Electric Current
Electricity and Magnetism

Current and charge

for 14-16

These activities will help students to relate the idea of current (as shown by an ammeter) to the idea of electric charge (more familiar from experiments in electrostatics). Electrolysis links these two by showing that, the greater the current that flows, and the longer it flows for, the greater the mass of material released.

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Series and branching circuits

Electric Current
Electricity and Magnetism

Series and branching circuits

Practical Activity for 14-16

Demonstration

Experiments to show that current (and therefore charge) is conserved around a circuit. They can be teacher demonstrations or student revision experiments.

Apparatus and Materials

  • Battery, 12 V or low-voltage DC power supply
  • Ammeters (0 - 1 A), DC, 6
  • Lamps (12 V 6 W) in holders, 2
  • Rheostat (10 - 20 ohms) rated at 5 A at least

Health & Safety and Technical Notes

Read our standard health & safety guidance


For this reason it is a good idea if they are mounted in the vertical plane (preferably attached to a board) so that the layout can readily be seen.

In a demonstration of this nature where a very clear display is required, it is advisable to use straight, stiff bare copper wire and to make T connections with crocodile clips.

If digital ammeters are used, it might be helpful with some classes to cover up the least-significant digit on the display.

Procedure

  1. Set up the circuit shown.
  2. Adjust the current to a suitable value (such as 0.4 A) by means of the rheostat. It would also be instructive to include a fourth meter between two of the cells in the battery if the battery permits this.
  3. As a simple branching circuit, set up the following:
  4. Point out that the currents in the two branches add up to the current in the main circuit.
  5. Finally, set up the following:

Specimen readings:

With a 12 V battery and 12 V, 6 W lamps, A1 will read 0.5 amps. Adjust R3 so that A3 reads 0.3 amps, and R2 so that A2 reads 0.2 amps. Then A4 reads 0.5 amps, A5 0.7 amps, and A6 1.0 amps.

Teaching Notes

  • Throughout these experiments, the idea of positive flow of charge is assumed, i.e. flow from the positive terminal of the power supply to its negative terminal.
  • We know from simple series circuits that something is the same all round the circuit. This is because similar lamps light equally all round it, and an ammeter changed from one place to another in a series circuit gives the same reading. That something, which is the same all round the circuit is called the current, because that experimental property corresponds to a similar property for water in pipes.
  • With the water analogy in mind, think of the wires of the circuit as full of something that can be made to move once a battery is applied. That something, which we suppose fills the wires and is ready to move, is called electric charge and it is measured in coulombs. See also the Water circuit experiment...
  • Water circuit


  • In parallel circuits, the total current entering a junction is equal to the total current leaving it. Current does not get lost when a circuit divides into branches which then re-unite.

This experiment was safety-tested in October 2006

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Electrolysis of copper sulfate solution

Electric Current
Electricity and Magnetism

Electrolysis of copper sulfate solution

Practical Activity for 14-16

Demonstration

To some students the feeling for coulombs of charge comes most easily from electrolysis.

Apparatus and Materials

  • Copper voltameters (exactly similar), 3
  • Ammeters (0-1 A), DC, 3
  • Rheostats (10-15 ohms), 3, rated to carry at least 1 A
  • Batteries, 12 V, 3
  • Top-pan chemical balance with a sensitivity of no less than 0.01 g
  • Electrolyte: saturated solution of copper sulfate to which is added 5 per cent IM sulfuric acid

Health & Safety and Technical Notes

Saturated copper sulfate solution is harmful. Wear eye protection and keep off the skin.

Read our standard health & safety guidance


The copper voltameter should be designed so that it is easy to remove and replace the cathode. A typical voltameter consists of two clean copper electrodes held to the sides of a rectangular glass jar by bulldog clips and each fitted with a soldered terminal. This arrangement facilitates easy replacement in the same place.

