School Science Lessons
UNPh32
2024-07-25
Electric circuits
Contents
32.1.0 Electric circuits
32.2.0 Electric conductivity
32.3.0 Electric fuses
32.4.0 Current electricity, electric current
32.5.0 Conventional current
32.1.0 Electric circuits
32.2.00 Circuit diagrams, electrical symbols, (gif)
Experiment
32.1.1 Cells in parallel
32.1.2 Cells in series
32.1.3 Cells in series and parallel, internal resistance
32.1.4 Circuit board
32.1.5 Electric circuit board
32.1.6 DC circuits, circuit diagrams, Kirchhoff's circuit laws
32.1.9 Electric current, Ampère
32.1.10 Electric shock
32.1.11 Electric light bulbs (lamps), resistors, in series and parallel
32.1.12 Electric torch (flashlight)
32.1.13 Lamps in parallel
32.1.14 Lamps in series and parallel
32.1.15 Resistors in parallel
32.1.16 Resistors in series
32.1.17 Resistors in series and parallel
32.1.18 Series and parallel (branching) circuits
32.1.19 Simple electric circuit
32.1.20 Simple switch
32.1.6.3 Six-volt batteries in series and parallel, (gif)
32.1.21 Switches in a circuit, tapping key
32.1.22 Switches in a circuit, knife switch
32.2.0 Electric conductivity
32.2.1 Conductivity of electrolytes
32.2.16 Conductivity of melted substances
32.2.3 Conductivity of solutions
32.2.4 Conductivity of water, deionized water, distilled water, tap water
32.2.5 Conductivity of salt water string, electrolytic conduction on chamois
32.2.6 Electrical conductivity, conductance
32.2.7 Electrical conductivity
32.2.8 Electrical conductivity of liquids
32.2.9 Electrical conductivity of different solutions
32.2.10 Electrical conductivity of different concentrations of solutions
32.2.11 Superconductors
Experiment
32.2.12 Bite on aluminium foil
32.2.13 Conduction from hot wire
32.2.14 Conduction of gaseous ions, electroscope
32.2.15 Conductors and insulators
32.2.17 Current electricity from frogs' legs
32.2.18 Current balance
32.2.19 Electric current detector
32.2.20 Electrical conductivity of melted solids, fused solids
32.2.21 Electrical conductivity of solids
32.2.22 High temperature and conductivity of sodium chloride and paraffin waxt
32.2.23 "Lead" pencil conductor
32.1.24 Liquids that conduct electricity
32.2.24 Solids that conduct electricity
32.2.25 Substances that conduct electricity
32.2.26 Test electricity with the tongue
32.2.27 Test materials for conductivity
32.2.28 TASER
32.3.0 Electric fuses
32.3.1 Fuse box
32.3.2 Power surge circuit breaker, fuses
32.4.0 Current electricity, electric current
Electric current, heat and light from electricity, direct current and alternating current, effects of a current: heat and light, magnetic, chemical, Q (coulombs) = I (amperes) × t (seconds), current, nature of electric current and DC / AC. EMF sources
Electric current is carried by discrete charge carriers; charge is conserved at all points in an electrical circuit.
Energy is conserved in the energy transfers and transformations that occur in an electrical circuit.
Current electricity, electric current, ampere or amp.
Current, I, of electricity exists when an electric charge is transported.
The directional movement of charges through a wire is called current, I, and it has the SI unit ampere or amp, symbol A.
The ampere is defined as the current in two parallel conductors one metre apart in a vacuum with a force between them of 2 × 10-7 newton per metre of conductor.
The direction of the current is the same as that of the movement of charges.
The size of the current, current intensity, equals the charge flowing through the cross-section of a conductor in unit time.
I = q / t, where I = current in amperes, q = the amount of charge that passes a point in the circuit, t = time interval.
Assume that the direction is in the direction of the flow of positive charge, so flow of electrons to the right means flow of current to the left.
When charge flows through a conductor, the rate of flow of charge through any section of the conductor is called the electric current.
1 amp = 1 coulomb per second.
In a copper plating tank, 1 amp carries 3.29 10-7 kg of copper across every second.
Ampere
(André-Marie Ampère, 1775–1836, France)
The flow of electrons through a conductor is called electric current and is measured in amperes, with the symbol I amp.
One ampere represents the flow of 6.28 × 1018 electrons per second past a fixed point in a conductor.
The unit quantity of electricity when one ampere of current flows for one second is called the coulomb, symbol Q.
So, I (ampere) = Q (coulomb) / t (second), amps = coulombs per second.
A "current of 1 amp" means 1 coulomb of electricity, charge, moves past each point in the circuit per second.
1 amp is the current flowing in two parallel wires one metre apart to produce a force of 2 × 10-7 newton on each metre of wire.
32.5.0 Conventional current
If "current" is the conventional positive current, in metal wires, the current is carried by electrons moving in the direction opposite to the current.
In conventional current, the cathode of a battery is the terminal where current flows out and the anode is the terminal where current flows in.
Conventional current passes from the positive terminal, through a circuit, to the negative terminal of the battery, from cathode to anode.
In a discharging battery or a galvanic cell, the cathode is the positive terminal since that is where the conventional current flows out of the device.
32.1.1 Cells in parallel
See diagram 32.1.6.1: Cells in parallel.
Connect two or three fresh dry cells or lead cell accumulators so that their positive terminals are joined and their negative terminals are joined.
The cells are now connected in parallel.
Set up a circuit on a circuit cardboard with the two, or three cells, in parallel.
Disconnect one of the cells. The circuit is not broken and the brightness of the light does not change.
The voltage drop in the circuit is the same if one, two or three cells are used.
So the voltage is 1.5 volts.
When cells are connected in parallel the total voltage is no greater than that of a single cell, but the total current available is increased in proportion to the number of cells.
If four cells in the circuit, the total current is 0.125 × 4 = 0.5 amps.
32.1.2 Cells in series
See diagram 4.60: Cells in series.
Cells connected in the same direction in series each add their own voltage, e.m.f., to the total voltage.
However, each cell has an internal resistance, Ri.
If connect three cells of voltage V1, V2 and V3, and current through the cells is I amps, then total voltage = (V1 +V2 + V3) - ( Ri1 + Ri2 + Ri3).
Connect two dry cells or lead cell accumulators so that the negative terminal of one is in contact with the positive terminal of the other.
They are connected in series.
Put a light bulb in the circuit.
Close the circuit with one cell, two cells, three cells in series.
Record the changes in the brightness of the light bulb.
The brightness of the light depends on the number of cells connected in series.
When you connect cells in series, the total voltage is the sum of the individual voltages of the cells.
If you use 1.5 V cells, then two cells give 3 V, and three cells give 4.5 V, four cells give 6 V.
The current will change.
32.1.3 Cells in series and parallel, internal resistance
See diagram 32.1.6.0: Cells in series, lamps in series.
See diagram 32.1.6.1: Cells in parallel.
See diagram 32.1.6.2: Cells in series and parallel.
If the terminals of a voltmeter are places on each end of a carbon zinc AA torch battery, the open circuit reading is emf of 1.5 volts.
However, if the battery in place in a circuit containing a resistor, and when current is drawn from the battery, you measure the voltage across the battery, it is < 1.5 volts, because of the internal resistance of the battery.
So each battery has its own internal resistance.
