Class practical

An opportunity for students to discover materials which will control the current.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamp with holder
• Crocodile clips, 2
• Leads, 4 mm, 5
• Miscellaneous materials

Health & Safety and Technical Notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Read our standard health & safety guidance

Miscellaneous materials could include:

• Stick of wood
• Strip of paper
• Strip of copper
• Thread of nylon
• Aluminium foil
• Pencil lead (preferably a soft grade)
• Bits and pieces from students’ pockets

Procedure

1. Connect up a circuit as shown.

2. In turn, insert samples of different materials in the circuit by clipping them between the crocodile clips.
3. Make lists of those which will carry current, and those which will not.

Teaching Notes

• This is an opportunity for you to offer your students many different components for them to investigate. Besides the variety of materials listed, it is possible to use diodes, light emitting diodes (LEDs), capacitors, motors and even transistors. However, it is wise to test the available components first: a determined class could destroy the stock of low-current diodes very quickly!
• The current passing through the material or component can be indicated by the brightness of a lamp or it can be measured with an ammeter. The variation in the conductance of the materials can be recorded. Include materials which have such a low conductance that they may be loosely termed insulators, at least for the voltage and sensitivity of the ammeter.
• You could raise the question of ‘fair testing’. Does this method give a fair comparison between different materials? (No; the samples should have the same dimensions for a fair comparison.)

This experiment was safety-tested in February 2005

Class practical

Introduces the idea of a variable resistor, or rheostat.

Apparatus and Materials

For each student group

• Crocodile clips, 2
• Cells, 1.5 V, with holders, 2
• Lamp with holder
• Ammeter (0 - 1 amp), DC
• Carbon resistor e.g. 1 W 3.9 ohms
• Variable resistor or rheostat e.g. 3 W 25 ohms
• Wire-wound resistor e.g. 3.9 ohm 3 W or 5 W
• Eureka wire 34 SWG, 1 m length
• Leads, 4 mm, 5
• Diode or LED

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator, causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Procedure

1. Set up a circuit of two cells and a lamp.

2. Make a gap in the circuit, and connect a length of Eureka wire between two crocodile clips in the gap.
3. Change the length of wire between the clips and observe the effect on the lamp.

4. In place of the wire, connect first the fixed resistor and then the variable resistor. Note what you observe. Reverse the connections to the resistor. Can you feel any difference between the two resistors?

5. With the variable resistor, make the lamp both brighter and dimmer.
6. In place of the resistor, insert a diode and note what happens when the connections to the diode are reversed. Also try placing the diode at different positions in the circuit.

Teaching Notes

• This is an opportunity to introduce students to the concept of resistance, though perhaps not yet as the ratio V/I.
• The variable resistor, or rheostat, shows how the brightness of a lamp can be varied. It is helpful if you indicate the path of the current through the resistance wire. (This may involve dismantling a rotary rheostat.)
• Resistance is a word that comes from the water-flow analogy. That idea was so strong in Ohm’s mind when he started his researches that he said that he was looking for ‘electrical resistance’ and trying to find its properties. When it was found that metal wires give a constant ratio for V/I, that constant was given the name resistance which Ohm had ready for it.
• In contrast with this case, the order of events in most physical discoveries is the other way round. Scientists first discover experimentally that some ratio has a constant value and then coin a name for it because it is constant (e.g. stress/strain = the Young Modulus).
• In class, this logical order sometimes gets obscured. Some students grab the name of the constant and take it for granted that the name itself assures the constancy, and thus takes away any need for experimental investigation. Then the practical experiment becomes a scheme for making one accurate measurement of that assured constant, instead of an interesting investigation to see what relationship is there. Of course there is nothing wrong in making these measurements: each has its own importance in physics but an organized series of such experiments can give beginners a wrong headed picture of science.
• In fact, when you compare a wide range of materials, metals are remarkable for their conductance (how easily current flows through them). Conductance is the inverse of resistance.
• The carbon resistor allows a current to flow in the circuit whichever way it is connected into the circuit. The diode only allows the current to pass in one direction. (LEDs light up and are more fun.)
• Students could repeat all the experiments with an ammeter in the circuit and observe how it steadily changes as the resistances are altered.

This experiment was safety-tested in April 2006

Class practical

Introduces two ideas: current is shared between parallel paths in a circuit, and brightness of a lamp depends on current.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamp with holders, 5
• Crocodile clips, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 8
• Variable resistor or rheostat e.g. 3 W 25 ohms

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Procedure

1. Connect the ammeter between the lamps and vary the variable resistor. Move the ammeter so that it is between one lamp and a cell and repeat the experiment. Explain what you observe.

