
15.6.0 Electrochemical cells, voltaic cell, galvanic
33.6.0 House circuits, circuit analysis
33.4.0 Incandescence, electric light
33.8.0 Thermocouple, thermoelectricity
33.4.0 Incandescence, electric
light bulb, filament lamp
Light, Light bulbs,
and lampholder, "Scientrific", (commercial website)
18.104.22.168 Carbon and tungsten light bulbs
33.5.12 Compare light from incandescent lamps
33.5.11 Compare power of incandescent torch
4.65 Incandescent lamp, electric light bulb, filament
lamp, light globe
22.214.171.124 Temperature of incandescent lamps
with silicon solar cells
126.96.36.199 Tungsten, (electric filaments)
33.7.0 Electric current, ammeter,
188.8.131.52 Electric current, ampere,
184.108.40.206 Calibrate a voltmeter
220.127.116.11 Connect a voltmeter
18.104.22.168 Convert a galvanometer to a voltmeter
22.214.171.124 Convert a galvanometer to an ammeter
126.96.36.199 Convert a galvanometer to an hot
wire current meter
33.7.0 Electric current detectors
188.8.131.52 Loading by a voltmeter
184.108.40.206 Make a galvanometer from a pocket
220.127.116.11 Potential difference and electromotive
18.104.22.168 Reduction factor k of a tangent galvanometer
22.214.171.124 Sensitivity and resistance of a galvanometer,
126.96.36.199 Voltmeter as cell counte
188.8.131.52 Loading by a voltmeter
33.5.0 Electric power and
Power supply, "Scientrific",
33.5.0 Power and energy, power transmission,
22.2.03 Megawatt-hour, MWh
2.2.04 Kilowatt-hour, kWh
30.5.06 Root mean square values,
RMS values, peak value
AC Chopstick fan
9.2.21 Bicycle ergonometer
33.6.0 Circuit analysis, house circuits
33.5.10 Current through a torch globe
33.5.3 Electric heater using steel wool
33.5.5 Electric jug, immersion heater
33.5.14 Electric kettle heating efficiency
33.5.4 Electric light and switch
33.5.17 Electrical appliances in the home
3.84 Electrical energy from the displacement
of copper by zinc
33.5.16 Exposure meter
33.5.01 Heat from current through a conductor
33.5.2 Heat from electrical energy
33.5.8 Heat wires in series
33.5.9 Hot dog / pickle cooker
33.5.7 kWh meter and loads, heating with current
33.5.1 Light from electrical energy
33.5.12 Light from incandescent
184.108.40.206 Power surge circuit
33.5.11 Power of incandescent torch
33.5.6 Voltage and current to a heating coil
in a calorimeter
33.6.4 Wheatstone bridge, bridge circuits, slide
wire, metre wire bridge
33.8.0 Thermocouple, thermoelectricity
22.6.1 Bunsen burner flame, (See
33.8.3 Copper iron junctions ring
220.127.116.11 Peltier effect, supercooled water,
33.8.11 Pyroelectric crystals, domains of electric
18.104.22.168 Seebeck effect, (thermoelectric effect)
22.214.171.124 Thermocouple, voltage produced by
33.8.4 Thermoelectric devices
22.7.0 Thermometers and temperature
126.96.36.199 Thompson effect
4.65 Incandescent lamp, electric
light bulb, filament lamp, light globe
Light, Light bulbs,
and lampholder, "Scientrific", (commercial website)
| See diagram 32.162: Heat and light from
| See diagram 28.65: Getting heat and light
A substance is incandescent if it emits light as a result of its temperature
Hot solids or liquids emit wavelengths of radiation depending on the
temperature as a continuous spectrum.
At lower temperatures they emit red wavelengths, so the metal appears
to be "red hot".
At higher temperatures, they emit the full visible spectrum as white
light, so the metal appears to be "white hot" or "incandescent".
The incandescent filament in an electric light globe, a filament lamp,
is "white hot".
1. Heat source.\
1.1 Remove the shade from a bed lamp containing a 100 W incandescent
electric light bulb.