In a circuit, V = V0 - Ri, where V = voltage of battery in the circuit, Vo = voltage of battery in an open circuit, I = current drawn from the battery and R = resistance of the circuit.
So in a circuit with resistance R, the more current drawn the greater the voltage drop.
Union Carbide Eveready 1.5 V (EP3) alkaline manganese cell has internal resistance, Ri = 0.18 ohm at 21oC.
Lead-acid batteries have an internal resistance, Ri, of about 0.02 ohms at 21oC.
1. Cells connected in the same direction in series each add their own voltage, emf, to the total voltage.
However, each cell has an internal resistance, Ri.
So if connecting three cells of voltage V1, V2 and V3, if current through each of the cells is I amps, then total voltage = [V1 +V2 + V3 - ( Rir1 + Rir2 + Rir3)].
Note that the internal resistance of this combination of cells is the sum of the internal resistances of the three cells.
Connect two dry cells or lead cell accumulators so that the negative terminal of one is in contact with the positive terminal of the other.
Connect them in series.
Put a light bulb in the circuit.
Close the circuit with one cell, two cells, three cells in series.
Record the changes in the brightness of the light bulb.
The brightness of the light depends on the number of cells connected in series.
When you connect cells in series, the total voltage is the sum of the individual voltages of the cells.
If you use 1.5 V cells, then two cells give 3 V, and three cells give 4.5 V, four cells give 6 V.
2. Total EMF of cells in series is the sum of each EMF = EMF1 + EMF2 + EMF3, e.g. 3 × 1.5 volt torch batteries in series produce 4.5 volts.
A group of similar cells is called a battery.
Cells in series:
If the EMF and internal resistance of each cell are e volts and r ohms respectively, and there are n cells in series, EMF of battery = ne volts and internal resistance of battery = nr ohms.
Cells in parallel:
If the EMF and internal resistance of each cell are e volts and r ohms respectively, and there are n cells in parallel, EMF of battery = e volts and internal resistance of battery = r / n ohms.
3. Connect two dry cells or lead cell accumulators so that the negative terminal of one is in contact with the positive terminal of the other.
They are connected in series.
Put a bulb in the circuit.
Close the circuit with one cell, two cells, three cells, in series.
Record the changes in the brightness of the lamp.
The brightness of the light depends on the number of cells connected in series.
When you connect cells in series, the total voltage is the sum of the individual voltages of the cells.
If you use 1.5 V cells, two cells give 3 volts, and three cells give 4.5 volts, four cells give 6 volts.
The current will change.
4. When three identical cells are connected in parallel the total voltage is as if for one cell.
However, the total resistance for one cell is 1/3 Rir.
For the three identical cells with internal resistance Ri1, Ri2 and Ri3, Ri1 = Rir2 = Ri3.
Let total internal resistance = Ri,
1/ Ri = 1/ Ri1 + 1/ Ri2 + 1/ Ri3,
1/ Ri = 3/ Ri1,
Rir = Ri1/3.
So total voltage = V -3I(1/3 Rir).
So motor car batteries may be connected in parallel to provide the extra current needed to start the engine.
Connect two or three fresh dry cells or lead cell accumulators so that you join their positive terminals and they join their negative terminals.
They are connected in parallel.
Set up a circuit on a circuit cardboard with three cells in parallel.
Disconnect one or two of the cells.
The circuit is not broken and the brightness of the light does not change.
The voltage drop in the circuit is the same if you use one, two or three cells.
The total current is unchanged.
If four cells in the circuit, the total current is 0.125 × 4 = 0.5 A.
5. Total EMF of identical cells in parallel is the same as for one cell,
e.g. 3 × 1.5 volt torch batteries in parallel produce 1.5 volts.
However, the effect of internal resistance is reduced, because total resistance = r / 3.
Total EMF = [EMF1- 3I( Ri / 3)].
6. Connect two or three fresh dry cells or lead cell accumulators so that their positive terminals are joined and their negative terminals are joined.
They are connected in parallel.
Set up a circuit on a circuit cardboard with three cells in parallel.
Disconnect one or two of the cells.
The circuit is not broken and the brightness of the light does not change.
The voltage drop in the circuit is the same if one, two or three cells are used.
The total current is unchanged.
If four cells in the circuit, the total current is 0.125 × 4 = 0.5 amps.
32.1.4 Circuit board
Use a piece of heavy cardboard measuring 30 × 30 cm as a base.
Fix clips on it to hold the cells, and sprung metal strips to provide connections between cells.
Screw brass curtain rod holders into the base.
Make spring connectors of varying lengths from curtain wire with hooks at each end.
Put light bulb holders into circuits with curtain wire connectors or heavy uninsulated copper wire.
Make other connections with lengths of uninsulated copper wire attached to crocodile clips.
32.1.5 Electric circuit board
Use a piece of heavy cardboard 30 x 30 cm as a base.
Fixed clips on it for holding the cells, and sprung metal strips for providing connections between cells.
Screw brass curtain rod holders for circuit making into the base.
Make spring connectors of varying lengths from curtain wire with hooks inserted at each end.
Put light bulb holders into circuits by using curtain wire connectors or heavy No. 16 uninsulated copper wire.
Make other connections with lengths of uninsulated copper wire attached to crocodile clips.
32.1.6 DC circuits, circuit diagrams, Kirchhoff's circuit laws
Electrical circuits enable electrical energy to be transferred efficiently over large distances and transformed into a range of other useful forms of energy including thermal and kinetic energy, and light.
An electric circuit is a complete conducting path around which the current can flow.
The EMF is the source of work per unit charge and is used up by the potential difference in the circuit to turn an electric powered device.
Circuit diagrams use a system of conventional signs.
Kirchhoff's circuit laws (Gustav Kirchhoff 1824 - 87, Germany)
Kirchhoff's laws state that the total current entering a junction in a circuit must equal the total current leaving it, and the sum of the potential drops around a circuit must be equal to the total EMF.
Law 1 (Junction law, Current law):
At any junction point in an electrical circuit, the sum of all currents entering the junction = the sum of all currents leaving the junction.
I = I1 + I2 + I3, where I = total current and I1, I2, I3 = separate currents.
Law 2 (Loop law, Voltage law):
For any closed loop in an electrical circuit, the sum of the voltages = zero.
V = V1 + V2 + V3, where V = total voltage and V1, V2, V3 = separate voltages.
Experiment
1. To connect a circuit first arrange the apparatus in the pattern shown in the circuit diagram.
Bare the ends of the connecting wires.
Connect the components with suitable lengths of wire.
Check that all connections are tight.
With large currents use thick connecting wires.
2. Volts / amps relationship for electrolytes, voltage / current relationship for gases
See diagram 32.2.54: Resistors in series and parallel.
Measure the voltage at different places in the circuit and show that the sum is zero.
There are many possible combinations of components.
3. Use three dry cells or 6 volt batteries or from a 12 volt battery.
By adjusting the rheostat a series of corresponding values of current and potential difference across the high resistance can be obtained.
Use both arithmetic and a graph to find the ratio potential difference / current.
4. Measure current and voltage in a simple circuit.
Change the voltage or resistance.
Connect an ammeter, voltmeter, rheostat and battery pack to show Ohm's law.
Place 2 V, 4 V, and 6 V across a resistor and measure the current then graph the results.
5. To observe resistance of a conductor using an ammeter and voltmeter, apply a potential difference to an electrical conductor and some current flows through it.
