School Science Lessons
(UNPh33)
2024-07-28
Electric Current
Contents
33.7.0 Electric current measurement
33.5.0 Electric power and energy
33.1.0 Electrochemical cells
33.6.0 House circuits
33.4.0 Incandescence
33.8.0 Thermocouple
33.4.0 Incandescence, electric light bulb, filament lamp
33.5.12 Compare light from incandescent lamps
33.4.65 Incandescent lamp
Tungsten, (electric filaments)
33.7.0 Electric current measurement
33.7.3 Ammeter
6.3.1.4 Electric current, ampere
Experiment
33.7.4.3 Calibrate a voltmeter
33.7.4.1 Connect a voltmeter
33.7.2.4 Convert a galvanometer to an hot wire current meter
33.7.01 Electric current detectors
33.7.2.0 Galvanometer, Tangent galvanometer
33.7.2.6 Make a galvanometer from a pocket compass
33.7.4.4 Potential difference and electromotive force
33.7.2.5 Reduction factor k of a tangent galvanometer
33.7.4.0 Voltmeters
33.7.4.2 Voltmeter as cell counter
33.5.0 Electric power and energy
33.5.01 Power and energy, power transmission, household power
33.2.03 Megawatt-hour, MWh
33.2.04 Kilowatt-hour, kWh
Experiments
33.6.8 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
33.5.02 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
33.5.1 Light from electrical energy
33.5.12 Compare light from incandescent lamps
33.5.13 Switch lights on and off
32.4.7.1 Power surge circuit breaker, fuses
33.6.4 Wheatstone bridge
33.8.0 Thermocouple, thermoelectricity
33.7.6.2 Peltier effect
33.7.6.1 Seebeck effect
33.7.6.0 Thermocouple
33.8.4 Thermoelectric devices
33.7.6.4 Thermopile
33.7.6.3 Thompson effect
33.4.65 Incandescent lamp
Incandescent lamp, electric light bulb, filament lamp, light globe
See diagram 32.162: Heat and light from electricity.
A substance is incandescent if it emits light as a result of its temperature being raised.
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".
Experiment
1. Heat source
* 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 paper.
* 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.
33.1.0 Electrochemical cells
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:
* Cations (positive ions, e.g. Fe2+), flow to the cathode where they are reduced, i.e. gain electrons, from the cathode.
* Anions (negative ions, e.g. SO42-), flow to the anode where they are oxidized, i.e. lose electrons, by depositing electrons onto the anode.
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. Electrochemical cells react spontaneously.
The anode is the negative terminal through which electrons enter and conventional current leaves.
The current formed by a chemical cell is from chemical reactions inside the cell.
Voltaic cells (galvanic cells) and common "batteries" are electrochemical cells that generate electric current.
In simple electrochemical cells, from copper and zinc metals in separate solutions of their sulfates, electrons move from zinc metal to copper metal through an external wire, an electrically conducting path, i.e. as electric current.
Electrochemical cells generate electric current by using two different metals that differ in their tendency to lose electrons.
For example, zinc tends to lose electrons more than copper, so by placing zinc and copper metal in solutions of their salts, electrons flow through a wire from the zinc to the copper.
A cathode is a negative electrode or terminal.
The cathode is usually labelled as the negative terminal, with "-" sign on it and painted black.
An anode is a positive electrode or terminal.
The anode is usually labelled as the positive terminal with "+" sign on it and painted red.
Zn (s) --> Zn2+ (aq) + 2e-
At the negative terminal, the anode, oxidation, because electrons are lost.
Cu2+ (aq) + 2e- --> Cu (s)
At the positive terminal, the cathode, reduction, because electrons are gained.
2. 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 one of its salts.
The half cells are separated by a porous membrane or salt bridge that allows movement of ions between the half cells.
3. Inside an electrochemical cell:
Cations (positive ions, e.g. Fe2+), flow to the cathode where they are reduced, i.e. gain electrons, from the cathode.
Anions (negative ions, e.g. SO42-), flow to the anode where they are oxidized, i.e. lose electrons, by depositing electrons onto the anode.
4. 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.
5. 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.
Experiment
See diagram 3.2.85.2: Voltaic Cell: Zinc/Copper.
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)
Experiment
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)
33.7.6.0 Thermocouple
See diagram 4.1.1.3: Voltage produced by heat, 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 hot end.
If the metals are connected, the electrons can cross from the iron to the copper at the hot junction, and from the copper through the current meter to the iron at the cold junction.
This device is called a thermocouple.
Use it to measure temperature, and as heat sensing devices in automatic temperature control equipment.
Thermocouples can be subjected to greater temperatures than thermometers containing liquids.
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.
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 metallic tube.
A thermocouple surface probe is used to measure the temperature of a solid surface.
