Voltage/Potential Difference
Electricity and Magnetism

Voltage - an activity

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

First of all, attention is focused on introducing what is meant by voltage. It is important that pupils have an understanding of what it is that they are measuring when they measure voltages in different parts of a circuit.

What to Prepare

  • 12 volt direct current power supply
  • 12 volt, 24 watt bulb in holder
  • connecting leads

What Happens During this Activity

It is a good idea to start with a demonstration of what happens in a simple circuit when the voltage of the supply is changed. You might start with the output of the power supply at about 6 volt and then switch it up to about 12 volt. You might also make links here to the practical work carried out by the pupils earlier in which they changed the number of batteries in a circuit.

Teacher: OK, so we start with the power supply set down here at around 6 volt. What do you notice about the brightness of the bulb?

Bill: It's dim.

Teacher: Yes, that's right. Now supposing I turn up the power supply here, increase the voltage on this scale. What do you think will happen? Predictions?

Jill: It'll get brighter.

Teacher: Well, let's give it a go [teacher turns up voltage]. Brighter yes. Where have you seen this kind of thing before?

Jill: If you add batteries it makes the bulb brighter.

Teacher: That's right! So if I add batteries or turn up the voltage, what happens with the bulb?

Bill: You get more energy from the bulb.

Teacher: Excellent! In fact the voltage is a measure of how much energy is provided by the battery for the charged particles in the circuit. If I increase the battery voltage more energy is shifted by the bulb. The bulb gets brighter.

Part 2: using a voltmeter

What the activity is for

The purpose of this activity is to introduce the voltmeter as an instrument that is used to measure voltages, and to develop further what those measurements actually mean.

What the activity is for

  • 12 volt direct current power supply
  • 12 volt, 24 watt bulb in holder
  • demonstration voltmeter
  • connecting leads

What happens during this activity

Following on from the previous demonstration and discussion, introduce the voltmeter as the instrument to be used in making measurements of the energy provided by the battery and shifted by the bulb.

Demonstrate how to connect the voltmeter by talking through the following kind of sequence with the pupils:

Step 1: make the complete circuit to light the bulb.

Step 2: connect the voltmeter, the right way around, across the power supply and take the reading.

Teacher: First of all let's make a measurement of the energy provided by the power supply. To do this we must connect the meter across the supply and we need to connect it the right way around, with the positive terminal of the voltmeter to the positive side of the supply.

Step 3: connect the voltmeter, the right way around, across the bulb and take the reading.

When connecting the voltmeter to the circuit emphasise the differences between using an ammeter to measure electric currents and using a voltmeter to measure voltages. The key point is that whereas the circuit must be broken to insert an ammeter, the voltmeter is connected across the relevant component in the complete circuit. The ammeter is connected in series; the voltmeter is connected in parallel.

Taking readings

The voltmeter reading across the power supply and the reading across the bulb will be the same. This is a fundamental point which needs to be talked through with the class:

Teacher: So the reading across the battery is 10.5 volt. The voltage value, 10.5 volt, is a measure of how much energy is shifted by the battery each time a coulomb of charge passes through the battery. A bigger voltage means more energy per coulomb of charge. What was the reading across the bulb?

Jill: Same.

Teacher: That's right, 10.5 volt. Why must it be the same?

Bill: Is it because the energy given by the battery equals the energy given out in the bulb?

Teacher: Exactly right! Excellent answer!

From here the ideas might be taken further:

Teacher: When we say that the voltage across the battery is 10.5 volt, this actually means that the battery shifts 10.5 joule of energy per coulomb of charge. The voltage is a measure of the number of joule of energy per coulomb of charge. With the bulb, the voltmeter reading is also 10.5 volt. This means that 10.5 joule of energy are shifted by each coulomb of charge as it passes through the bulb.

An energy hill diagram for the circuit might also be introduced as a focal point for further discussion and to support pupil understanding.

Voltage/Potential Difference
can be measured using Voltmeter

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