Cover the bulb with very thin aluminium sheet, e.g. aluminium cooking
1.2 Insert a 100 watt pearl electric light bulb in a holder so that
you can dangle the light bulb down.
2. Push the ends of two pieces of copper wire 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.
3. Examine a manufactured electric light bulb.
It contains a mixture of argon and nitrogen, but no oxygen.
It has a tungsten carbide wire filament that glows without burning
when heated to a high temperature.
The argon restrains the blackening of the inside of the bulb by deposition
of tungsten vapour.
However, fluorescent lamps containing mercury vapour or neon gas are
much more energy-efficient than incandescent lamps.
15.6.0 Electrochemical cells,
voltaic cell, galvanic cell
Electrochemical cells form electricity from chemical reactions.
The cell is made up of two half cells.
Each half cell consists of an electrode in contact with an electrolyte.
It is usually a metal in contact with a solution of its salts.
The half cells are separated by a porous membrane or salt bridge that
allows movement of ions between the half cells.
In an electrochemical cell:
1. Cations, (positive ions, e.g. Fe2+), flow to the cathode
where they are reduced, i.e. gain electrons, from the cathode,
2. Anions, (negative ions, e.g. SO42-), flow
to the anode where they are oxidized, i.e. lose electrons, by depositing
electrons onto the
The anode is usually labelled as the positive terminal with "+" sign
on it and painted red.
The cathode is usually labelled as the negative terminal, with "-" sign
on it and painted black.
When electrochemical cells are part of a circuit, electrons carry
electric current through the external electrical circuit and ions carry
current through the solutions in the cell.
Oxidation at the anode and reduction at the cathode occur in both
electrochemical and electrolytic cells.
However, an electrochemical cell can react spontaneously, but an electrolytic
cell needs external electrical energy to force electrons
around the circuit.
1. Put a piece of zinc metal in a zinc sulfate solution to form a Zn
/ Zn2+ half cell.
The zinc metal atoms dissolve as zinc ions, leaving negative charges
on the electrode until the increased charge stops the process.
Connect the zinc foil to the negative terminal of a 5 V voltmeter.
Zn (s) ---> Zn2+ (aq) + 2e-
2. Put a piece of copper foil in a concentrated
copper (II) sulfate solution to form a Cu / Cu2+ half cell.
The copper metal ions in solution take electrons from the electrode
and deposit on the copper electrode as copper atoms.
Cu2+ (aq) + 2e- ---> Cu (s)
3. Connect copper foil to the positive terminal
of the 5 V voltmeter.
Record any changes in the voltmeter reading.
Note the maximum reading.
Note any changes at the copper foil and the zinc foil.
The voltage falls to zero after a short time because copper deposits
on the zinc and causes the reaction to stop.
Zinc metal becomes zinc ions and copper ions become copper metal.
Electrons transfer from the zinc metal to the copper ions by moving
from the zinc to copper along a wire.
The potential difference or voltage reflects the greater activity
of zinc over copper.
The current flowing depends on the size and rate of the reaction.
Zn (s) + Cu2+ (aq) ---> Zn2+ (aq) + Cu (s)
188.8.131.52 Thermocouple, voltage
produced by heat
| See diagram 184.108.40.206: Voltage produced
by heat, thermocouple
| See diagram 220.127.116.11: Thermocouple
Voltage is produced by heating the joint (junction) where two unlike
metals are joined.
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
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
A thermocouple consists of two dissimilar metals, joined together
at one end.
When the junction of the two metals is heated or cooled a voltage
is produced that can be correlated back to the temperature.
Thermocouples are cheap and available in different combinations of
metals or calibrations, but may not be accurate enough or
consistent enough for some applications.
The alloys used in thermocouples are commonly available as wire.
A beaded wire thermocouple consists of two pieces of thermocouple
wire joined together with a welded bead.
A thermocouple probe consists of thermocouple wire housed inside a
A thermocouple surface probe is used to measure the temperature of
a solid surface.
The thermocouple is based on the Seebeck effect (thermoelectric effect),
(Thomas Johann Seebeck, 1770-1831, Germany), is that a
conductor generates a voltage when in a thermal gradient.