Procedure

  1. Before the lesson, set up three exactly similar circuits as shown and adjust the rheostats to give suitable currents.
  2. In the case of electrodes with an immersed area of 8 x 5 cm, currents of 1 A in one case and 0.5 A in the other two cases would be very suitable: this corresponds to current densities of about 0.025 A per cm2 and 0.012 A per cm2.
  3. Switch off the circuits and remove the cathodes. Wash, dry, and then lightly clean them with emery cloth or paper.
  4. When the demonstration is to be performed, weigh the cleaned dry cathodes and replace them in the three circuits. Then switch on the 3 circuits together. After ten minutes, switch off the currents in the circuit carrying 1 A and one of the circuits carrying 0.5 A. After twenty minutes, switch off the third current. Remove the cathodes. Wash, dry and reweigh them, taking care to record which is which. You should find that the copper carried across is proportional to current x time.

Teaching Notes

  • Like all metal ions, and hydrogen, the blue copper ions carry positive charges and the sulfate ions carry negative charges. So the plate on which the copper collects is connected to the negative of the battery.
  • From these experiments you can extract the idea that the mass of copper carried across the cell is proportional to the current and to the time and hence to current x time.
  • Each coulomb passing through copper sulfate solution deposits...
  • 0.000 000 329 kg of copper
  • The experiment which determined that number, had to have a standard ammeter and a stop watch. But once that is settled, you can reverse the argument and use that number in testing your own ammeters. That number can be sent on a post-card to a new laboratory elsewhere. This is much easier than sending a delicate standard ammeter by parcel post!

This experiment was safety-tested in October 2006

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Electrolysis of water and the concept of charge

Electric Current
Electricity and Magnetism

Electrolysis of water and the concept of charge

Practical Activity for 14-16

Demonstration

The electrolysis of water yields oxygen and hydrogen gases and simple calculations estimate the charge of the ions.

Apparatus and Materials

  • Gas voltameter kit (see Technical notes and illustration)
  • Ammeter, (0-1 A), DC
  • Power supply, low voltage, variable, or rheostat (10-15 ohms) and battery, 12 V
  • 0.4 M sulfuric acid

Health & Safety and Technical Notes

Do not ignite the hydrogen gas directly in the burettes but sample it as described below.

0.4 M sulfuric acid is not hazardous.

Read our standard health & safety guidance


The Worcester voltameter kit consists of two 250-ml burettes mounted vertically so that they will fit inside a tall gas jar (see diagram). The electrodes consist of stiff wires covered with plastic insulation except where they are within the inverted burettes. Ideally, these tips are plated with platinum. The advantage of this design (which many schools have) is that the glass items are standard and easily replaced when broken.

The Hofmann voltameter is an alternative piece of apparatus. It is expensive and easily broken, but many schools have one. There is also a mini-version consisting of two small test-tubes in a small trough.

Although this is traditionally described as electrolysis of water, showing that an electric current can split water into two volumes of hydrogen to one of oxygen, it is a little more complicated. Pure water contains few ions so the process is very slow. To obtain results in a reasonable time, sulfuric acid or another electrolyte is added. See the CLEAPSS Laboratory Handbook (section 11) for a fuller discussion (in colour on the CD}. For the oxygen problem, see teaching note 2.

CLEAPSS


In this context, the object is to indicate that each hydrogen ion is associated with only half the electric charge associated with each copper ion in the electrolysis of copper.

Procedure

  1. The circuit is connected as shown.
  2. Only one of the two 250-ml burettes need be used. This should be filled with the 0.4 M sulfuric acid after it is placed over the cathode. One way to achieve this makes use of a squeezy plastic bottle, connected to the narrow outlet tube of the burette.
  3. Squeeze the plastic bottle firmly, and connect a tube between the bottle outlet and the top of the burette. Open the tap of the burette and allow the bottle to regain its normal shape. Air is removed from the burette. The tap is then closed, the bottle removed, emptied again of air and the procedure repeated until the burette is completely filled.
  4. Switch on the current and adjust it to a value of 1 A. Note that the position of the burette relative to the electrode has a marked effect upon the current.
  5. Allow the current to flow for, say, 20 minutes.
  6. After switching off, slide the burette in its holding clip until the levels of the water inside and outside the tube are the same. Read off the volume of the hydrogen gas. The mass liberated by the current in twenty minutes can be determined (density of hydrogen is about 0.1 kg /m3).
  7. Repeat part 4 for the volume of the oxygen (density of oxygen is about 1.43 kg /m3).
  8. Compare with the results for part 4.

