Electrical Circuit
Electricity and Magnetism

Simple electric circuits

for 14-16

This collection contains important introductory experiments, often qualitative. They should not be hurried. Students will enjoy these and learn basic but difficult concepts, provided they are accompanied by sensitive questioning, sufficient time and encouragement.

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Problem circuit

Electrical Circuit
Electricity and Magnetism

Problem circuit

Practical Activity for 14-16

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

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Experiments with switches

Electrical Circuit
Electricity and Magnetism

Experiments with switches

Practical Activity for 14-16

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

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Circuits from circuit diagrams

Electrical Circuit
Electricity and Magnetism

Circuits from circuit diagrams

Practical Activity for 14-16

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

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Using ammeters

Ammeter
Electricity and Magnetism

Using ammeters

Practical Activity for 14-16

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

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Investigating the current around a circuit

Electric Current
Electricity and Magnetism

Investigating the current around a circuit

Practical Activity for 14-16

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

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Water circuit

Electrical Circuit
Electricity and Magnetism

Water circuit

Practical Activity for 14-16

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

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Learning to use voltmeters

Voltmeter
Electricity and Magnetism

Learning to use voltmeters

Practical Activity for 14-16

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

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Working with simple electrical components

Electrical Circuit
Electricity and Magnetism

Working with simple electrical components

Teaching Guidance for 14-16

It is often up to a teacher and a particular class to decide what equipment to use to introduce electric circuits. There are two general types of equipment used in schools for experimenting with electric circuits:

  • Circuit boards (such as the Worcester Circuit Board) are designed with simple components so that the shape of the circuit which is constructed looks like a circuit diagram. This helps students to work from a circuit diagram or draw one themselves as a record of the work they have done. Some teachers find circuit boards can confuse less able students - they don’t realize that parts of the board without anything connected are not part of the circuit. Circuit boards have an advantage in that the connection of the cells in parallel is discouraged.
  • Separate components connected by wires. This can be a cheaper solution, but it can also produce a tangle of wires so that the circuit becomes confusing.

Give students simple instructions on how to use the kit. As work progresses, make simple testing devices available, to test whether a cell is flat, a lamp is broken, or a lead not providing a good connection. These are easy to assemble with the item to be tested being the missing component in a simple series circuit consisting of lamp, cell and connecting wires. Learning how to trouble-shoot a circuit probably teaches more than circuits which give the predicted result the first time.

Good maintenance is essential

Time spent in checking the equipment before a lesson will pay dividends in the students’ understanding.

Some agreement must be established within the class so that the brightness of one lamp used with one cell is ‘normal’ brightness. In more complex circuits the brightness of the lamps can then be compared to this standard.

For this to be clear, students need to be given cells which have the same voltage (checked when they are driving a current through a lamp and not on open circuit), and all the lamps in a student’s collection need to produce the same brightness with the same cell. This is quick to do if three cells are connected in series to three rows, each consisting of three lamps, so that all lamps glow with normal brightness. If possible, new cells should be used at the beginning of each year and the old cells used up doing other jobs. The quality control, during production, on simple lamps is not good and even new lamps from the same packet can vary widely.

The difference in brightness of the lamps might be difficult to see in bright sunlight or with laboratory lighting and so the laboratory should be dimmed a little.

What type of cell is best?

The cost of cells has led some teachers to try rechargeable cells, which have their own problems. They have low internal resistance, so, if shorted, allow a large current. And they need to be completely flat before they are recharged. Cheap zinc-chloride cells are best for elementary work. Alkaline-manganese cells may be used where shorted cells are unlikely.

Some teachers even use power supplies. However, power supplies suffer from their internal resistance, just like cells. They may give unexpected, but entirely correct, results when the simple story about electric circuits is being told and internal resistance is being neglected. In order to avoid running down some of the cells and not others during experimenting, students should be issued with switches, or asked to disconnect the circuit when they are doing other things.

Terminology

The language may vary in different teaching programmes with an insistence on cell for the simple 1.5 volt (approximately) simple cell and battery being reserved for several cells in series. Bulb may also be used instead of lamp.