2. Set up circuits like those shown. Describe and explain the patterns you observe in the brightnesses of the lamps.

Teaching Notes

• Once students have an understanding of resistance, practice in using resistors in circuits and attempting to explain their results is good reinforcement.
• Step 1: The variable resistor is connected in parallel with one or other lamp. It allows part of the current to by-pass that lamp, so that the other lamp will be brighter.
• Step 2: Explanations of these circuits should be in terms of current-sharing when lamps are in parallel. When lamps are in series, they have the same current, so they are of equal brightness.
• By the end of this series of experiments, students should be confident in understanding series and parallel circuits which include both fixed and variable resistors.

This experiment was safety-tested in April 2006

Class practical

A simple investigation of the factors affecting the resistance of a wire.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 2
• Crocodile clips, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 5
• Wire available for class use (see technical notes)
• Power supply, 0 to 12 V, DC (OPTIONAL)
• Metre rule (OPTIONAL)
• Insulating tape (OPTIONAL)
• Digital and analogue ammeters, 0-1 A (OPTIONAL)
• Digital and analogue voltmeters, 0-12 V (OPTIONAL)
• Micrometer (OPTIONAL)

Health & Safety and Technical Notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

When using a power supply, high currents will cause the safety cut-out on the power packs to automatically switch it off. If short lengths of wire are used with relatively high currents and voltages, then significant electrical heating may also occur. Students should be encouraged to adjust the voltage to keep currents small with every set of readings. At each stage they can connect the circuit, take readings quickly and then disconnect the power supply.

If you use a mains power supply, use one that is designed to limit the output current to about 1 amp, and preferably with a current overload indicator.

Read our standard health & safety guidance

The following apparatus should be available for class use:

• Selection of reels of Eureka wire (also known as Constantan or Contra) of different gauges, e.g. 0.71 mm (22 SWG), 0.46 mm (26 SWG), 0.32 mm (30 SWG) and 0.24 mm (34 SWG).
• Selection of reels of different wires (e.g. copper, Eureka, iron) of same gauge (e.g. 34 SWG).

Procedure

1. Connect up a series circuit of two cells, and the ammeter, with a 30 cm length of one of the wires closing a gap between two crocodile clips. Note the reading on the ammeter.
2. Replace the specimen of wire with another of the same length but different gauge or material.
3. Investigate how the current depends on the thickness of the wire, its length and the material from which it is made.

Teaching Notes

• Use fine gauge wires. If too thick a wire is used, the results may be affected by warming of the wires.
• If coils of copper and Eureka wires of the same gauge can be prepared so that they have equal resistances, the effect is very striking. However, this would then lose its value as an open investigation.
• Students should come to understand that the resistance of a wire depends on its length, its cross sectional area, and the material out of which it is made. With some students you could go further and introduce the concept of resistivity ρ, through the relationship R = ρ l / A where R = resistance, ρ = resistivity, l = length and A = cross-sectional area.
• This may also be an opportunity for a large scale demonstration of the effect by the teacher. But note: if the current is too large, the voltage of the cells will fall due to their internal resistance. For this reason, it is important to keep the current very low - copper wire is effectively a short.
• How Science Works extension: This experiment can be used as a more open-ended investigation. Students can select the variables, the ranges of results and the equipment used. The amount of guidance will depend greatly upon the teaching group. Investigating the effect of length on resistance is common but some students may wish to investigate the effect of the thickness of wire. In either case, different wires should be made of the same material. Students may need to know the conversion between SWG (standard wire gauge) and wire diameter/radius.
• Students will find it easier to measure at a prescribed length if they tape the wire to a metre rule with insulating tape and make connections with flying leads rather than crocodile clips.

This experiment was safety-checked in August 2007

Class practical

Illustrates two ideas: electric current causes a heating effect; temperature affects the resistance of a wire.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamp with holder
• Crocodile clips, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 5
• Eureka wire 34 SWG, 15 cm length

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Procedure

1. Set up a series circuit of three cells and a lamp. Include two crocodile clips in the circuit.
2. Wind the length of bare Eureka wire into a coil (perhaps on a pencil). Clamp the ends of the wire into the two crocodile clips. (Make sure that the turns of wire do not touch each other.)
3. Stand back! Carefully, hold your hand above the wire coil. Can you feel hot air rising?