The thermocouple is based on the Seebeck effect (thermoelectric effect): 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.
Experiment
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 voltmeter.
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 thermocouple current.
33.7.6.1 Seebeck effect (thermoelectric effect)
(Thomas Johann Seebeck, 1770-1831, Germany)
"Thermoelectric device" ("Seebeck Effect", bismuth telluride), Peltier Device
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 series.
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.
Experiment
Constantan wire
Experiment
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 thermoelectric generator is made from 150 constantan / nickel molybdenum thermocouples in series.
33.7.6.2 Peltier effect
(Jean Charles Athanase Peltier, 1785-1845, France)
Thermoelectric ("Seebeck Effect", bismuth telluride) ("Peltier Effect", connect 5 V DC)
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.
Experiment
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
Send current through a copper-iron-copper circuit for several seconds and immediately disconnect and switch to a galvanometer.
3. Make an antimony-bismuth junction and an apparatus to show heating and cooling due to the Peltier effect.
33.7.6.3 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.
Experiment
A flame moved along a long wire will push ahead current.
33.7.6.4 Thermopile
A thermopile is a set of joined thermocouples.
A thermopile is made of thermocouple junction pairs connected electrically in series.
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 thermopile.
Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient.
Experiment
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 the thermopile.
33.5.01 Power and energy
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 / R
2. Power and Energy, watt W = 1 joule per second, watts = volts × amps, filament lamps, fluorescent lamps, radiant electric fires, 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 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.
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 as heat.
To reduce this loss power, engineers use the best conductors, large diameter wires, the shortest possible route and the highest possible voltage.
If Ploss = I2R, where Ploss = power dissipated as heat (watts), I = line current (amperes), R = line resistance, (ohms).
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 / V2.
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 fenced-off areas.
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.
5. DC is preferred for long distance transmission hauls, because of the economics of a DC line.
DC is more efficient than AC at high voltages and has lower line losses.
The DC line has lower losses, no reactive power is needed, and the power flow can be fully controlled.
Wire diameter is limited for AC transmission lines, because of the "skin effect" that prevents an AC current from penetrating to the centre of a large wire, whereas a DC line can be arbitrarily thick.
At 60 Hz, the skin effect becomes significant for wires greater in diameter than about an inch (2.54 cm).
Because of the skin effect in part, multiple wires arranged in a circular pattern and separated by polymer spacers are often used in high capacity high voltage AC transmission lines.
Overhead HVDC powerlines can transport significantly more power for greater distances than AC lines, for two main reasons:
* the effective voltage can be higher, and 2. the wires can be bigger.
* DC transmission lines become cost effective with AC lines at around 300 miles for overhead and 30 miles for underground power transmission.
This is because the the cost of converter stations for a DC line are much greater than the cost of a transformer.
At those distances, the line costs equalize and for longer lines, DC is more efficient.
33.2.03 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 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 day.
33.2.04 Kilowatt-hour, kWh
The kilowatt-hour, kWh, is the energy used 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 electricity consumption.
32.4.6.5 Battery, source of emf
A battery is a source of electrical energy with electromotive force, emf, measured in volts, equal to the potential difference between its terminals, assuming no loss of internal energy in the battery.
A current whose direction does not change with time is called direct current.
The current whose current intensity is invariable in the circuit is called constant current.
The end of the resistor where current enters is the high potential end.
Current flows through a resistor from high potential to low potential.
The positive terminal of a battery is always the high potential terminal assuming the internal resistance is small.
In the external circuit of the electrical source, the constant current flows from the high potential to the low potential.
In the internal circuit of the electrical source, the current flows from the low potential to the high potential.
32.4.6.8 Power wasted inside a battery
The three accumulators with negligible internal resistance are enclosed in a suitable box.
Connect the terminals to two external terminals on the box.
The high resistance is coiled and put in series with the accumulators inside the box to provide the "internal resistance".
Record the readings of the ammeter and the voltmeter.
33.5.02 Heat from current through a conductor
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.
Experiment
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.
33.5.1 Light from electrical energy
See diagram 32.5.4: Light switches circuits.
Experiment
Short lengths of wire with small bulldog clips soldered at each end are useful leads for electrical connections.
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 energy
See diagram 32.5.4: Jug element wire circuit.
Experiment
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.
33.5.3 Electric heater using steel wool
See diagram 32.5.3: Steel wool heater.
Experiment
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.
Fasten another bare copper wire to the brass end of the centre 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.
33.5.4 Electric light and switch
See diagram 32.5.4: Light switches.
Experiment
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 heater
See diagram 32.5.5: Immersion heater circuit.
See diagram 32.5.5a: Electric jug.
Experiment
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 the 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.
