Class practical

An extra wire can shed light on the problem.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 2
• Lamps with holders, 2
• Leads, 4 mm, 6

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.

Experiments on shorting cells should be restricted to used cells which are not quite flat. The old ammonium chloride cells would polarize under short-circuit conditions and the current would then drop so that overheating and explosion did not occur.

Read our standard health & safety guidance

Procedure

1. Set up the circuit shown. Why do the lamps not light?

2. What will happen if an additional link is added as shown below?

Teaching Notes

• Two lamps are set up with two cells whose polarities face opposite ways: the lamp does not light. The idea that opposing cells cancel each other out is clear to most students.
• The second part of this experiment is designed to make students think about individual loops within a circuit. Students should try to predict the outcome before trying out the circuit in practice. The effect of adding a connection from a position between the cells to a point between the lamps, and so dividing the circuit into two circuits, is quite startling.
• Other effects of shorting out other components can be discussed.

This experiment was safety-tested in April 2006

Class experiment

Many interesting circuits can be set up with switches in series and in parallel with components.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamps with holders, 3
• Switches, 2
• Leads, 4 mm, 7
• Ammeter (optional)

Health & Safety and Technical Notes

Various kinds of switches can be used such as a swivelling link, SPST (single pole-single throw) toggle switches, bell pushes etc.

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

Procedure

1. Set up the circuit shown. What happens when you open and close the switch?

2. Investigate the effect of moving the switch to different points in the circuit.
3. Investigate some more complex circuits including two switches.
4. Summarize your findings.

Teaching Notes

• Many interesting circuits can be set up with switches in series and in parallel with components. Students should be encouraged to think of a switch as ‘breaking’ a circuit. Can they identify the relevant circuit?
• Simple logic circuits can also be set up. With two switches in series, both switch 1 AND switch 2 have to be closed to produce the desired effect. Connect switches in parallel to each other, and then connect this ‘unit’ into a series circuit. This will show the desired effect if one OR the other switch is closed.
• It is also valuable to include an ammeter in the circuit. This may show something interesting when there are branches of several lamps or sets of lamps in a circuit, with a switch in each branch.

This experiment was safety-tested in June 2006

Class experiment

Circuit diagrams allow students to make simple records. They also allow teachers to give students summaries of their instructions.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamps with holders, 3
• Leads, 4 mm, 6

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

Procedure

1. Copy the first circuit diagram. Set up the circuit using the equipment provided. Note down what you observe.
2. Repeat this procedure for each of the circuits shown.

Teaching Notes

• Before this experiment, you may wish to discuss how to draw circuits neatly and what symbols to use. Students need to learn how to draw circuits conventionally using standard symbols and straight lines for the connecting wires. They might also start a glossary of component symbols which will grow as they meet more and more components.
• For each circuit, a simple note of the brightness of the lamp is all the record that is needed. Recording should not get in the way of early experimenting when familiarity with what the electrical components do is all important. Too many breaks for recording may cause some students to lose the thread of the argument.
• Students should learn how to construct circuits from circuit diagrams. Too often students copy slavishly from circuit diagrams without beginning to understand what is happening in the circuit. They should be encouraged to discuss the meaning of a circuit diagram. Students should also be encouraged to predict how each circuit will behave, giving a reason, before trying it out.

This experiment was safety-tested in January 2005

Class practical

An opportunity to measure the electric current and introduce the ampere unit. There is no need to define an ampere.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 2
• Lamps with holders, 3
• Ammeter (0-1 amp), DC, preferably moving-coil
• Leads, 4 mm, 6
• Digital and analogue ammeters with varying ranges (optional)
• Digital multimeter with multiple current ranges (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.

Read our standard health & safety guidance

Procedure

1. Set up a circuit in which a cell, a lamp and an ammeter are connected in series.
2. To record what you observe, draw a circuit diagram. Beside the lamp, note its brightness. Beside the ammeter, note its reading.

3. Set up a second circuit with two lamps connected in series with the cell and ammeter. Record your observations.
4. Repeat this with the two lamps connected in parallel with each other (side-to-side).
5. Repeat these observations using two cells in place of one.
6. How does the reading on the ammeter relate to the brightness of the lamps?
7. Investigate how the reading on the ammeter depends on its position in the circuit.