Teaching Notes

  • Pure water will not conduct very well and so a little acid (about 2%) has to be added to provide plenty of ions, but it is ultimately the water that is used up to provide the gases.
  • The masses of hydrogen and oxygen collected can be calculated if you know the density of the gases and the volumes in the tubes. (Density of hydrogen is about 0.1 kg /m3, and the density of oxygen is 1.43 kg /m3). Oxygen dissolves in water, and so the water should be saturated with oxygen before the collection of gases begins. Do this by running the experiment without the collecting tubes initially.
  • The fun of this experiment is identifying the gases. You can collect a syringe full of gas by doing this: connect a syringe to the top of the burette, open the tap of the burette, and withdraw the syringe plunger gently so that the syringe fills with gas. You can then transfer the gas to a test-tube, collecting the gas by downward displacement of water. (A full test-tube of water is inverted in a small trough of water. A tube from the syringe leads into the test-tube, and the plunger of the syringe pushes gas into the test-tube.)
  • The test for hydrogen uses a lighted splint. The test for oxygen uses a glowing splint. Do not test the gas directly in the burettes - use a test-tube as described above.
  • The comparison between hydrogen and copper suggests that only half as many copper atoms have to travel across compared with hydrogen for the same amount of current x time or charge. From simple chemical measurements we know that a copper atom is 64.5 times as massive as a hydrogen atom. If copper ions were Cu+ with (one + e charge), you should expect electrolysis of copper sulfate to deliver a mass of copper 64.5 times the mass of hydrogen delivered by the same current x time. In fact you only get 32.25 times. This shows that copper ions are Cu++ not Cu+ each with a double charge of +2e (where e is the charge on one hydrogen ion). Copper ions make copper sulfate solution blue instead of the usual brown colour of copper metal; that charge makes all the difference.
  • How Science Works Extension: This experiment can be made more quantitative by recording the volumes of gases collected at intervals of time as electrolysis proceeds. Note that, in principle, the volumes of the gases should all be measured at the same pressure (as described above); however, you should be able to obtain reasonable results simply by reading off the burette scales. You should find that volume is proportional to time. Repeating with a different current should show that volume of gas is also proportional to current.
  • Hence mass of gas released is proportional to current x time. If your students think of current as a flow of electrons, you can point out that current x time is a measure of the number of electrons which have passed through the solution.

This experiment was safety-tested in January 2007

Up next

Electric charge using capacitors

Electric Current
Electricity and Magnetism

Electric charge using capacitors

Practical Activity for 14-16

Demonstration

Shows that electrostatic charges are the same as the charges passing through wires and components in electric circuits.

Apparatus and Materials

Capacitors and resistors as follows

  • 500 μF electrolytic capacitor (50 V working)
  • 50 μF electrolytic capacitor (350 V working)
  • 0.001 μF capacitor (15 kV working) See technical note 3
  • 4.7 kΩ. resistor (2 W)
  • 100 kΩ resistor (2 W)
  • Ammeters (preferably 2.5-0-2.5 mA.), 2
  • EHT power supply (0–5 kV), output current limited to 2 mA. or 5 mA. on old models
  • HT power supply (0-300 V), VERY HAZARDOUS BECAUSE OUTPUT CURRENT IS HIGH
  • Low voltage power supply (0-12 V)
  • Electrostatic generator, Van de Graaff or Wimshurst
  • Lamp holder (BC) on base
  • Lamp, 240 V, 15 W,in holder
  • Safety screen
  • Leads, including shrouded leads

Health & Safety and Technical Notes

Safety screens should also be set up in case the capacitor explodes.

Read our standard health & safety guidance


With electrolytic capacitors, correct polarity should always be observed, and, if they have not been used recently, they should be re-formed: see CLEAPSS Lab. Handbook...