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Introducing electric current

Electric Current
Electricity and Magnetism

Introducing electric current

Teaching Guidance for 14-16

Once your students understand how circuits can be set up, it is time to introduce them to the idea of the lamp as an informal ammeter. A fully-lit lamp connected to one cell can be said to indicate ‘one lamp’s worth of current’ (whatever current really is). Two lamps lit by one cell will each have less than ‘one lamp’s worth’ of current through them. One lamp connected to two cells will have more than ‘one lamp’s worth’ of current through it.

Some students may wish to know more about the electric current. At an introductory level, discussion along the following lines might be suitable.

You cannot see an electric current, or hear it, or know about it, by anything except by what it does.

How do you know that your Uncle George has a bad temper? Because he talks very crossly when you annoy him. You only know he has a bad temper by its effects. You cannot see a warning light on his head labelled ‘bad temper’ or a tribe of little demons dancing in his stomach to keep him irritable.

Some of you may have heard that when there is an electric current there are little electrons running along in the wire but you cannot see them any more than you can see the demons in Uncle George’s stomach. If we behave as good scientists and stick to the evidence, we can say that we see a lamp that's lit and a wire that’s hot. From this we infer there is an electric current.

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Electric charge and current - a short history

Charge
Electricity and Magnetism

Electric charge and current - a short history

Teaching Guidance for 14-16

Electrical phenomena result from a fundamental property of matter: electric charge. The atoms that constitute most matter we encounter contain charged particles. Protons and electrons each have one unit charge, but of opposite sign. Atoms are generally neutral because the number of electrons and protons are the same.

Electric charges at rest have been known much longer than electric currents.

The amber effect

The property now called static electricity was known to the philosophers of ancient Greece. In fact the word electricity comes from ‘elektron’, the Greek name for amber. Amber is a resinous mineral used to make jewellery. It is probable that small fibres of clothing clung to amber jewels and were quite difficult to remove. Trying to rub the fibres off made the situation worse, causing early philosophers to wonder why.

William Gilbert mentioned the amber effect in his ground-breaking book On Magnetism, published in 1600. He noticed that the attraction between electrics was much weaker than magnetism and wrongly said that electrics never repelled.

Benjamin Franklin

A giant leap of understanding was required to explain observations like these in terms of positive and negative electrical charge. In the 18th century, Benjamin Franklin in America tried experiments with charges. It was Franklin who named the two kinds of electricity ‘positive’ and ‘negative’. He even collected electric charges from thunderstorm clouds through wet string from a kite.

Franklin was an advocate of a ‘single fluid’ model of electric charge. An object with an excess of fluid would have one charge; an object with a deficit of fluid would have the opposite charge. Other scientists had advocated a ‘two fluid’ theory, with separate positive and negative fluids moving around. It took over a century for the debate to come down on Franklin’s side.

It is interesting to note that Franklin coined several electrical terms which we still use today: battery, charge, conductor, plus, minus, positively, negatively, condenser (= capacitor), among others.

Electric currents

Electric currents were not fully investigated until batteries were invented in about 1800. Passing currents through salt solutions provides evidence that there are two kinds of charge carriers, positive and negative. The charge carriers that boil out of white hot metals are negative electrons, and movements of electrons produce current in a cool, metal wire.

For a time electric currents seemed so different from electric charges at rest that the two were studied separately. It seemed as if there were four kinds of electricity: positive and negative electrostatic charges, and positive and negative moving charges in currents. Now scientists know better. There are just two kinds, positive and negative, exerting the same kind of forces whether they were ‘electrostatic charges from friction’ or ‘moving charges from power supplies’.

A modern view

Electric forces are what hold together atoms and molecules, solids and liquids. In collisions between objects, electric forces push things apart.

Today we understand that electrons may be transferred when two different materials contact each other and then separate. You can list materials in order, from those “most likely to lose electrons” (gaining positive charge) to "those most likely to gain electrons” (gaining negative charge). This is called the triboelectric series.