A thermocouple has two different wires or semiconductors joined at
the ends to be a thermoelectric source of EMF.
1. Connect a voltmeter to the iron wires of two copper-iron wire junctions
hanging from a stand.
Light the Bunsen burner and heat one of the junctions, watching the
You can also immerse a junction in ice.
2. Make a thermocouple coil magnet.
Heat a thermocouple loop and the current produces a magnetic field
that can be detected by a compass needle.
3. 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
18.104.22.168 Seebeck effect
"Thermoelectric device", ("Seebeck Effect", bismuth telluride) Peltier
The Seebeck effect, when the wires are at different
temperatures is used to detect very small differences in temperatures.
The Seebeck effect, an electromotive force is generated in a circuit
containing junctions between dissimilar metals if these junctions are
at different temperatures.
A thermocouple consists of two such dissimilar metals connected in
A thermocouple is a thermoelectric device for measuring temperature,
consisting of two different metals joined at two points so that the
junction develops a voltage dependent on the amount by which its temperature
differs from that of the other end of each metal.
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 iron-copper 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.
22.214.171.124 Peltier effect,
Thermoelectric device, for "Peltier Effect", connect 5 V DC, (commercial)
Thermoelectric, ("Seebeck Effect", bismuth telluride), ("Peltier Effect",
connect 5 V DC)
The Peltier effect, (Jean Charles Athanase Peltier, 1785-1845, France),
heat is given out or absorbed when an electric current passes
through the junction of two materials.
The Peltier coefficient is the quantity of heat given out or absorbed,
depending on the direction of the current at a junction between two
conductors or between a given material and a reference conductor when
a unit of charge passes between them.
The Peltier coefficients represent how much heat current is carried
per unit charge through a given material.
The Peltier effect is used in thermoelectric refrigeration or heating.
1. Cool a small test-tube of water in a Peltier device or dry ice
/ alcohol bath and use a thermocouple to record the temperature.
Shake to freeze and the temperature will rise.
A Peltier device is a very small solid state device that functions
as heat pump made of layers of ceramic plates and bismuth telluride.
Apply DC current to move heat from one side to the other so the cold
side can cool a small electronic device.
They contain no moving parts and no Freon refrigerant.
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
3. Make an antimony-bismuth junction and an apparatus to show heating
and cooling due to the Peltier effect..
126.96.36.199 Thompson effect
The Thompson effect, (Lord Kelvin, 1851, England), the existence of
a temperature gradient along an electrical conductor causes a
potential gradient in the same direction or contrary direction.
A flame moved along a long wire will push ahead current.
A thermopile is a set of joined thermocouples.
A thermopile is made of thermocouple junction pairs connected electrically
The absorption of thermal radiation by one of the thermocouple junctions,
called the active junction, increases its temperature.
The differential temperature between the active junction and a reference
junction kept at a fixed temperature produces an electromotive
force directly proportional to the differential temperature created.
This effect is called a thermoelectric effect.
For more sensitivity, thermocouples are joined in series to make a
Thermopiles do not respond to absolute temperature, but generate an
output voltage proportional to a local temperature difference or
A thermopile consists of four iron and four copper wires twisted to
form seven junctions to produce a thermoelectric EMF.
The end terminals connect to a galvanometer.
Use a hand held hair drier is a suitable source of heat to activate
188.8.131.52 Constantan, Eureka
wire, CrAl (Kranthal) and NiCrm (Nichrome wire)
Constantan wire 15.6 ohm / m, 6.9 ohm / m, 0.98 ohm / m.
Constantan is an alloy, about 40% nickel and 60% copper, having high
volume resistivity and negligible temperature coefficient.
So resistance hardly changes with change in temperature.
It is used for resistance wire.
Also Eureka wire, CrAl (Kranthal) and NiCrm (Nichrome wire) have very
An optical pyrometer is used to measure very high temperatures from
the colour of the radiant heat source.
1. Attach a piece of copper and a piece of constantan to two wires.
Heat lead to above its boiling point of 327oC.