Teaching Notes

• With two lamps in series, less light is produced and the ammeter will show that the current is less. This is where high power cells are needed. Otherwise the result is spoilt by internal resistance.
• When two lamps are connected in parallel, twice as much light will be produced (two lamps of equal brightness), and the ammeter will show that twice as much current is flowing.
• Note that the results of this experiment may not match up to an idealized view of current flow. There are two reasons for this:
• when two lamps are connected in series, the voltage across them is halved but the current is likely to be more than half the previous value (because the lamp's resistance is lower when it is cooler)
• the voltage provided by a cell is likely to be less when it is making a bigger current flow, as a consequence of the internal resistance of the cell.
How Science Works extension:
• Use this demonstration as an opportunity to raise some of the issues relating to the selection of appropriate equipment for practical work. Students may think that there is only one type of ammeter. By demonstrating that there are ammeters with different ranges, you can reinforce the importance of selecting appropriate equipment. With two or more meters on different ranges, take the same measurement: the pointer of a meter with a more sensitive scale will be deflected further.
• In some experiments, students will not get a single, fixed reading on their ammeter but will get a constantly fluctuating value. Deciding what the 'right' reading is provides an excellent opportunity to discuss:
• the relative merits of analogue and digital meters
• uncertainty in measurements
• how to select a meter with an appropriate range and sensitivity.
• Point out that care needs to be taken to avoid meters being overloaded or damaged. If an electrical meter has more than one range, students should always select the highest range first and then more sensitive ranges, if appropriate. They should also select the lowest voltage on the power supply.

This experiment was safety-checked in December 2006

Class experiment

The fact that current is the same all round a series circuit may have been discovered when using ammeters informally. It is so important that this activity may be needed to be reinforce it.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 2
• Lamps with holders, 2
• Ammeter (0-1 amp), DC, preferably moving-coil
• Leads, 4 mm, 6

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

Procedure

1. Set up a circuit in which a cell, a lamp and an ammeter are connected in series. Draw a circuit diagram.
2. Connect the ammeters in different parts of the circuit: between the lamps, between the cells, between the cells and the lamps. Each time note the current at the appropriate point on your circuit diagram.
3. Draw a conclusion from your observations.

Teaching Notes

• This result, that the current is the same all round a series circuit, may well be surprising. Even when students have seen it for themselves they find it hard to believe. Many people think that the current is used up as it passes around a circuit.
• So what is ‘used up’? From an early stage it is helpful to understanding if the idea of the cell storing energy chemically is discussed. Once the circuit is connected, energy can be transferred by the electrons in the wires to the lamps; the wires are warmed up and energy is transferred away from the lamp by the electromagnetic waves (visible and infra red).

This experiment was safety-tested in December 2004

Demonstration

An introduction to what voltmeters measure and how they are connected in circuits.

Apparatus and Materials

• Water circuit board
• Variable voltage supply (0-12 V, AC or DC as required by pump)
• Leads, 4 mm

Health & Safety and Technical Notes

Read our standard health & safety guidance

The water circuit board should be set up vertically. The electric motor, which drives the water pump, should be connected to the terminals of the variable voltage supply. The tubes should be filled with water: a little fluorescein or a few drops of methyl orange can be added to make the water more clearly visible. The water is conveniently poured in at the funnel. The pump will drive water round the circuit of glass tubing attached to the board, the pressure being dependent on the voltage applied to the motor.

At one point, the tube divides. The two sections represent different resistances: one tube has a much finer bore than the other. Clips enable one or other or both sections to be opened at once: thus the effect on the current of different 'resistances ' can be seen.

The pressure gauge consists of a U-tube connected as illustrated and filled with coloured water.

Where there is a break in the circuit, the funnel catches the water flowing down from the tube above. The rate of flow of water is apparent and this indicates the current. Alternatively, if there is a pool of water in the funnel, the faster the flow of water the more rapid the swirling motion in the funnel. A small piece of cork floating on the water in the funnel acts as an indicator of the rate of swirling. This shows the current.

You can show the flow of water more clearly like this: push a large sewing needle into a small cube of expanded polystyrene so that the cube is half way along it. Put one end of the needle into the tube at the base of the flow meter so that the polystyrene floats on the water. If the funnel is inclined a little then the water will swirl more easily.