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Models of electric circuits

Electrical Circuit
Electricity and Magnetism

Models of electric circuits

Teaching Guidance for 14-16

At some point during the early teaching of electric circuits, students will want to know what an electric current is. Indeed students may already have their own ideas about what it is and how it behaves. There has been much research into the ideas students bring to their lessons, and the misunderstandings they develop during the teaching/learning process.

Electric current is known only by its heating, magnetic or chemical effects. Beyond this there are only models which explain such effects and make possible reliable predictions.

Misconceptions common among students

  • the ‘clashing currents model’ in which electric current is thought to leave both ends of the cell and meet at a component, for example a lamp, and make it operate;
  • the ‘single lead model’ in which students see the need for only one connecting wire leading from the cell to the lamp (this is often exacerbated if all the connections are not clearly visible on the circuit itself);
  • the ‘current is used up around the circuit model’ in which the current is thought to leave one terminal of the cell and is used up in the components; nothing returns to the other terminal. In fact, why have a return wire?

Teachers' models

There are many models which teachers use to describe electric circuits. Different ones are useful in different situations. Three of these are listed here:

  • the water circuit in which the flow of water is likened to the electric current;
  • a grid of wide and narrow streets, complete with car parks and one way systems, on which cars pass at speeds determined by the density of traffic;
  • the pupil circuit in which sweets are given up by the ‘cell pupil’ (energy) and ‘pupil charges’ transfer them to ‘component pupils’.

When discussing the water circuit as a model for an electric circuit, you could say to students:

There is something the same all the way round the circuit, the same reading with a simple ammeter, or the same brightness of a series of lamps. One of the lamps could even be placed in series between the two cells and will be just as bright as the others.

That is why scientists say, “There is a current; there is something running round the circuit which stays the same all along, just like a current of water in a river.” If a river is carrying 1,000 litres per minute past one place, it must be carrying 1,000 litres a minute past any other place farther down the river unless there is some side stream or a mysterious hole in the ground. Some scientists like to think of this electric current story as rather like water being pumped round a closed ring of piping.

Bring out the analogy between:

  • the pump and the cell
  • the tubing and connecting wires
  • the wide and narrow tubes and resistances
  • the flow meter and an ammeter, and
  • the pressure gauge and a voltmeter

Once students have used other components then the model can be extended in imagination to the idea of one-way valves representing diodes, and reservoirs representing capacitors. Stress that the flow of water is the same all round the circuit, unless of course you have a leak!

Once the model has been described then discussion can return to the electric circuit.

Is there really something that moves round through the copper wires and through the lamp and makes the lamp light or pulls the magnet? As far as you or I can tell, this electric circuit behaviour is rather like the behaviour of a current of water flowing that makes the same thing happen all the way round. We do not know, yet, whether anything is really flowing and certainly not what it is. If it flows it might be some kind of juice flowing this way round the circuit (positive juice) or it might be some opposite juice (negative juice) running the other way round the circuit. Or it might be both of those each running its own way.

Instead of some smooth juice flowing like water in a pipe the current might be a movement of little particles, moving along like a line of rabbits in a burrow or an army on a road. Again this might be a row of positive bits travelling this way or negative bits travelling that way or both kinds each travelling its own way.

Which of all these things do you think is right? Nothing travelling at all, or a juice travelling one way or another, or little bits of electricity travelling one way or another?

Whatever the answers at this stage students need to wait for further evidence. Nowadays scientists know that there are things which move when an ‘electric current’ happens, in some cases several kinds of things. In fact, contrary to wishful hopes, nothing in elementary physics teaching, even cathode ray tubes, requires a view that electric charges come in small particles. Continuous (negative) juice would do just as well. Only when students meet Millikan’s experiment do they require particles of electric charge to explain the data.

For the moment stick to the standard agreement, used by all electrical engineers, which is the idea of bits of positive electricity coming out of the red knob of the cell and going round the circuit in one direction to the negative end of the cell. That was settled long before anyone knew about electrons and is used to put arrows on the electric circuit drawings. Later on you will be able to decide for yourselves what is really going on and you might find it even more complicated than you think.

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