Attach the wires to a galvanometer and insert the copper and constantan
into the boiling lead.
The galvanometer can be calibrated to read temperature and act as a
thermocouple to read the temperature of the molten lead.
33.5.0 Power and energy,
electrical equivalent of heat, power transmission, household power
1. 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 × amps
Power, P in watts = current2 × resistance = I2R
Power, P in watts = voltage2 / resistance = V2
2. Power and Energy, watt W = 1 joule per second,
watts = volts × amps, filament lamps, fluorescent lamps, radiant electric
three heat switch, fuses, W (joules = Q (coulombs) × 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) × time (t) so work done = QV = VIt joules.
3. Electric power is the rate of energy use, measured
in watts, W, the amount of energy that flows in one second.
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 × Amperes, VI = joule / coulomb
× 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
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.
Some circuit elements, e.g. vacuum diode, do not have uniform I-V
graphs and do not have a constant value for resistance.
Power systems, UK, 50 Hz 240 volts, RMS.
4. In high-voltage power lines, high tension power
lines, the ohmic resistance of the wires causes some power to be dissipated
To reduce this loss power, engineers use the best conductors, large
diameter wires, the shortest possible route and the highest possible
If Ploss = I2R, where Ploss = power
dissipated as heat, (watts), I = line current, (amperes), R = line resistance,
If R remains constant, Pload I2, where I = the
net load, i.e. the total current used by all the power users.
For voltage, I = Pload / V, where Pload = total
power demanded by users.
At a fixed power load, Ploss = I2R, Ploss
= (Pload / V)2R, = (Pload)2R
So increasing the voltage decreases the power loss.
Power loss could be further decreased by using direct current for
long distance transmission because direct current does not produce
electromagnetic fields, another source of power line inefficiency,
electromagnetic radiation loss.
However, direct current power transmission is costly to install because
it requires high voltage rectifiers at a generating plant and DC to
AC inverters where high voltage has to be connected to low voltage
lines to consumers.
In USA where households use alternating current at 117 volts and 234
volts RMS, step-down transformers reduce the voltage of high
tension lines carrying 100 K volts or more to a "municipal voltage",
using large step-down transformers in buildings or on the ground in
Then smaller transformers on utility poles or underground, step down
the "municipal voltage" to "household voltage" of 234 volts RMS
as three separate AC waves, called phases.
Each wave of the three-phase power runs 120o out of phase,
with the two other phases to form the three-phase AC power needed for
large appliances, e.g. ovens and washing machines.
However, in USA the conventional wall outlets carry single phase AC
electricity at 117 volts RMS.
22.2.03 Megawatt-hour, MWh
The megawatt-hour, MWh, is a unit of energy that measures the power
consumed or produced by one megawatt for one hour.
The megawatt, (MW), is a unit of power that measures the rate at which
power is consumed or produced at any given moment in time.
Megawatt hours, (MWh) = megawatts, (MW), x hours, (h)
Megawatts, (MW) = megawatt hours, (MWh) / hours, (h).
One megawatt, (MW) = one million watts, joules/second, W
One megawatt, (MW) = 1000 kilowatts, kW
One gigawatt, (GW) = 1000 megawatts
In the State of Queensland, the long-term average wholesale price
of electricity is AUD 45 to 50 per megawatt hour, MWh.
The baseload generating capacity is about 7 600 MW, but more than
8 000 MW is required to to power the State at peak demand.
Spare capacity of about 1 500 MW comes from higher cost peak generators.
For domestic general consumption, (14/01/2013), the charge is 25.37810
cents / kWh and the supply charge is 28.78700 cents per
22.2.04 Kilowatt-hour, kWh
The kilowatt-hour, kWh, is the energy used when when an appliance
with the power of one kilowatt runs for one hour.
A power of one watt = one joule per second, so a kilowatt-hour = 3,600,000
J, (1000 watt-hours = 1 kilowatt-hour,
1 kWh or 3.6 megajoules, 3.6 MJ), about the energy used by one bar of
an household electric heater.
The kilowatt-hour is a common billing unit for household and commercial
184.108.40.206 Carbon and tungsten
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.