Procedure

1. Initially, it is not advisable to draw attention to the analogy with an electric circuit. Start by showing how the rate of flow of water (the current) depends on the pressure pushing it. To do this, change the speed of the pump by altering the voltage of the supply. The U-tube indicates the pressure difference across the tube at the top of the board.
2. Go on to show the effect of changing the bore of the tube at the top. A narrower tube allows a smaller current to flow, provided the pressure difference is kept constant. Emphasize the need to adjust the pumping rate to ensure that the pressure difference is constant.
3. Now you can discuss the analogy with an electric circuit. The flow of water represents the current, and must be conserved around the circuit (unless you have a leak!). The pump represents the battery pushing it round. Greater pressure difference corresponds to greater voltage (or potential difference). A narrower tube corresponds to greater resistance.
4. Test your students' understanding by asking them to predict the effect of having two narrow-bore tubes in parallel with each other at the top of the board. (The current should double, compared with a single tube.)
5. Once students have used other components, they might be able to suggest how the water circuit analogy can be modified to represent diodes (one way valves) and capacitors (reservoirs).

Teaching Notes

This experiment was safety-tested in November 2006

Demonstration

An introduction to what voltmeters measure and how they are connected in circuits.

Apparatus and Materials

• Cells, 1.5 V, with holders, 3
• Lamps with holders, 3
• Leads, 4 mm, 8
• Demonstration voltmeter (0-5 V)
• Digital multimeter with multiple voltage ranges (optional)
• Digital and analogue voltmeters with varying ranges (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.

Read our standard health & safety guidance

Procedure

1. Connect three cells in series. (Don’t complete the circuit.)
2. Attach two leads to the demonstration voltmeter of a different, distinctive colour, e.g. green.
3. Connect the meter, reading 0-5 volts, first across one cell, then across two, then across three. Show that the meter reading increases in equal steps – the meter is ‘counting the cells’. (You might wish to mark the meter face to indicate ‘1 cell’, ‘2 cells’, ‘3 cells’.)
4. Now connect three lamps in series. Connect one cell across the three lamps - the demonstration meter should read approximately ‘1 cell’. Repeat with two and three cells.

5. Finally, with three cells and three lamps, make readings across one, two and then three lamps to show how the voltage of the cells is shared between the lamps when they are in series.

Teaching Notes

• As a first introduction to the voltmeter there is no need to define the volt. Instead use it like a ruler or a watch is first used - without defining the metre or the second. Eventually the volt will be used as a measure of the energy which a cell is able to provide. (A potential difference in volts is defined from the energy transferred to or from each coulomb of charge flowing between the two points in the circuit, where the voltmeter is connected.)
• You could start by introducing the cell as a device that stores energy chemically. This may help students to understand that the current is not used up, but the chemicals in the cell are used up. So the current is the same all round a series circuit and is not used up in devices. (The current does electrical work - the lamps heat up and warm up the surroundings.) We are paying for the chemicals in the cell, and, on a large scale, for the coal (and infrastructure) that enables a current to flow when appliances are connected to the mains. We pay for the fuel, power stations, and National Grid when we pay an electricity bill.
• Students could carry out a similar experiment, but they are likely to be less confused by their observations if the experiment is performed as a demonstration. They could go on to practise using voltmeters to measure voltages in any circuits which they have previously studied.
• How Science Works extension: Use this demonstration as an opportunity to raise some of the issues relating to the selection of appropriate equipment for practical work. Students may think that there is only one type of voltmeter. By demonstrating that there are voltmeters with different ranges, you can reinforce the importance of selecting appropriate equipment. With two or more meters on different ranges, take the same measurement: the pointer of a meter with a more sensitive scale will be deflected further.
• In some experiments, students will not get a single, fixed reading on their voltmeter but will get a constantly fluctuating value. Deciding what the ‘right’ reading is provides an excellent opportunity to discuss:
• the relative merits of analogue and digital meters
• uncertainty in measurements
• how to select a meter with an appropriate range and sensitivity
• Point out that care needs to be taken to avoid meters being overloaded or damaged. If an electrical meter has more than one range, students should always select the highest range first and then more sensitive ranges, if appropriate. They should also select the lowest voltage on the power supply.

This experiment was safety-tested in December 2006