CLEAPSS


It is useful to have two identical demonstration meters fitted with d.c. dials (2.5–0–2.5 mA.)

A 0.001 μF capacitor (15 kV working) is probably suitable. These are available from RS Components Ltd (117-473 or 119-097).

Safety HT power supplies must always be connected with shrouded plug leads and never with bare crocodile clips.

When high voltages are used, high-voltage leads should be used and all bare terminals securely insulated. EHT supplies are limited to 2 mA., but when the capacitor is charged to 5,000 V it is ready to discharge a very high initial current. The charge stored is only about 5 μC and the current would die out in a few microseconds, yet the shock would be unpleasant. So well-insulated flying leads are essential.

Procedure

Charging a 500 μF capacitor (no resistance in circuit)

  1. Set up the series circuit shown, setting the low voltage supply to 4 V and using the 500 μF electrolytic capacitor (50 V working).
  2. On completing the circuit, the transient nature of the current will be revealed by the two galvanometers. Students will see the momentary pulses of current. Allow the capacitor to discharge by disconnecting the lead N from the supply and connecting it to M.
  3. Charging a 500 μF capacitor through a large resistor:
  4. Modify the circuit above by including a 4.7 kΩ. resistor in series with the capacitor. Use 12 V from the supply.
  5. Charge the 500 μF capacitor as before. In this experiment, the slow charging process will be apparent. Students will see the current dying exponentially as the charge rises to the full value.
  6. Again connect lead N to lead M to show the discharge through the resistor.
  7. Charging a 0.001 μF capacitor to a high voltage
  8. Set the EHT power supply to provide 5 kV and then use it to charge the 0.001 μF capacitor. This is best done by holding the capacitor horizontally in a clamp and connecting the stud mounting end to the earthed negative terminal of the power supply.
  9. Connect the positive terminal to the capacitor through a 100 kΩ. resistor. (If the power supply has a built-in 50 MΩ. safety resistance, you could use this in place of the 100 kΩ. resistor. But, as the safety resistor is less obviously part of the circuit, it is better to use a separate 100 kΩ resistor.) Care should, of course, be taken when working at these voltages.
  10. Connection between the end of the resistor and the top of the capacitor is made with a well-insulated flexible lead, held by hand.
  11. After a moment or two, remove this lead. Use another insulated lead to short-circuit the capacitor.
  12. Charging a 0.001 μF capacitor using an electrostatic generator
  13. Again it is best to clamp the capacitor in position.
  14. Where a Wimshurst is used, the connection can be made directly. Where a Van de Graaff is employed, flying leads must be used. In the latter case, the well-insulated flying lead is best held by hand against the sphere, so that it can readily be removed from contact and used to short circuit the capacitor. Care must, of course, be exercised: 1- or 2-cm sparks can be obtained from a capacitor charged in this way.
  15. The circuit should have a high resistance included in it, such as a piece of wet string, in order to slow down the charging and avoid overstraining the capacitor.
  16. Once the capacitor is charged, then bring another well-insulated lead from one of the terminals round to the other terminal of the capacitor. There will be a small spark.
  17. You could also connect the terminals of the charged capacitor to a gold leaf electroscope.
  18. Immediately afterwards, set up the Van de Graaff with its discharging ball connected to earth through a sensitive galvanometer. Each time a spark leaps across to the ball then a deflection will be shown on the galvanometer. Take care - the capacitor stores more charge than the dome of the Van de Graaff, and so the initial current will be larger and the spark fatter.
  19. Note: The capacitors used are not intended for use at these voltages and may break down. If the capacitor is damaged replacement may be impossible._
  20. 500 μF capacitor discharged through a lamp
  21. Charge a 50 μF electrolytic capacitor (350 V working) from an HT power supply set to give 240 V. A safety resistor of 100 kΩ. should be included in the charging circuit. Allow thirty seconds for charging.
  22. Disconnect the capacitor and allow it to discharge through a 240 V, 15 W mains lamp. (The safety resistor should not be included when discharging.)

