Use of a voltmeter

Class practical

Experimenting with a voltmeter leads to a discussion of the meaning of potential difference or voltage.

Apparatus and Materials

• Power supply, low-voltage
• Lamp (12 V, 6 W) in holder
• Ammeter, (0-1 A), DC
• Voltmeter, (0-15 V), DC

Health & Safety and Technical Notes

Choose a low voltage power supply so that the meters are unlikely to be damaged, e.g. use 2 x 1.5 V cells.

Read our standard health & safety guidance

Procedure

1. Connect up a simple series circuit using the battery, a lamp in its holder, and an ammeter.
2. Now add the voltmeter to the circuit. Use it to measure the voltage across the lamp.

Teaching Notes

• Allow students to decide for themselves how to connect the voltmeter. They should be allowed to connect the voltmeter in series in the circuit. They will soon discover that there appears to be no current left in the circuit. This is because the voltmeter has a very high resistance.
• Putting the ammeter across the lamp could damage the ammeter. Make sure that it can withstand the shock if students do it accidentally! The reason for this is that the ammeter has an extremely low resistance, and so most of the current takes the easy route and avoids the lamp.
• When considering how fast energy is transferred in an electric circuit, the output energy is directly proportional to charge, or current x time, or the number of coulombs that pass, measured by ammeter and clock.
• However, charge cannot be the only factor. You also need a device that shows how many joules of energy are delivered by each coulomb going through the lamp. That device is called a voltmeter.
• Just as a mass of one kilogram can transfer more energy if the cliff it falls off is 100 metres high than if it is only 10 metres high, so a coulomb of electricity delivers more energy if it falls through a potential difference of 100 volts than if it falls through a potential difference of 10 volts. Also, more obviously, more energy is transferred by several kilograms or coulombs than by one. Analogies of helter-skelters or loaves of bread on delivery vans may help to get the idea across.
• An analogy for a voltmeter might be the following.
• The flow of coulombs in a circuit can be likened to the flow of cars along a main road. Suppose that every driver arrives at point A on the road with the same amount of money in his pocket and has spent all of it at point B. To find out how much money that is, you need not hold up every car and examine the driver's finances. Instead you could arrange to divert just a few cars out to a lay-by at A and back onto the main road again at B. Somewhere along the lay-by install a hold-up gang who empty the pockets of each driver in the diverted stream. That is a model of a working voltmeter (actually a milliammeter in disguise).
• The water circuit with its pressure gauge as a model for a voltmeter could also be used here. The pressure gauge is connected across the tubes to measure the pressure difference between the two sides.
• Whatever explanation you give, end up by saying that the voltmeter measures the energy transferred to the lamp by each coulomb, as it passes through part of the circuit across which the voltmeter is connected.
• The voltmeter is connected across the lamp so that it can measure the joules per coulomb transferred to the lamp. The ammeter measures how many coulombs per second pass through the lamp.
• Hence the energy transferred to the lamp = potential difference (measured in volts) x charge (measured in coulombs).
• Energy transferred = V x Q joules
• = V x I x t joules
• Energy transferred per second = power = V x I watts
• where V volts is the potential difference across the lamp, Q coulombs is the charge flowing through the lamp, I amps is the current through the lamp and t seconds is the time for which the charge flows.
• In discussing this experiment with students, continue to reinforce these new ideas at every opportunity:
• 5 volts means 5 joules per coulomb: a volt is just shorthand for joules per coulomb which is itself an abbreviated form of joules of energy transferred from the energy supply to a component in a circuit for every coulomb passing through it. So 5 volts (joules per coulomb) carried by a current of 2 amps (coulombs per second) transfers 10 joules of energy per second or 10 watts of power.

This experiment was safety-tested in January 2007