Teaching Notes

• When an electric current passes through a material, the material warms up. The open coil of wire will be warm to the touch. Blowing on the wire will reduce its temperature and the lamp will glow brighter. Try using an electronics freezer spray to reduce the temperature of the coil even more and the lamp will glow brighter still.
• It is important that the coils must not touch each other, or the coil will become a short circuit.
• This experiment can be demonstrated in order to explain how a filament lamp works. The filament is just a short piece of wire which gets so hot that it glows red for low currents, becoming whiter as the current increases, until the filament finally melts and the lamp lights no more.
• Students often ask why you can’t just use the filament; why does it have to be in a glass envelope? It is fairly easy to break the glass envelope of a torch lamp by squeezing the lamp in the jaws of a small clamp. (Cover the lamp in tissue paper or a fine cloth if necessary though the cloth might damage the filament.) The lamp, minus its glass envelope, can then be connected to a cell. The filament may disintegrate in a puff of smoke without there being time to see that it heated up. The air has oxidized the filament and it has broken up. So whatever the gas is in a lamp, it is not oxygen. (Frequently it is argon.)
• Some students may have noticed that, when a circuit consisting of a cell, a lamp and an ammeter is connected, the current is momentarily greater when the connection is made, and then the current settles down to a steady lower value. This is because the resistance of the cold wire is less than the resistance of the hot wire.

This experiment was safety-tested in April 2006

Demonstration

Model a fuse using fine strands of steel wool. Too large a current causes the steel fibres to heat and melt, breaking the circuit.

Apparatus and Materials

• Cells, 1.5 V, with holders, 3
• Lamp with holder
• Leads, 4 mm, 7
• Variable resistor or rheostat
• Crocodile clips, 4
• Heat-resistant mat
• Steel wool (wire wool), grade 2
• Ammeter, 0-1 A

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Procedure

1. Set up a simple circuit as shown, with three cells, a lamp, an ammeter and a variable resistor in series. Include two crocodile clips.
2. Before completing the circuit, connect a 5 – 8 cm length of one strand of the steel wool between the clips. Place this part of the circuit on a heat-resistant mat.
3. Complete the circuit. The current through the wool should cause it to heat up; it may even catch fire. Reduce the variable resistance to increase the current.
4. You can also reduce the resistance by removing the bulb from the circuit.

Teaching Notes

• If the steel wool does not fuse, it can be used to show in a graphic way the variation in resistance with temperature. Blowing on the wire is sufficient to reduce its resistance and increase the current very considerably.
• Holding the wire between the fingers has the same effect, and gives the students a clue as to what is happening.
• By sliding the crocodile clip down the steel wool, a point will be reached when it fuses quite spectacularly. If the lamp is not removed, it may fuse first!
• You could attach a small lump of wire wool between the two crocodile clips (which are placed together). A small firework display ensues. Take care to do this behind a safety screen.

This experiment was safety-tested in April 2006

Class practical

An opportunity to test the manufacturer's fuse rating.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Crocodile clips, 4
• Lamps with holders, 6
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 12
• Cartridge fuses, 0.25 A, (3 cm long), preferably glass type with visible wire, 2
• Variable resistor or rheostat

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Procedure

1. Connect up the circuit shown, but with only one pair of lamps in circuit diagram. Note the current.
2. Add a second pair of lamps and note the current.
3. Add the third pair of lamps and again note the current. Does the current ever exceed the rating of the fuse?

4. Now, set up a new circuit to determine the current needed to blow (melt) the fuse. Connect the three cells in series with the fuse, the ammeter and the variable resistance (rheostat). Gradually decrease the resistance in the circuit, watching the current reading on the ammeter until the fuse blows.

Teaching Notes

• Students could try to determine whether the manufacturer's rating on the fuse is right. The rating is the current which the fuse can carry for some time (say several hours) but not for ever. If the current rises to twice the rating, the fuse should blow in less than one second.
• If the current could rise to ten times the rating (e.g. in a mains circuit), the fuse could explode. Fuses for this application have ceramic tubes, not glass, and the space inside is filled with dry sand.
• It is often surprising how difficult it is to blow a fuse - the internal resistance of the cells limits the current.

This experiment was safety-tested in December 2004

Class experiment

This activity models an archaeological survey (as students in the UK might have seen in TV programmes such as Time Team).

They learn that the resistance between two points depends on the shape and resistivity of the material(s) with which they are in electrical contact.

