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

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

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

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.
Charging a 500 μF capacitor through a large resistor:
3. Modify the circuit above by including a 4.7 kΩ. resistor in series with the capacitor. Use 12 V from the supply.

4. 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.
5. Again connect lead N to lead M to show the discharge through the resistor.
Charging a 0.001 μF capacitor to a high voltage
6. 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.
7. 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.
8. Connection between the end of the resistor and the top of the capacitor is made with a well-insulated flexible lead, held by hand.
9. After a moment or two, remove this lead. Use another insulated lead to short-circuit the capacitor.
Charging a 0.001 μF capacitor using an electrostatic generator
10. Again it is best to clamp the capacitor in position.
11. 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.
12. 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.
13. 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.
14. You could also connect the terminals of the charged capacitor to a gold leaf electroscope.
15. 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.
16. 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._

500 μF capacitor discharged through a lamp
17. 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.
18. 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

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