Teaching Notes

  • The electric charge measured by current x time is the same kind of thing as the electric charge that you gather by rubbing plastic with wool, or pile up on the dome of a Van de Graaff machine.
  • These demonstrations show that charges can be obtained from:
    • current sources such as batteries, power packs and dynamos;
    • electrostatic sources such as friction - charged insulators or a Van de Graaff.
  • They also show two kinds of behaviour from charges:
    • electric current behaviour, such as lighting a lamp or moving the pointer on a sensitive meter;
    • electrostatic behaviour, such as sparks or attracting small pieces of paper.
  • The capacitor is simply a pair of metal plates, separated by an insulator, rolled up and housed in a box. The demonstrations show charges running to those plates, and from them, and the charges accumulated on those plates producing sparks or making an electroscope leaf rise.
  • In step 1 when the current is turned on, there will be a momentary pulse. Positive charge will be piled up on one plate and negative charge on the other. When the battery is removed from the circuit and the charged capacitor is shorted through the meters, the pulse will be in the opposite direction.
  • In step 3 the charging currents are smaller, the charging process is slower, and you can see the currents dying away exponentially as the charges on the capacitor rise to their full value.
  • Unfortunately the charges stored on the plates by a low voltage battery will not create sparks, and so the charging voltage must be increased.
  • In step 3 the high resistance will reduce the currents and slow the charging process down so that the higher voltage can be used.
  • In step 6, you can show current effects by removing the EHT supply and letting the capacitor discharge through the high resistance and the meters.
  • You can show electrostatic effects by taking one terminal of the charged capacitor round to the other terminal and producing a spark. The energy dissipated by the spark is 1/2 CV2, where C is the capacitance and V is the potential difference.
  • Step 10 shows that the charges which are now resting on the plates can produce an electrostatic effect.
  • In step 17, lighting the lamp reinforces the idea that the charges piled up on the plates of a capacitor behave just like the charged plates of a battery, albeit a battery which can only supply a limited amount of charge.

This experiment was safety-tested in January 2007

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From galvanometer to ammeter

Ammeter
Electricity and Magnetism

From galvanometer to ammeter

Practical Activity for 14-16

Class practical

Adding a shunt in parallel with a galvanometer converts it to an ammeter with a higher range. This is a trial and error method, not one involving calculations.

Apparatus and Materials

For each student group

  • Galvanometer
  • Ammeter, 0 to 1 A, DC
  • Cell, 1.5 V in holder
  • Power supply, low voltage, DC
  • Lamp in holder, 12 V, 36 W or 24 W
  • Eureka wire, 28 SWG or thicker
  • Leads, 4 mm, 6

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

Making an ammeter:

  1. Your galvanometer is designed to measure small currents of a few milliamps. When the pointer is at the end of the scale, the current through the little coil which moves with the pointer must be, say, 0.01 A (or whatever your galvanometer is built to measure there). Suppose you wish to use it to measure much larger currents, say 1 A, at the end of its scale. The rest of that large current (1 A minus 0.01 A, for example) must travel by an alternative route a loop line in parallel.
  2. For that loop line or shunt, connect a short piece of alloy wire across the terminals of your galvanometer, as in the diagram. Take care! If, when adjusting the shunt, you let the whole big current go through the galvanometer, even momentarily, you might damage the galvanometer badly.
  3. Start with a very short shunt, straight across from terminal to terminal. Make a very rough test of that by connecting in series a lamp, your shunted galvanometer, a commercial ammeter (for comparison) and one 1.5-volt cell - just for a safe first trial.
  4. Switch on the current just for a moment, to see whether the pointer moves too far or too little.
  5. Adjust the length of shunt by trial and error. Shorten or lengthen the shunt until your home-made ammeter seems to read roughly what you want it to read.
  6. Disconnect the battery from your test circuit and replace it with the power supply, set to give 12 V. Adjust the shunt more carefully till you have a good ammeter.
  7. A commercial ammeter is constructed like this. It is a milliammeter with a shunt. Sometimes the basic instrument has several removable shunts to make it an ammeter with a choice of several ranges-as in the case of multimeters where you can select a range by turning a dial.