Ohm's Law states that, provided the conductor does not get hot, the current is proportional to the applied potential difference, so the ratio (PD applied to the conductor) / (current through the conductor), is a constant called the resistance, R of the conductor.
Connect the circuit as shown in the above diagram.
Close switch S.
Adjust the rheostat Rh so that a small current passes through the conductor of unknown resistance R ohm.
Record the current I amps and the potential difference V volts between the ends of R.
Adjust the rheostat Rh to get of five pairs of readings of current I amps and potential difference V volts.
Calculate R = V / I for each pair of readings.
32.1.9 Electric current, Ampère
(André-Marie Ampère, 1775-1836, France)
I (ampere) = Q (coulomb) / t (second) An ampere is the current which, if maintained in two parallel straight conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.
Proposed alternative definition: The ampere is such that the elementary charge is exactly 1.60 217 653 × 10-19 coulombs.
(1 coulomb = 1 ampere / second).
Abbreviations:
Ampere, A,
milliampere, mA, 1 mA = 0.001A,
microampere, µA, 1µA = 0.001mA = 10-6A,
nanoampere, nA, 1nA = 0.001mµA = 10-9A.
32.1.10 Electric shock
Electric shock:
> 1mA, tingling feeling
> 5mA, electric shock, but can let go hold of source of electricity,
10 mA, electric shock, let go threshold, cannot let go of source of electricity,
100-2000 mA, electric shock, abnormal heart beat, possible death,
>2000mA, electric shock, heat stops, certain death.
32.1.11 Electric light bulbs (lamps), resistors, in series and parallel
See diagram 32.157: Resistors in series.
See diagram 32.158: Resistors in parallel.
See diagram 4.62: Electric light bulbs in series and parallel.
If resistors with resistance R1, R2 and R3 are connected in series, they have the same current, I, passing through them.
Total resistance of the circuit = R1 +R2 + R3 ohms.
If resistors with resistance R1, R2 and R3 are connected in parallel, they have a common potential difference across them, V.
Total current through them is the sum of the separate currents = I1 + I2 + I3.
If total resistance is RT, then 1 / RT = I / R1 + I / R2 + I / R3.
So the total resistance will be less than the smallest resistance in parallel.
Experiment
1. Connect one, two and three identical light bulbs in series.
Record the brightness for each combination of the light bulbs.
2. Connect one, two and three light bulbs in parallel.
Record the brightness for each combination of the light bulbs.
If you connect six light bulbs in series in a circuit containing a 6 V battery, each light bulb receives 1 V.
If you connect six light bulbs in parallel in a circuit containing a 6 V battery, each light bulb receives 6 V.
When light bulbs are connected in series, the total voltage is divided between them.
For example, if three bulbs are connected in series to a 3 volt battery, each bulb receives 1 volt.
When light bulbs are connected in parallel, each light bulb receives the full voltage of the supply.
For example, if three bulbs are connected in parallel to a 3 volt battery, each light bulb receives 3 volts.
32.1.12 Electric torch (flashlight)
See diagram 4.57: Electric torch, flashlight.
A Glass screen in front to protect the light bulb, B Small incandescent light bulb (lamp) C Reflector, D Electric switch, E Batteries, F Cover that can be gripped in the hand and containing part of the electric circuit, G Spring to keep batteries tightly together, H Screw opening at the end for battery replacement
See diagram 32.154.1: Electric torch.
Take apart an electric torch to see the following different parts.
Note the directions of insertion of batteries.
The batteries must be inserted in series.
Note the rating on the side of the light bulb, e.g. 2.4 V, 0.5 A.
Larger light bulbs are rated in volts, V and watts, W, e.g. in Australia, 240 V 40 W.
Note the lamp type, fitting, e.g. screw or bayonet.
1. Trace the circuit in an electric torch.
Use a torch with metal sides and a torch battery opened with the back of an axe.
Take a torch to pieces and put it together again.
Show the class the torch, turn it on and off with the sliding switch.
Dismantle the torch.
Show the class the different parts: switch, metal case.
Use the arrows to show them how these parts are part of a circuit.
Show the children the opened battery.
The electricity comes from the zinc case when some zinc dissolves in the black chemical.
In the very old batteries, so much zinc dissolves that holes in the zinc case let some chemical leak out.
The carbon rod does not dissolve.
Why should the two batteries be in the same direction in the circuit?
[Otherwise they would push electric currents against each other.]
Take out the batteries in a radio.
Are they all in the same direction in the circuit? [Yes.]
The electrical strength of a battery is measured in volts.
How many volts in one battery? [1. 5 volts.]
If the batteries are put end to end in the circuit, how many batteries do you need for a total of six volts? [Four.]
2. Take apart an electric torch to see the following different parts.
Note the directions of insertion of batteries.
The batteries must be in series.
Note the rating on the side of the light bulb, e.g. 4.4 V, 0.5 A.
Larger light bulbs are rated in volts, V and watts, W, e.g. in Australia, 240 V 40 W.
Note the lamp type, fitting, e.g. screw or bayonet.
3. The flashlight is an electrical device that makes use of a switch, insulators and conductors, dry cells and a bulb.
Examine various kinds of flashlights and take them apart.
Connect the bulb to the dry cell without using the flashlight case.
Reassemble the flashlight.
Find the circuit in a flashlight and to determine where the circuit is completed and broken.
In metal flashlights, the case is part of the circuit.
In a two cell flashlight, the cells must be inserted so that the bottom of one cell touches the top of the other to provide the electrical circuit.
Place the cells in various positions to discover which way works best.
32.1.13 Lamps in parallel
See diagram 32.1.4.5: Lamps in parallel.
The lamp holder bases and the single pole switches should be fitted with 4 mm insulated terminals.
Connect the ammeter, the four lamp holders and switches to the mains supply and note the current as you switch on more lamps.
This shows that the rate of obtaining the output energy in joules / second is proportional to the rate at which coulombs pass, coulombs / second, as shown by the readings on the ammeter.
32.1.14 Lamps in series and parallel
See diagram 32.1.4.6: Household lamps in series and parallel.
See diagram 32.1.4.6a: Lamps on series and parallel boards.
1. Connect one, two and three identical bulbs in series.
Record the brightness of the bulbs.
When you connect bulbs in series, the total voltage is divided between them, e.g. if three bulbs are connected in series to a 3 volt battery, each bulb receives 1 volt.
Connect one, two and three bulbs in parallel.
Record the brightness of the bulbs.
When lamps are connected in parallel, each bulb receives the full voltage of the supply.
When lamps are on boards single lamps can be screwed out and disconnected to the circuit.
2. Make up two boards containing three 60 W household lamps, one board wired in series and the other board wired in parallel.
When plugged into the mains the series wired lamps will be dimmer than the parallel wired lamps.
If you substitute a 15-W lamp for one lamp in the series board, the other two lamps are dimmed.
If you substitute a 15 W lamp for one lamp in the parallel board, the other two lamps are not dimmed.
So parallel wiring is used in household electrical circuits for lighting.
32.1.15 Resistors in parallel
Resistors in parallel have a common potential difference across them, and the total current through them is the sum of the separate currents.
Total current of 3 resistors in parallel = I1 + I2 + I3.
Total resistance of 3 resistors in parallel = 1 / R1 + 1 / R2+ 1 / R3
So total resistance of 3 resistors in parallel < smallest resistance in parallel.