33.5.8 Heat wires in series
Experiment
Solder together several lengths of different wires of the same length in series.
33.5.9 Hot dog / pickle cooker
Experiment
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 longer.
33.5.12 Compare light from incandescent lamps
Two lamps using the same current emit different amounts of light.
Experiment
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.
33.5.13 Switch lights on and off
When light is switched on the tungsten element expands and contracts when the light is switched off.
At high temperature some of the tungsten filament evaporates, but it evaporates unevenly.
Places of greater evaporation become thinner, their electrical resistance becomes greater, they will have less surface area to radiate heat, so have a higher temperature and will melt before the rest of the filament.
Repeatedly turning a light on and off causes early light globe fatigue.
If you know the power of the light bulb and the cost of electricity, you can calculate the cost per hour of leaving the light bulb turned on.
If you know the cost of a light bulb and the average life time, you can calculate a "break-even point", when it is as cheap to leave the light bulb turn on as turn it off.
For some commercial bulbs, the time is as much as 20 minutes.
33.5.14 Electric kettle heating efficiency
See diagram 32.2.3: Electric kettle heating efficiency.
Any kettle used to heat water can lose heat to its surroundings and to the materials from which it is constructed.
So the heat produced by the heat source does not only heat the water.
Experiment 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 steam!
* Record the power rating of the heater element.
* Measure and record the temperature of 500 mL of water and pour it into a kettle.
* Switch on the power supply to the kettle and start timing how long it takes the kettle to bring the water to boil.
* Switch off the power supply when the water boils, and record the time it took for the water to come to boil.
* 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, 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 and Ti = initial temperature.
Divide this value by the time it took to bring the water to boiling and you get the power consumed in boiling the water.
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.
So the power consumed to boil the water = 163, 254 / 94 = 1, 737 Watts.
For 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 % value, i.e. (1, 737 / 2, 200) × 100 = 79%.
The efficiency of the kitchen kettle is 79%, or 21% of the power output is wasted.
33.5.16 Exposure meter
See diagram 32.5.13: Exposure meter circuit.
Experiment
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 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 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 heating efficiency.
See 19.3.5: Microwave cooking.
Experiment
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.
7. Relevant times and terms of use, e.g. guarantee period, scrap period, 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:
* Be clear on how to switch off the power in an emergency and the exact position of the appliance.
* 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 connect it to mains supply.
* 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!
* Some 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 flying glass.
* Do not use exposed wires to connect a circuit and note 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.
* Use only ammeters, voltmeters and power meters authorized for use in schools.
* 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.
* Plug the three pin plug into a normal three pin socket and do not change the pin and the socket.
33.6.0 Circuit analysis, house circuits
1. Measure the voltages around a three resistor and battery series circuit.
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 circuit board laid out so meters can be plugged in and readings taken for demonstrations of series and parallel circuits and Kirchhoff's laws.
33.6.4 Wheatstone bridge
Wheatstone bridge, bridge circuits, slide wire, metre wire bridge
See diagram 32.2.60: Metre wire bridge.
Experiment
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 the following:
* Measure a resistance with a Wheatstone bridge, metre wire bridge, post office box.
* 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).
3. 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.
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
Experiment
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.
33.7.01 Electric current detectors
See diagram 32.163.: 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.
Experiment
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 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.
33.7.2.0 Galvanometer, Tangent galvanometer
See diagram: 32.3.01: Moving coil galvanometer.
See diagram 32.0.1.1.6: 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 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 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.
However, 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 DC 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.
Experiment
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 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.
33.7.2.4 Convert a galvanometer to an ammeter, hot wire ammeter
See diagram 32.7.2.4: Hot wire ammeter.
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.
33.7.2.5 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 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 θ, (Greek theta θ). 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.
Experiment
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 and θ2o).
Reverse the current with the reversing switch S2 and again read both ends of the pointer (θ3o and 4o).
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.
33.7.2.6 Make a galvanometer from a pocket compass
See diagram 33.7.2.6: 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.
33.7.3 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.
33.7.4.0 Voltmeters
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 placed across part of a circuit it does not divert a lot 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.
Experiment
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 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.
33.7.4.1 Connect a voltmeter
See diagram 32.3.03.1: Voltmeter circuits.
Experiment
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.
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.
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.
33.7.4.2 Voltmeter as cell counter
See diagram 32.3.03.2a: Voltmeter circuits.
Experiment
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.
33.7.4.3 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.
33.7.4.4 Potential difference and electromotive force
See diagram 32.3.03.4: Voltmeter circuits.
Potential difference is like "electrical pressure difference" between the ends of a part of a circuit, where energy transfer occurs, e.g. electrical energy to heat.
Experiment
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.
33.8.4 Thermoelectric devices
Experiments
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 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.