Teaching Notes

  • Those who are keen to use their knowledge of resistance can have a go at converting a milliammeter to an ammeter. This is the job which a meter shunt does. The correct resistance has to be connected in parallel with the milliammeter in order to allow it to register amps. It does this by sending most of the circuit’s current through the shunt and tapping a small fraction of it to send through the meter.
  • The commercial ammeter enables students to adjust their home-made one to read as they wish, by a trial and error method rather than one in which the resistance of the shunt is calculated.

This experiment was safety-tested in October 2006

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Quantitative ideas in electricity

Electrical Circuit
Electricity and Magnetism

Quantitative ideas in electricity

Teaching Guidance for 14-16

Introductory level ideas

At introductory level, the descriptions of what happens in electric circuits are simply qualitative. It is not appropriate to discuss concepts quantitatively.

Intermediate level ideas

Defining current

Current can be described as a flow of charge measured in coulombs. You then describe and define the coulomb in terms of copper plating. You can even state that unit current, one ampere (or amp), means one coulomb per second in terms of copper plating (0.000 000 329 kg of copper carried across every second in a copper plating bath). Although that does not agree with the present fashion of defining currents by forces, it gives students a much easier way of picturing currents. They already have, from common knowledge, a strong feeling for currents as streams of little electrons, and if you bunch those electrons into large coulombs of charge you can easily persuade them to think of currents being measured in coulombs per second.

Defining Potential difference

Once students understand energy transfers, potential difference can be discussed clearly, and the volt defined as a joule per coulomb. Discussing power supplies as sources of energy, and electric charges as carriers of energy helps the beginner to understand why a current in a series circuit does not diminish as it flows through energy transfer components such as lamps. You can treat potential difference as a fundamental measurable quantity, described as energy transfer for each coulomb that passes through the region in question; e.g. the energy transferred from the battery to the lamp and hence into the environment.

It is of course unscientific fantasy to picture coulombs carrying loads of energy on their backs and disgorging some of the load in each part of the circuit, then gathering a fresh load each time they pass through the battery. Yet if you warn students from time to time that this is an artificial picture, with unrealistic details, they can use the model to develop a useful insight into potential difference.

Then resistance, which may well be more convenient in developing a professional scheme of electrical units, takes a secondary place as [potential difference]/[current] with one ohm merely defined as a name for one volt/amp. That is just dictionary work.

With these descriptions and definitions of potential difference and current, it is obvious that potential difference x current gives us the power, the rate at which energy is transferred. In slang terms ‘volts x amps = watts ’.

And when you generate an e.m.f. you can give a clear description of that concept too.

Advanced level ideas

In more formal treatments of electricity, unit current is chosen as the fundamental quantity (defined in terms of the force between parallel currents). Resistance is a useful derived quantity, a secondary standard that can be preserved and copied easily. Then the unit of potential difference is derived from the units of charge and energy (or current and power).

However convenient that scheme may be, it leaves the nature of potential difference itself without a clear description. Certainly at introductory level, students find ‘voltage’ a mysterious concept, often vaguely described as an electrical pressure, and frequently described as multiplying current by resistance. When the use of potential difference is extended to cases where there is no current, or cases where there is no Ohm’s law resistance, it remains very puzzling.

Developing electrical knowledge – from introductory to advanced level

There is a danger here of confusion between several different purposes in building electrical knowledge. There is the matter of careful definition of fundamental units and the deriving of secondary units; that is a matter for advanced level courses. There is the matter of describing and defining physical quantities to be measured in those units. There you need to know the physical relationship, extracted from experiments, such as thermal transfer varies as the current2, or rate of copper plating varies as the current. There are ‘operational’ definitions, in the technical sense of that word, which describe the scheme of measurement in terms of actual apparatus that could be used.