220.127.116.11 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
33.5.01 Heat from current
through a conductor is proportional to: 1. time, and 2. current2
See diagram 32.2.63d: Calorimeters
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
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
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
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.
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.
33.5.1 Light from electrical
See diagram 32.5.4: Circuits
Connect a 2.5 volt torch globe to a single 1 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?
33.5.2 Heat from electrical
See diagram 32.5.4: Jug element wire circuit
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
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.
33.5.3 Electric heater using
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
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.
33.5.4 Electric light and
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-on 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.
33.5.5 Electric jug, immersion
| See diagram 32.5.5: Immersion heater circuit
| 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
The number of the wires depends on the electric current provided by
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
33.5.6 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.
33.5.7 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
33.5.8 Heat 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.
33.5.9 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.
33.5.10 Current through
a torch globe
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
33.5.11 Compare 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.
33.5.12 Light from incandescent
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 AC 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.
33.5.14 Electric kettle
See diagram 32.2.3
Any kettle used to heat water can lose heat to its surroundings and
to the materials from which it is constructed.
The heat produced by the heat source does not only heat the water.
You can measure the heat efficiency of an electric kettle by doing
a simple experiment.
BE CAREFUL! Be sure that water cannot come into
contact with the power supply.
Some simple heating elements are bare wire and should not be used
for this experiment! Do not operate an electric kettle with
wet hands! Be sure that students and teachers cannot be scalded by
1. Record the power rating of the heater element.
2. Measure and record the temperature of 500 mL of water and pour
it into a kettle.
3. Switch on the power supply to the kettle and start timing how long
it takes the kettle to bring the water to boil.
4. Switch off the power supply when the water boils, and record the
time it took for the water to come to boil.
5. Empty the kettle and allow the element to cool to room temperature
then repeat steps 2, 3 and 4 and find the average time to
bring the water to boil.
To calculate the efficiency of the kettle you need to find how much
energy the water absorbed to bring it to boiling point.
Use the formula Q = mc (Tf - Ti), where m =
mass of water, c = specific heat of water, Tf = final temperature
Ti = initial temperature.
Then divide this value by the time it took to bring the water to boiling
and you get the power consumed in boiling the water.
Finally you divide this value by the power rating of the element to
give the efficiency of the kettle.
The following example is based on a kitchen kettle with an element
rating of 2,200 watts:
m = 0.5 kg, c = 4186 J / kgoC, Tf = 100oC,
Ti = 22oC.
So Q = 0.5 × 4186 × (100 - 22) = 163,254 Joules
The time taken to bring the water to boil was 94 seconds.
Therefore the power consumed to boil the water = 163,254 / 94 = 1,737
To find the efficiency of the kettle divide the power used to boil
the water by the power output of the element and multiply by 100 to
give a percentage value, i.e. (1,737 / 2,200) × 100 = 79%
The efficiency of the kitchen kettle is 79%, or 21% of the power output
33.5.16 Exposure meter
See diagram 32.5.13: Exposure meter circuit
If the lamp is lit from the AC terminals of the variable voltage supply
and not the DC then AC meters will be required.
Place the exposure meter 15 cm from the lamp.
Record readings of the light meter reading for various currents through
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
For a 100 watt lamp, a variac provides a suitable supply used with
an AC meter giving I amp full scale deflection or better 500 mA
full scale deflection.
33.5.17 Electrical appliances
in the home
See diagram 32.2.3: Electric jug
Before preparing to teach this topic, select, examine and report on
a useful electrical appliance, e.g., air conditioner, boiler,
calculator, charger, clock, dishwasher, doorbell, fan, freezer, fryer,
hair dryer, heater, iron, mixer, motor, printer, radiator, refrigerator,
roaster, shaver, telephone, television, toaster, torch, toy, vacuum
cleaner, Xmas tree lights.
Examine the nameplates and study the instruction manuals.