Kirchhoff's laws state that the total current entering a junction in a circuit must equal the total current leaving it, and the sum of the potential drops around a circuit must be equal to the total EMF.
To calculate the total resistance of three or more resistors in parallel, convert all resistance values to ohms, calculate the reciprocal of each resistance value to give conductance in siemans, add the individual conductances to get the net conductance, calculate the reciprocal of the net conductance to get the net resistance in ohms.
32.1.16 Resistors in series
Resistors in series have the same current flowing through them.
The total potential difference across them is the sum of the separate potential differences.
Total potential difference of 3 resistors in series = V1 + V2 + V3
Total resistance of 3 resistors in series = R1 + R2 + R3
32.1.17 Resistors in series and parallel
See diagram 4.157 Resistors in series.
See diagram 4.158: Resistors in parallel.
See diagram 32.2.2.1: Resistors in series and parallel.
For resistors in series R = R1 + R2.
For resistors in parallel 1 / R = 1 / R1 + 1 / R2.
Resistors in series:
Connect two resistors in series, e.g. R1, 2 ohms and R2, 4 ohms, with combined resistance, R = 6 ohms.
Adjust the rheostat Rh to get of five pairs of readings of current I amps and potential difference V volts.
Calculate R = V / I for each pair of readings.
Resistors in parallel:
(The ammeter should read about 6 amps.)
Adjust the rheostat Rh to get of five pairs of readings of current I amps and potential difference V volts.
Calculate R = V / I for each pair of readings.
1. Connect one, two and three identical bulbs in series.
Record the brightness of the bulbs.
When bulbs are connected in series, the total voltage is divided between them.
If three bulbs are connected in series to a 3 volt battery, each bulb receives 1 volt.
Connect one, two, and three bulbs in parallel.
Record the brightness of the bulbs.
When lamps are connected in parallel, each bulb receives the full voltage of the supply.
32.1.18 Series and parallel (branching) circuits
See diagram 32.1.4.4: Branching circuits.
In a series circuit, the current is the same in all parts of the circuit.
In a branching or parallel circuit, the total current = the sum of currents in the branches.
Experiment
1. For a series circuit, adjust the current to 0.4 amps with the rheostat.
Can you include a fourth ammeter between two of the cells in the battery?
2. With a 12 volt battery and two 12 volt watt lamps, A1 reads 0.5 amps.
Adjust R3 so that A3 reads 0.3 amps.
Adjust R2 so that A2 reads 0.2 amps.
Now A4 reads 0.5 amps (same as A1), A5 reads 0.7 amps (A2 + A4), and A6 reads 1.0 amps (A3 + A5).
32.1.19 Simple electric circuit
Connect an electric bulb, e.g. 2.4V, 0.5A, and lamp holder, to the +ve and -ve terminals of a dry cell or lead cell accumulator or low voltage power supply.
Notice the filament made of tungsten carbide.
Passage of the electric current through the tungsten carbide wire causes it to become very hot and give off light.
Reverse the connections to the source of electricity and the lamp still operates although the electricity is flowing in the opposite direction.
Draw a diagram to show the path of the current through the bulb and around to the other end of the cell.
This is a simple electric circuit.
Circuit diagrams are used to represent the electrical components in a circuit.
32.1.20 Simple switch
See diagram 4.55: Simple switch.
Fasten the end of a piece of wire to a pencil with two rubber bands.
A second wire makes a connection.
32.1.21 Switches in a circuit, tapping key
See diagram 32.152: Simple switch.
1. Make a simple switch.
Fasten the end of a piece of wire to a pencil with two rubber bands.
A second wire makes a connection.
2. Insert switches in a circuit.
Put a knife switch in a circuit with a dry cell and a light bulb.
Turn the light on and off by operating the switch.
Replace the light bulb with a bell or buzzer and operate the switch.
Replace the knife switch with a push button switch.
Examine the construction of different switches, e.g. household tumbler switch, rocker switch.
Use them in a circuit.
3. Collect materials to be tested for electrical conductivity, and to suggest answers to this question.
Try paper, eraser, plastic button, key, coins, cloth, string, chalk, glass, nail, nail file, insulated wire, bare wire.
Test these in a circuit across an open knife switch, or in a tester made as shown in the diagram.
Materials that carry electricity are called conductors.
Materials that do not carry electricity are non-conductors (insulators).
The copper of a wire is a conductor; its covering is an insulator.
32.1.22 Switches in a circuit, knife switch
Put a knife switch in a circuit with a dry cell and a light bulb.
Turn the light on and off by operating the switch.
Replace the light bulb with a bell or buzzer and operate the switch.
Replace the knife switch with a push button switch.
Examine the construction of different switches, e.g. household tumbler switch, rocker switch.
32.1.24 Liquids that conduct electricity
1. Test melted substances.
If you heat the following substances, heat very gently and cautiously, because they may ignite and burn: sulfur, wax, naphthalene, polyethylene, a low melting point salt, e.g. lead bromide, mp 488oC, or potassium iodide, mp 682oC.
To test the conductivity of the melt, dip the electrodes in the melt and wait for the electrodes to reach the same temperature as the melt.
Make sure that the electrodes are in contact with the liquid melt and not the solidified melt.
Scrape and clean the electrodes between each test.
2. Test methylated spirit, acetone, vinegar, sugar solution, copper (II) sulfate solution, sodium chloride solution, and other substances dissolved in water.
Clean and dry the electrodes between each test.
3. Test demineralized water.
Put the electrodes into a container of distilled water.
The light bulb does not light.
Slowly add small crystals of sodium chloride to the demineralized water.
Observe the light bulb as the salt dissolves.
4. Test tap water.
Note whether you get the same result as for distilled water.
32.2.1 Conductivity of electrolytes
Classify substances into the following groups:
* Substances that conduct electricity in the solid state.
* Substances that conduct in the liquid state.
* Substances that conduct electricity when dissolved in water.
Solids with ionic bonds have high melting points and low electrical conductivity.
However, when molten or in solution their electrical conductivity is high.
Solids with covalent bonds within the molecules with van der Waals forces or weak dipole to dipole intermolecular forces have low melting points and no electrical conductivity.
The SI unit of conductivity is siemens per metre, S/m.
Siemens is the SI derived unit of electric conductance, conductivity, S or Ω-1.
1. Dip two metal electrodes in series with a light bulb in various solutions of electrolytes.
Immerse two copper plates in series with a lamp in deionized water, then add barium hydroxide, then add sulfuric acid.
Put two copper plates in series with a lamp in deionized water and add salt or acid.
Dip two electrodes in series with a 110 V lamp into deionized water, salt water, sugar solution, vinegar solution and tap water.
2. Volts and amps relations for electrolytes
Use a 12-volt batteries must allow tapping off intermediate voltages.
Connect the copper voltmeter in a series circuit.
Find the voltage / current relationship by connecting 1, then 2, 3, 4, 5, 6 cells from a 12-volt battery across the voltmeter.
Draw a graph of voltage against current.
The technique of changing the number of cells without introducing a rheostat is essential to avoid difficulties with polarization.
Use the same procedure for the gas voltmeter.
With the copper voltmeter a rheostat could be used.
Use 4 mm sockets.
32.2.2 Conductors and non-conductors of electricity
See diagram: 4.155: Testing for conductivity.