Apparatus and Materials

For each student group

• Model field (see technical notes below)
• Resistance meter (or ammeter, voltmeter and low voltage power unit or cell)
• Connecting probes (e.g. clean 4 mm plugs), 2
• Leads
• Graph paper (A4 sheets)
• Computer running a spreadsheet/graphing package (optional)

Health & Safety and Technical Notes

Read our standard health & safety guidance

To make the model field you need:

• hardboard sheet, about 30 cm square, preferably with a grid of ready-drilled holes with a spacing of about 2 cm (or drill your own grid)
• Teledeltos paper, same size as hardboard
• metal cooking foil strip a few centimetres wide
• cork tile, same size as hardboard or slightly larger
• strips of wood to make frame

Cut a piece out of the Teledeltos paper to represent a buried wall. The exact shape does not matter, but it needs to be at least 5 cm wide in any dimension.

Glue a strip of metal onto the Teledeltos paper to represent a buried waterlogged ditch.

Glue the Teledeltos paper onto the cork tile.

Construct the model as shown in the diagram.

Procedure

1. Connect the probes to the meter(s).
2. Measure the resistance between pairs of holes in the grid. This is best done by a systematic survey, e.g. by moving the pair of probes along two parallel rows of holes.
3. Display the results on graph paper and/or using a computer graphing package.
4. Identify regions of anomalously high and low resistance and hence locate the hidden wall and ditch. Discuss the factors that determine the resistance measured between a pair of probes.

Teaching Notes

• This is intended as a fun introduction to the idea of resistivity and the dependence of resistance on sample dimensions.
• You will probably want to follow this activity with further activities or demonstrations, in which students explore the resistance of simple shapes and carry out calculations.
• The resistance measured between two probes depends on the resistivity and shape of the underlying material. A uniform sample of length l and cross-sectional area A has resistance R such that
R = ρl / A where ρ is the resistivity of the material.

This experiment comes from University of York Science Education Group...

Salters Horners Advanced Physics

Diagrams are reproduced by permission of the copyright holders, Heinemann

This experiment was safety-tested in January 2005

Demonstration

This activity models a railway track-circuit signalling system. Train wheels and axles provide an electrical connection between the two rails on which they travel. This enables a track signalling circuit to detect the presence of a train on a given section of track.

Students use their existing knowledge of resistors combining in parallel, and of internal resistance, to explain its operation.

Apparatus and Materials

• Model rail section
• Track circuit relay and signals unit
• Power supply, 6 V, AC
• Variable resistor, 20 ohm, 3 W
• Dry cells, 1.5 V, 4
• Resistors (rated for 7 W power) from 0.22 ohm to 10 ohm
• Multimeter
• Crocodile clips, 2
• Leads
• Metal foil (steel or copper), thick strip about 1 cm x 5 cm

Health & Safety and Technical Notes

Read our standard health & safety guidance

See apparatus entries for construction details.

Procedure

1. Connect the units together as shown in the diagram.

2. Simulate a train on the right-hand section by holding a piece of steel or copper foil across the rails. Observe what happens to the lamps.
3. Simulate different ballast conditions by placing various resistors across the rails. (‘Ballast’ is the loose pieces of stone spread between and around the railway sleepers.) Do not leave them in contact for long, as they will get hot and exhaust the battery.
4. Note the smallest resistance that will cause the signal to switch from green to red.
5. Connect a voltmeter across the relay solenoid and connect a 20 Ω. variable resistor across the tracks.
6. For various settings of the variable resistor, find the drop-away voltage (when the signal switches from green to red) and pick-up voltage (red to green).
7. After each set of measurements, disconnect the variable resistor and find its resistance using a resistance meter.
8. Study the circuit diagrams and explain as precisely as possible how the track signalling circuit works.
9. Discuss what would happen if the power supply had no internal resistance.

Teaching Notes

• This activity is intended for use with students who are already familiar with the idea of internal resistance and with resistor combinations in parallel. The intention is that they study the behaviour of the circuit and use their existing knowledge to explain its operation.
• A key feature of the circuit is that the power supply has some internal resistance.
• The relay is connected across the terminals of the power supply. When a low resistance is connected in parallel with the relay, the net resistance falls. The terminal potential difference falls, and hence the current in the relay also falls.
• If the relay current drops below a certain threshold, the switch is released and the signal shows red.
• If the power supply’s internal resistance were zero, reducing the resistance in the external circuit would not affect the terminal potential difference. The current in the relay would be unchanged and the signal would permanently show green.
• It is important to avoid explanations along the lines of ‘most of the current goes along the short circuit and by-passes the relay’. That implies, incorrectly, that the total current remains constant, and is merely divided up differently between the relay and other resistors in parallel with it.

This experiment comes from University of York Science Education Group...

Salters Horners Advanced Physics

Diagrams are reproduced by permission of the copyright holders, Heinemann. This experiment was safety-tested in June 2007