Report on the following:
1. Correct name and use of each electrical appliance
2. Normal or allowed working voltage and current
3. Working principles including a circuit diagram
4. Power input or useful power output, resistance and other properties
5. Operating method and points for attention
6. Safety characteristics, including the safety operating conditions
so that the operator will not be hurt and apparatus not to be damaged .
date of production, continuous operating time.
8. Examine how the appliances convert electric energy to other forms
of energy and think about how to design an experiment project
to measure the efficiency of energy transformation.
Points for attention before preparing to teach this topic:
See 19.3.5: Microwave cooking
1. Be clear on how to switch off the power in an emergency and the exact
position of the appliance.
2. Any old or discarded appliance that requires mains power to operate
should be inspected and repaired only by a qualified electrician.
If you have any doubt about the operating status or safety of any
electrical appliance, do not under any circumstance connect it to
As a rule, all appliances that require mains voltage to operate should
be tested periodically by a qualified electrician.
Check with your local electricity supply authority about how often
these checks should be done.
Be careful! Mains electricity can kill!
Other pieces of equipment contain high vacuum tubes, such as television
sets and microwave ovens.
Breaking the glass container that is evacuated can cause injury from
Do not use exposed wires to connect a circuit.
Pay special attention to whether the leads of the electrical appliance
discarded for a long time are exposed or ageing.
You must wrap with electrical insulating tape or replace all exposed
or ageing wires.
Check for damaged three pin plugs, exposed flex wire and exposed ends
before the experiment.
3. Use only ammeters, voltmeters and power meters authorized for use
Use only low voltage power packs up to 12 V.
Check the circuit before connecting the last lead to the source of
power, especially if an ammeter is in the circuit.
Make the first connection to the source of power by switching on and
off very quickly to check whether you have connected
ammeters and voltmeters correctly with correct deflection and reading
not off the scale.
4. Plug the three pin plug into a normal three pin socket.
Do not change the pin and the socket.
5. Teachers should check all experiments involving electricity no matter
the voltage before they allow students to energize circuits.
6. Never allow students to work unsupervised on electrical experiments.
7. Ensure that no other appliances are working before starting the test.
33.6.0 Circuit analysis, house
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
1. Measure the voltages around a three resistor and battery series
2. To show continuity of current, insert an ammeter into any branch
of a circuit to show currents in and out of a node.
3. To show superposition of currents, measure the current from one battery,
the current from a second battery in another position and
the combination in a circuit.
4. Study a 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.
5. 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.
6. To show series and parallel equivalent resistance,
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
Use a circuit board laid out so meters can be plugged in and readings
taken for demonstrations of series and parallel circuits and
33.6.4 Wheatstone bridge,
bridge circuits, slide wire, metre wire bridge
See diagram 32.2.60: Wheatstone bridge
1. 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
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.
2. Measure the value of an unknown resistance with
a metre wire bridge.
When switch S is closed and the resistance is 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 AC 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 × (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.
3. Wheatstone bridge with a human galvanometer.
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.
4. Use a Wheatstone bridge configuration with 4
light bulbs for resistors using 110 ac.
Use four 60 W lamps in a diamond-shaped 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.
33.6.8 AC 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
In a regular electric light bulb, the metal filament continues glowing
between the peaks in current.
33.6.9 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
If some day you want to decorate or fit up your house, it will help
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
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.]
33.7.0 Electric current detectors
| See diagram 32.163.1: Electric current
detector - Compass in a coil
| See diagram 32.163.2: Plotting compass
in a match box
Current electricity is electricity flowing as a current.
It is a form of energy caused by charged particles, e.g. protons, electrons,
accumulating dynamically as a 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.
Make a simple instrument to detect electric current.
Wrap 50 to 60 turns of bell wire to form a coil around a jar 8 cm in
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
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
Reverse the connections, and observe the deflexion of the compass needle
Make a more sensitive instrument by putting a compass in the tray of
a match box then winding the coil wire over the box.
| See diagram: 32.3.01 Moving coil galvanometer
| See diagram 18.104.22.168.6: Right hand motor
rule for electron flow
A galvanometer is an instrument for detecting small electric currents
by observing the deflection of a magnetic needle by an electric
current in a magnetic field.