1. Use a dry cell, switch, lamp, wire, two crocodile clips, battery box and lamp socket to connect in series a simple DC circuit.
The lamp will show if there is electric current flowing through the circuit.
Note if the lamp lights.
Do not let the two crocodile clips touch.
Connect two ends of a wool thread, 50 cm long, folded repeatedly and twisted together.
Put the wool thread connected by crocodile clips into water.
Put the wool thread connected by crocodile clips into thick salt water.
2. Use a simple electric circuit to test whether different substance conduct electricity.
Use paper, rubber eraser, plastic, key, coin, cloth, string, chalk, glass, pin, nail file, insulated wire, bare wire, finger, water.
Test these in a circuit across an open knife switch.
Materials that carry are called electricity conductors.
Materials that do not carry electricity are called non-conductors or insulators.
The copper core of bell wire is a conductor.
Its covering is an insulator.
3. Use six volts direct current, a low power bulb and electrodes made of carbon or steel, and mounted in a cork to keep them at a constant distance apart.
Use the carbon centres of old six volt dry cells as electrodes.
Test the conductivity of solids by making a good contact between the surface of the solid and the two electrodes.
The surface of the solid must first be cleaned.
All metals conduct electricity.
Carbon conducts electricity.
Test whether non-metallic solids conduct electricity: plastics, naphthalene (moth balls), wax, sugar, sodium chloride, sulfur.
4. Use a simple electric circuit to test whether different substances conduct electricity, e.g. paper, rubber eraser, plastic, key, coin, cloth, string, chalk, glass, pin, nail file, insulated wire, bare wire, finger, water.
Test these in a circuit across an open knife switch.
Materials that carry electricity are electrical conductors.
Materials that do not carry electricity are non-conductors, insulators.
The copper core of bell wire is a conductor, but its plastic covering is an insulator.
5. Use a dry cell, switch, lamp, wire, two crocodile clips, battery box and lamp socket to connect in series a simple DC circuit.
The lamp will show if there is electric current flowing through the circuit.
Note if the lamp lights.
Do not let the two crocodile clips touch.
Connect two ends of a wool thread, 50 cm long, folded repeatedly and twisted together.
Put the wool thread connected by crocodile clips into water.
Put the wool thread connected by crocodile clips into thick salt water.
6. Use six volts direct current, a low power bulb and electrodes made of carbon or steel, and mounted in a cork to keep them at a constant distance apart.
Use the carbon centres of old six volt dry cells as electrodes.
Test the conductivity of solids by making a good contact between the surface of the solid and two electrodes.
The surface of the solid must first be cleaned.
All metals conduct electricity and carbon, a non-metal, conducts electricity.
Test whether non-metallic solids conduct electricity, e.g. plastics, naphthalene, (moth balls), wax, sugar, sodium chloride and sulfur.
32.2.3 Conductivity of solutions
Test ethanol or methylated spirit, acetone, vinegar, sugar solution, copper (II) sulfate solution, sodium chloride solution, and other substances dissolved in water.
Clean and dry the electrodes between each test.
32.2.4 Conductivity of water, deionized water, distilled water, tap water
Put the electrodes into a beaker of deionized water.
Students find that the bulb does not light up and therefore pure water does not conduct.
Slowly stir small crystals of common salt into the water.
Note what happens to the bulb as the salt dissolves.
Repeat the experiment by testing distilled water and tap water.
In experiments to find the conductivity of solutions, "conductivity water" is used as a standard, because it is water of high purity and very low electrical conductivity.
32.2.5 Conductivity of saltwater string, electrolytic conduction on chamois
Suspend a chamois between ring stands to show no conduction with a battery resistor meter then soak in deionized water repeat, then sprinkle on salt and repeat again.
32.2.6 Conductance, electrical conductivity
Conductance, electrical conductivity, electric current conductance = 1 / electrical resistivity, 1 /R, unit siemans, S
Conductance, electrical conductivity, G or σ or mho (ohm backwards) = amps / volts
Electrical conductivity SI unit is siemens per metre (S / m).
32.2.7 Electrical conductivity
Conductance or conductivity or is the ratio of the current flowing through a conductor to the potential difference between its ends, i.e. the electric field causing the current to flow.
Conductance or conductivity is the reciprocal of resistance or resistivity.
The SI unit for conductance is the "siemens", S.
The SI unit for its reciprocal is ohms (ω).
Pure substances that are gases or liquids at room temperature are not good conductors, e.g. water, alcohol, and olive oil., but the liquid metal, mercury, is an exception.
Fused solids vary in their conductivity.
Molten metals, alkalis and salts are good conductors.
Other fused solids are not good conductors.
The salts sodium chloride and sodium nitrate, as fused liquids, are good conductors, but fused sugar and sulfur are non-conductors.
Experiment
Use a 6 volt battery and two crocodile clips to grasp the cleaned surface of different substances.
Put a light bulb in the circuit to show when current is flowing.
Record the solids, melted solids, liquids, and aqueous solutions that do or do not conduct electricity.
32.2.8 Electrical conductivity of liquids
See diagram 3.59: Electrical conductivity apparatus.
Pure substances that are gases or liquids at room temperature are not good conductors, but the liquid metal mercury is a good conductor.
1. Clean and dry the carbon electrodes between each test.
To test the conductivity of liquids, immerse the ends of carbon electrodes 3 mm apart in acetone, copper (II) sulfate solution, methylated spirits, liquid paraffin, olive oil, peanut oil, sodium chloride solution, sugar solution, turpentine (mineral turps), vinegar.
2. Test the conductivity of solutions, e.g. 2 M concentration of the following:
* strong electrolytes, e.g. copper (II) sulfate, hydrochloric acid, potassium hydroxide, sodium chloride, sodium hydroxide, sodium nitrate, sulfuric acid.
*2 weak electrolytes: ammonia solution, benzoic acid, ethanoic acid (acetic acid).
Always wash the electrodes thoroughly after testing each solution.
Solutions of acids alkalis and metallic salts are generally good conductors.
Solutions of sugar and alcohol are non-conductors.
Solutions of other types of substances in water and in other liquids are generally non-conductors.
3. Test demineralized water for conductivity.
The bulb does not light.
Very gradually stir small crystals of sodium chloride into the water.
Note any light from the light bulb as the salt dissolves.
Similarly, test distilled water, tap water and mineral water.
32.2.9 Electrical conductivity of different solutions
1. Make a standard electrical conduction apparatus.
Connect two carbon rods from used torch batteries to a light bulb and a source of direct current.
Dip the carbon rods into the following solutions and record the brightness of the light bulb.
Rinse the carbon rods with deionized water after each test.
The brightness of the bulb is a measure of the conductivity of the solution.
2. Make more accurate measurements of conduction with an ammeter or a galvanometer.
Record which solutions are good, fair, poor conductors, or are not conductors.
Test demineralized water, deionized water, mineral water, and tap water.
Test 0.2 M solutions of: aqueous ammonia, copper (II) sulfate, ethanoic acid (acetic acid) potassium hydroxide, sodium chloride, sodium hydroxide, and sucrose.
Aqueous ammonia solution, ethanoic acid (acetic acid) and sucrose solutions are poor conductors.
3. Use a battery-powered conductivity device.
The difference between weak conducting solutions, e.g. distilled water, sucrose solutions, and moderately to strong conducting solutions, e.g. sodium chloride solution, copper chloride solution, is shown by the intensity of light emitted by bulb that is directly related to the conductivity of the solution.