In earlier days, scientists sometimes used concepts that could not be given an operational definition. Nowadays they are more careful and try to define, or at least describe, concepts of physical quantities in terms of possible, or at least conceivable, methods of measuring them. Such definitions should yield a clear knowledge of the concept; but they do not always lead to the most convenient unit in which to measure the physical quantity. The unit chosen may be defined quite separately – you often find it was chosen earlier in the history of the subject.

There is no logical objection to defining the unit of current in terms of the mass of copper deposited per second in electrolysis, although current is formally measured in terms of force between wires or coils carrying currents.

Up next

Electric current

Electric Current
Electricity and Magnetism

Electric current ideas

Teaching Guidance for 14-16

You can define one coulomb as one ampere-second but that is of little value in giving students a picture of it. Talk of coulombs as things that go round the circuit, the things that you might count flowing past any point you might choose in the circuit, much as a policeman might count ‘cars per minute’ for traffic flow or a hydraulic engineer ‘gallons of water every minute’. In electric circuits scientists and engineers count coulombs per second and call them amps.

Introductory level ideas

At introductory level, students need only have the idea that something flows around a complete circuit, transferring energy from the power supply to devices such as lamps.

Worries about absolute standards and units belong in advanced level teaching and not in an introductory scheme of activities. Students accept the kilogram as a well understood unit when it is no more than a copy of some chosen standard.

If students ask how anyone knows the size of a coulomb, the best reply at introductory level is that you simply read the ammeter which tells the rate at which coulombs are passing and multiply by the time in seconds. That puts the blame for the definition on the ampere, and the ampere you could say is defined by the reading of a standard ammeter kept at some standardizing laboratory in each country in the world. You may need to point out to students how one ammeter can be compared with another and that in turn with a standard ammeter.

Intermediate level ideas

At intermediate level, instead of saying what things are travelling in a circuit, electrons or electric charges, emphasize a cruder view. Say that something travels that you measure in chunks called coulombs. Coulombs travel along a wire in a circuit and you can count them as they go by with an ammeter and a clock.

Students need to be equally confident in expressing some energy transfers in joules per coulomb. You can price oranges in pence per dozen, or milk in pence per litre, knowing quite well what kind of things a dozen and litre are. So, make a coulomb too almost real, by tracing it round the circuit pushed by coulombs behind it and pushing the coulomb in front (by means of electrostatic fields), slipping smoothly through low resistance, banging its way through high resistance, shoving against the edges of the armature-wires in a motor, carrying the material of chemical ions across with it in electrolysis; always paying out joules as it goes.

Say a current of 2 amps means that 2 coulombs of electric charge pass each point in a circuit every second. Then whenever you give pupils data, ask them to interpret it: the current is 2 amps; that means 2 coulombs per second. If the current is 5 amps, how much charge passes a given point in 10 minutes?

Advanced level ideas

In advanced courses, 1 amp is defined as the current which when flowing in each of two infinitely long parallel wires one metre apart produces a force of 2 x 10-7 newtons on each metre of either wire. That definition has two virtues: it reduces the number of arbitrary standard units and it makes it possible to carry out some very important calculations; e.g. the force on a current-carrying coil placed in a region near another current-carrying coil. This should not worry beginners to the subject.

Students may ask if a coulomb is the same kind of thing as an electron. Say that you can think of the electron as a very tiny particle which has a mass like any other piece of matter but also carries a charge. A coulomb is the charge of a vast number of electrons. In fact the size of an electron’s charge can be measured in coulombs; that means experimentally comparing two sizes, one electron charge and one coulomb of charge, i.e. Millikan’s experiment.

One electron charge = 1.6 x 10-19coulomb

One coulomb is 6 x 1018electron charges

A coulomb is always 6 x 1018 electron charges but in many cases of currents (e.g. in conducting solutions, in gases and in some semiconductors) some of those charges are negative and moving one way and the rest are effectively positive and are moving the opposite way.

If we were beginning the development of the science of electricity all over again in the twenty-first century, we might well take the electron charge as our basic unit instead of the coulomb. But electricity is too well established for us to make that change comfortably. Also the electron charge would prove far too small for convenient use in many practical applications.

Up next

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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