Meters for measuring voltage or current are made from moving coil
To keep the maximum force acting on the moving coil, the magnetic
field is drawn inwards by a "soft" iron core, making the field
The moving coil turns against springs that 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.
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
made to "bypass" the galvanometer through a "protective shunt" as in
A short length of fine, cotton covered copper wire serves as a convenient
This moving coil meter works on the same principle as a simple DC electric
motor and is called the D'Arsonval movement after its
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.
1. 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
In some galvanometers a coil spring below the moving coil, with an
attached pointer, controls how far the coil turns and measures the
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.
22.214.171.124 Sensitivity and
resistance of a galvanometer, voltmeter, electroscope
Determination of galvanometric constants.
Use external resistors to measure the resistance and sensitivity of
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.
126.96.36.199 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.
188.8.131.52 Convert a galvanometer
to an ammeter
Knowing the resistance and sensitivity of a galvanometer add a shunt
resistance and then measure a current.
184.108.40.206 Convert a galvanometer
to an ammeter, hot wire ammeter
See diagram 220.127.116.11: Hot wire ammeter
In a hot wire metre you pass the current through a platinum alloy
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
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
Finally, silk, pulley and pointer move in turn.
In view of energy, first the electric energy transforms into heat
Then heat energy transforms into kinetic energy of pointer and potential
18.104.22.168 Reduction factor
k of a tangent galvanometer
See diagram 32.2.66: Tangent galvanometer
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
The galvanometer is made horizontal with adjustable legs and a spirit
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 θ (Greek) is the angle of
deflection of the pivoted magnet, H = H1 tan θ.
H, is a constant at a particular place so H is proportional to tan θ.
H is proportional to I, so I is proportional to tan θ or I = k tan θ.
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 (θ1o
Reverse the current with the reversing switch S2 and again
read both ends of the pointer (θ3o and θ4o).
Record the current I amps.
Repeat to give more deflections between 30o and 60o,
each time reversing the current through the galvanometer and recording
current I amps.
22.214.171.124 Make a galvanometer
from a pocket compass
See diagram 126.96.36.199: Compass galvanometer
Wind 50 turns of fine insulated wire over a pocket compass on a square
of cardboard so that with the needle of the compass pointing
north and the axis if the wire points east west.
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
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.
See diagram: 32.3.03: Voltmeter
A voltmeter measures electric potential in volts.
A voltmeter measures "electron pressure", potential or electromotive
force (EMF), the ability of a cell to move electrons around a
circuit, potential difference, with SI unit the volt.
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
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 measures 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 × I = volt × 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 × length / area, e.g. resistivity copper = 1.7 × 10-8
ohm metre, rubber = 1013 ohm metre.
188.8.131.52 Connect a voltmeter
See diagram 32.3.03.1: Voltmeter circuits
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 circuit should always be switched off during changes in wiring.
Wire the lamp fitted to the lamp base into a series circuit with the
Switch on 12 volt battery.
Switch off and connect the voltmeter parallel with the lamp.
184.108.40.206 Voltmeter as cell
See diagram 32.3.03.2a: Voltmeter circuits
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
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
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.
220.127.116.11 Calibrate a voltmeter
See diagram 32.3.03.3: Voltmeter circuits
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.
18.104.22.168 Potential difference
and electromotive force
See diagram 32.3.03.4: Voltmeter circuits
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.
22.214.171.124 Loading by a voltmeter
Measure the voltage across a high resistance circuit with high and low
33.8.3 Copper-iron junctions
Use a Bunsen burner to simultaneously heat sixty copper-iron junctions
in series and arrayed in a ring to produce 90 mA current
33.8.4 Thermoelectric devices
1. 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.
2.Thermoelectric effect in a wire.
A piece of soft iron wire connected to a galvanometer has little thermoelectric
effect until the wire is kinked.
3 Thermoelectric heat pump.
Mount aluminium blocks with digital thermometers on either side of
a Peltier device.
Run the current both ways.
4. 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.
33.8.11 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
126.96.36.199 Battery, source
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
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
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.
188.8.131.52 Power wasted inside
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.