32.2.10 Electrical conductivity of different concentrations of solutions
Use different concentrations of acids, bases and salts and compare the conductivity.
More dilute solutions are better conductors.
Test the conductivity of glacial ethanoic acid (acetic acid).
Test again after adding different amounts of water.
32.2.11 Superconductors
An electric current can pass through a superconductor with almost no resistance.
Aluminium, mercury, lead, zinc cooled close to close to 0 Kelvin with liquid nitrogen have superconductor properties.
Some ceramics, may be superconductors at higher temperatures.
The critical temperature, Tc, for superconductors is the temperature at which the electrical resistivity of a metal suddenly drops to zero:
Gallium 1.1K, Aluminium 1.2 K, Indium 3.4 K, Tin 3.7 K, Mercury 4.2 K, Lead 7.2 K, Niobium 9.3 K.
Liquid nitrogen, 77 K, can be used to maintain a superconducting state in some materials.
32.2.12 Bite on aluminium foil
If you bite on a piece of aluminium cooking foil or a lump of foil from a packet of chocolate you may feel a sudden pain, especially if you bite on the foil with your molars that have a large amalgam tooth filling.
The pain is caused by current flowing between the foil and the metal amalgam through the saliva containing salts to stimulate the dental nerve ending in the tooth.
If you have perfect teeth with no tooth filling the experiment does not work!
32.2.13 Conduction from hot wire
Constantan, constant resistance wire, Eureka wire, CrAl (Kranthal) and NiCrm
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.
Resistance hardly changes with change in temperature.
It is used for resistance wire.
Also Eureka wire, CrAl (Kranthal) and NiCrm (Nichrome wire) have very high resistivity.
An optical pyrometer is used to measure very high temperatures from the colour of the radiant heat source.
Experiment
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.
2. Hold a constantan wire near a charged electroscope to cause discharge when it is heated red-hot.
32.2.14 Conduction of gaseous ions, electroscope
A nearby flame will discharge an electroscope.
Insert a flame connected to a high voltage source between charged parallel plates.
Use compressed air to blow ions from a flame through the area between charged parallel plates onto a mesh hooked to an electrometer.
Connect electrodes at the bottom, middle and top of a tube to an electrometer while a Bunsen flame burns at the bottom.
32.2.15 Conductors and insulators
1. Use the ends of two conducting wires in the circuit, or use two 4 mm plugs, to act as probes.
Test the material by noting whether the light bulb lights.
Also note whether the brightness is the same, for different materials,
for different distances between the probes on the same material.
Select common materials: string, live plant, plastic ruler, pencil, rubber, fork, knife, pipe, paper, soil, brick, bread, clothes, deionized water, tap water, milk.
After testing a liquid, wash and dry the probes.
2. Test a clean dry matchstick.
The light bulb does not light.
The matchstick is an insulator.
Soak matchsticks in water, vinegar and sodium chloride solution.
When the matchstick is soaked, the light bulb will light so this piece of match will become to a conductor.
3. Test a burnt matchstick.
If the light bulb is not light, shorten the distance between the probes or increase the voltage of the circuit.
The light bulb lights.
32.2.16 Conductivity of melted substances
Melt the following substances, but heat very gently, because they may ignite: sulfur, wax, naphthalene (moth balls), polyethylene material, tin, lead and, a low melting point salt such as lead bromide, mp 488oC, or potassium iodide, mp 682oC.
Test the conductivity of the melt by dipping in the electrodes and wait for the electrodes to reach the same temperature.
This ensures that the electrodes are in contact with the liquid and not the solidified melt.
Scrape and clean the electrodes between each test.
32.2.17 Current electricity from frog's legs
Put one wire on the body of a dead frog and with the other, probe the frog near the pit of its stomach.
You can make the legs twitch.
The electric current from the wires set up electric currents in the nerves of the frog and these currents run along the nerves to the muscles causing them to contract and move just as if the current was coming along the nerve of a live frog.
This experiment seems to have nothing to do with physics.
However, Luigi Galvani, (1737-1798), was one day cutting the legs off dead frogs to make soup.
To dry the legs, he hung them on an iron fence using copper hooks.
He noticed that the dead frog legs started shaking when the toes of the legs touched the iron fence.
He concluded that "animal electricity" was in the muscles of the frog legs.
Later, Alessandro Volta, (1745-1827), repeated the observation as an experiment and concluded that the electricity came from the copper and the iron.
He experimented with other metals and dipped pieces of copper and zinc into a container of salt solution.
To get more electricity he made a pile of these containers and so invented the voltaic pile, a battery.
Although Luigi Galvani was wrong about the frogs' legs, we still use the terms galvanized, galvanometer and even galvanic.
However, Alessandro Volta is better remembered as the inventor of the volt.
32.2.18 Current balance
A current balance measure the force of repulsion between two straight, parallel wires.
One wire is fixed and the other wire can move to allow measurement of the gravitational force that balances the magnetic force.
This force is proportional to the product of the currents in the two conductors, and depends on the permeability of free space.
It can be use to measure the forces between electric fields and a current-carrying wire.
32.2.19 Electric current detector
See diagram 31.66.1: Compass in a coil.
See diagram 31.66.2: Compass in a match box.
Wrap 50 to 60 turns of bell wire to form a coil around a container 8 cm in diameter.
Remove the coil from the container 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 note the deflexion of the compass needle.
Reverse the connections, and note the deflexion of the compass needle again.
Make a more sensitive instrument by putting a compass in the tray of a match box then winding the coil wire over the box.
32.2.20 Electrical conductivity of melted solids, fused solids
Be careful! Do not let the carbon electrodes ignite and burn.
Grip two carbon electrodes from used dry cell batteries with the crocodile clips.
Test the conductivity of the melt by dipping in the electrodes.
Wait for the electrodes to reach the same temperature.
This ensures that the electrodes are in contact with the liquid and not the solidified melt.
Scrape and clean the electrodes between each test.
1. Melt substances that are solids at room temperature, but heat very gently, otherwise they may ignite and burn, e.g. candle wax, cellulose acetate (acetate rayon) lead metal, lead bromide, naphthalene, nylon, octadecanoic acid (stearic acid) polyethylene, polythene, Perspex, potassium iodide (mp 682oC) sodium chloride, sodium nitrate, solder, sulfur, tin metal.
Melted solids vary in their conductivity.
Only molten metals, alkalis and salts are good conductors.
Sugar and sulfur are non-conductors.
2. Glass can be a conductor.
Heat a glass rod until it becomes very hot and begins to soften.
Test the hot, soft part with the conductivity apparatus.
When molten, glass is a good conductor of electricity.
32.2.21 Electrical conductivity of solids
See diagram 3.59: Electrical conductivity apparatus.
Use two carbon electrodes from torch batteries, a non-conducting support for the electrodes, crocodile clips or crunched aluminium foil for connections, light bulbs to show when current flows, and a 6 V dry cell power source.
Test the conductivity of solids by making a good contact between the cleaned surface of the solid and the two electrodes.
Confirm that metals and carbon conduct electricity.
Test the conductivity of non-metallic and crystals, e.g. calcite (crystalline calcium carbonate) candle wax, copper (II) sulfate-5-water, ethanedioic acid-2-water (oxalic acid) glass rod, naphthalene, plastics, octadecanoic acid, sucrose (cane sugar), sodium chloride crystals, sodium nitrate, sugar crystals, sulfur, wax.
None of these solid compounds is a good conductor.
32.2.22 High temperature and conductivity of sodium chloride and paraffin wax
1. Place a small amount of salt in the bottom of the crucible.
Support two stiff copper wires so that they reach the crucible and make electrical contact with the salt.
Close the switch.
The solid sodium chloride does not conduct electricity.
Remove the electrodes from the salt and heat the crucible strongly until the salt melts.
Replace the electrodes and adjust the rheostat to current of 1 amp.
Remove the burner and let the salt cool.
The current rapidly falls to zero.
Repeat the experiment using paraffin wax.
The paraffin wax fails to pass a current when melted.
2. To make a liquid rheostat attach leads to the carbon rods from two dry cells.
Dip the ends of these carbon electrodes in a dilute sodium chloride solution.
Put a switch, a torch globe and 1.5 volt battery in the circuit.
Close the switch and adjust the distance between the carbon electrodes or add more sodium chloride until the torch globe glows.
Changing the distance between the carbon electrodes changes the strength of the current in the circuit just like a rheostat.
Instead of using carbon electrodes, attach leads to two metal milk bottle tops floating in the sodium chloride solution in a Pyrex dish or earthenware dish.
Do not use a metal dish.
32.2.23 "Lead" pencil conductor
In a "lead pencil", the "lead", or "blacklead", is graphite (plumbago) + some iron, but no lead.
When graphite is put under stress, e.g. press down on a "lead" pencil, weak van der Waals forces break, leaving layers of graphite on the writing page to mark the paper.
Connect a flashlight bulb with a battery by means of a pair of scissors and a pencil, so the bulb lights up.
Current then flows through the graphite in the "lead" shaft of the pencil to the positive pole of the battery.
32.2.24 Solids that conduct electricity
Use a 6 V dry cell or lead cell accumulator and a 1.5 V light bulb.
Fix electrodes from old 6 V dry cells in a cork to keep them at a constant distance apart.
Test the conductivity of solids by making a good contact between the surfaces of the test solid and the two electrodes.
Test metals and non-metals, e.g. scissors, nails, plastic, paper, naphthalene, wax, sugar, sodium chloride, and water.
Record which substances are conductors and non-conductors, insulators.
Test conductivity of glass.
Test the conductivity of a glass rod at room temperature.
Heat the glass rod until it becomes very hot and begins to soften.
Test the hot soft part with the conductivity apparatus.
Molten glass can be a good conductor of electricity.
32.2.25 Substances that conduct electricity
See diagram 3.59.1: Substances that conduct electricity.
An electrode conducts electricity into or out of an electrolyte or gas.
An electrolyte conducts electricity as a solution, e.g. in a battery, or when molten, melted.
Insert two carbon rods each through a 13 mm one-hole stopper.
Bind the two stoppers together so that the wider end of one stopper (up) is next to the narrower end of the other stopper (up).
The carbon rods in the stopper should be 5 mL apart.
Use crocodile clips to attach a conducting wire between one battery terminals and the bulb and the other battery terminal and one of the carbon rods.
The bulb should light when a current passes so lightly touch both carbon rods with copper wire to make the bulb light and show that the circuit is works.
Prepare separate beakers of sugar, sodium carbonate, sodium chloride and laundry starch.
Dip in the carbon rods in each beaker and record whether the bulb lights up to show that the solution is conducting electricity between the carbon rods.
Add water to each beaker.
Dip in the carbon rods in each solution beaker and record whether the bulb lights.
Wash the rods thoroughly under the tap after dipping in each solution.
Note any signs of chemical reaction in the beaker.
None of the original solid substances conduct electricity.
Sodium carbonate solution and sodium chloride solution conduct electricity.
These solutions are electrolytes.
Solutions of sugar, starch and methylated spirit do not conduct electricity.
They are non-electrolytes.
Repeat the experiment by testing dilute hydrochloric acid and dilute sodium hydroxide.
Acids, salts, and alkalis are electrolytes.
When dissolved in water to form solutions or melted into liquids by heating, they conduct electricity.
Electrolytes are usually decomposed when electric current passes through them, electrolysis.
In electrolysis, the carbon rod (electrode) connected to the negative (-) terminal of the battery is the cathode, and the electrode connected to the positive (+) terminal is the anode.
Gases from the decomposition of electrolytes may be seen as bubbles on the electrodes.
32.2.26 Test electricity with the tongue
1. Touch two wires from a 1.5 volt battery with the tip of your tongue.
Do not let the wires touch each other.
You can feel or "taste" something.
The electric current has set up a current in the nerve cells of your tongue and these are carried to the brain causing the sensation you feel.
This is an old method of testing whether there was still any "juice" in the battery.
Do not try it with a car battery or mains!
2. Some people routinely test a 9 volt battery by licking the terminals and report that a near flat battery has almost no taste, but a full battery has a sour, acid taste.
Some people report a slight jolt and metallic taste.
This test is not recommended in this publication and note that the battery must be clean and without any signs of damage or degradation.
Sir William Thompson, Lord Kelvin (1824-1907), reported: "I measured the current that circulates when you touch a 9-V battery with your tongue.
It begins at about 3 mA and increases rapidly to 4.6-4.8 mA.
After a few seconds the sensation goes from displeasing to painful."
3. To test whether an electric fence is working, wear thick-soled, dry, rubber boots and thick, dry socks.
Hold about one centimetre of dry grass of small cross-section near the fence wire and note the strength of the electric shock.
The voltage from an electric fence is intermittent.
Wet skin has much less electrical resistance than dry skin, so never grasp the electric fence with the bare hand.
32.2.27 Test materials for conductivity
Connect a torch globe and two torch cells with metal wire, leaving a break AB in the wire.
Connect A to B with the material to be tested.
If the material is a conductor, the lamp will glow.
If the material is an insulator, it will not glow.
Test the following substances: 1. metals, e.g. iron, brass, aluminium, copper, 2. plastics, e.g. PVC, 3. sulfur, 4. rubber, 5. wood, 6. graphite, 7. glass, 8. cork, 9. textile fibres.
32.2.28 TASER
A taser (conducted electrical weapon, used by police), steps the voltage up to around 10, 000 V, but the high voltage is oscillated at a frequency that is not supposed to affect life functions, only localized muscle.
(TASER: "Thomas A. Swift's electronic rifle", named after Thomas Swift, hero of US boy's book series.)
32.3.1 Fuse box
See diagram 32.159: Fuses.
See diagram 32.160: Household fuse.
See diagram 32.161: Fuse with increasing load.
See diagram 32.163.3: Miniature fuses used in electronics, audio.
A fuse is a safety device that protects electrical appliances by preventing too much electricity flowing into them.
The fuse is a thin wire of easily melted metal as part of an electric circuit inside a protective case.
If the flow of electricity becomes too powerful, the wire melts and stops the current flowing and so it interrupts the circuit.
The fuse wire is a length of wire with a given current rating at which the wire would melt if that current is exceeded.
A fuse is a wire that melts at a certain temperature and so breaks the circuit preventing damage to other components of the circuit caused by excessive current.
The choice of fuse is restricted by the electrical source and conducting wire used in the circuit.
In the installed circuit, the allowable current is fixed, so it is very dangerous to use a large capacity fuse that allows more than the allowable current to pass.
Any device that opens a circuit, because of abnormal electric current is called a circuit breaker.
A fuse wire will eventually fail when the load on the circuit is increased.
Use mains-operated circuit breakers (MOCBs) instead of fuses to eliminate the possible use of inappropriate fuse wire.
Be aware, MOCBs do not act in the same way as safety switches and should not be confused with them.
Experiment
1. Check the fuse box.
Know where the fuse box for your premises is and know how to turn off the power supply in case of an emergency.
Check that each switch, circuit breaker and fuse is correctly labelled and call a licensed electrical contractor if there is any confusion.
Use only the correct size fuse wire to rewire fuses.
The fuse box is the equivalent of the circuit breaker's electrical service panel in that it is a metal box with a hinged cover that houses and controls the incoming electrical service and distribution to branch circuits within the house.
It provides overcurrent protection through the use of fuses.
The fuse box will have threaded sockets into which the fuses will be screwed.
These large threaded sockets look like light bulb sockets and are called Edison sockets, named after Thomas Edison who invented them.
There are several types of fuses that go into these sockets.
Some fuses have Edison bases and some fuses have a socket adapter that screws into the Edison base, but the fuse itself screws into the adapter base.
The latter are called "S" fuses and are also called "tamper-proof" fuses with rejection bases.
2. Open the fuse box at your school or home.
Note the different kinds of fuses, how to trip a fuse and how to replace the fuse wire.
A fuse box should contain spare fuse wire.
When you use several appliances simultaneously, the wires carrying the current may become overheated and cause a fire.
Be careful! Putting a coin behind a fuse to allow more current to flow is a very dangerous practice.
Use the correct fuse wire.
A 30A fuse in a circuit designed for a 15A fuse is unsafe.
Do not repair a fuse in a fuse box by wrapping a burnt out fuse with a metal foil gum wrapper, where the metal in the shiny part of the gum wrapper acts as a replacement conduit for the burned out fuse.
It may not burn out under excess current as a proper fuse does and result in a house fire.
Do not use any wire, pins, staples or hair pins to replace burnt out fuses.
Always replace burnt out fuses with the correct fuse wire.
3. Examine normal and burnt out fuses.
See diagram 32.163: How a fuse works.
Use fuses to protect electric circuits against overloading.
The fuse wire melts and breaks the circuit when an unsafe amount of current is flowing.
Use a thin strip, no more than 0.5 mm wide, of metal foil cut from a chocolate wrapper or a thread of steel wool.
Fasten it between the ends of two wires projecting through a cork.
Pass electric current through the fuse until the fuse wire melts and breaks.
A short circuit is the deviation of a current from the planned path along a path of less resistance.
However, this excess current can be stopped if a suitable fuse exists in the circuit.
4. Insert a fuse.
See diagram 32.164: Circuit with fuse and light bulbs.
Place a fuse in a circuit with three lamps in parallel.
Use a crocodile clip to short circuit the lamps.
If the fuse does not melt, cut a thinner strip of foil.
Experiment with different kinds and widths of foil until the foil carries the current when connected properly, but melts when a "short" occurs in the circuit.
Then replace the fuse and add more lamps in parallel until the fuse burns out.
32.3.2 Power surge circuit breaker, fuses
Circuit breaker. A "spike" or power surge can move through any of the three electrical mains connections, i.e. active, neutral and earth, to damage electrical equipment.
However, a circuit breaker can shut off power in the event of overloading across the three connections with built-in devices to absorb the spikes and protect the equipment, e.g. computer, domestic equipment.
A circuit breaker can be part of a multi-outlet power board.
Example specifications for a power board used in Australia are as follows:
Input: 240 volt, 50 hertz, Maximum 10 amps,
Surge capacity: To 4500 amps,
Maximum continual voltage: 275 volt,
Reaction time: < 25 nanoseconds,
Clamping voltage: 750 volt, 50 amps.
(The maximum voltage the surge protector will allow to pass through it before it suppresses the power surge and blocks any further current from flowing into a computer or domestic equipment.)
Energy absorption factor: 75 joules.
4. Insert two identical bare copper rods, 1.5 mm diameter and 30 mm length into a cork of a thermos bottle and use the residual 15 mm to 20 mm long part of each of them out of the cork to make a stand for fuse installation.
Use the twist method to install the fuses on the tops of the rods, and then connect the conducting wires to the ends of the rods.
Use four dry cells, several 6 V and 3 A bulbs, connecting wire with 0.4 mm diameter, a 0.25 A, 0.5 A and 1.0 amp fuse.
The surface lacquer of the lead connecting with the binding posts must be cleaned off with a knife.
Switch off the electric switch K before operation and switch on after operation.
Connect the 0.25 A fuse and a bulb in the circuit.
After several minutes, use the back of hand to feel the temperature of the lead.
The limiting current of the 0.25 A fuse is 0.5 A, so the fuse can allow the working current of the bulb, 0.3 A.
Observe the condition that the fuse is burned out, when you two bulbs in parallel.
Substitute the 0.25 A fuse by a 0.5 A fuse whose limiting current is 1 A.
Connect three bulbs in parallel, close the circuit then check if the lead is very hot.
Then connect four or even more bulbs in parallel until the 0.5 A fuse is burned out.
5. Use a 1 A fuse to keep all the four bulbs light.
Note the temperature of the lead and peculiar smell emitted.
Examine normal and burnt out fuses.
Use fuses to protect electric circuits against overloading.
The fuse wire melts and breaks the circuit when an unsafe amount of current is flowing.
Use a thin strip, no more than 0.5 mm wide, of metal foil cut from a chocolate wrapper or a thread of steel wool.
Fasten it between the ends of two wires projecting through a cork.
Pass electric current through the fuse until the fuse wire melts and breaks.
6. Place a model fuse in a circuit in series with three cells and a lamp.
Use a crocodile clip to short circuit the lamp.
If the fuse does not melt, cut a thinner strip of foil.
Experiment with different kinds and widths of foil until the foil carries the current when connected properly, but melts when a "short" occurs in the circuit.
Then replace the fuse and add more lamps in parallel until the fuse burns out.
Open the fuse box at your school or home.
Note the different kinds of fuses, how to "trip" a fuse, and to replace the fuse wire.
7. Short a low voltage high current transformer with zinc coated iron wire then vaporize wire with 500 amp surge.
Use fuse wire in a miniature house circuit, S.33 fuse wire and 8 Eh-5 fuses.
Connect fuse wires of different sizes across a heavy copper buss then determine which of the fuse wires of different diameters connected in parallel will burn out first.
Two resistance wires substituting for house wiring glow when they power a load of lamps and heaters.
Copper and nichrome wires in series show different amounts of heating due to current, and a paper rider on the nichrome wire burns.
8. The fuses in the mains supply are usually 5 or 10 amperes.
Connect a 60 watt lamp to the 3-pin socket and turn on the mains power.
The 2 ampere fuse will sustain this load since the current is about 0.25 A.
Replace the lamp with a 1 000 W radiator.
The fuse melts, because of the current overload.
Short the 3-pin socket with a piece of thick bent wire.
When the current is turned on the fuse will melt immediately without harm to the mains fuse.
Use a piece of 5 amp fuse wire connected in series in a circuit with a car headlamp operated from a 6 volt storage battery.
The fuse should not "blow" (melt) with a 20 watt lamp, but should melt when you connect a 36 watt lamp.