Predicting and measuring electric currents

What the Activity is for

Having introduced what is involved in measuring electric currents, the pupils are now given the opportunity to make some measurements for themselves. This is an important practical activity for directly addressing the idea that the electric current does not get used up.

The approach taken is to encourage the pupils to think and talk about the electric circuit model and teaching model before taking the actual current measurements. To this end, the pupils are asked to make predictions of current values before they make each measurement.

What to Prepare

• batteries
• bulbs
• ammeters
• connecting leads
• support sheet: predicting and measuring currents

Support sheet

For the purposes of this activity, it is much easier for the pupils to use digital meters. Given the aim of establishing that the current is the same around each circuit, it makes sense to use less sensitive ammeters. It is not helpful for pupils to measure current to one-hundredth of an ampere and then to worry about differences between readings in the second decimal place. The scenario to avoid is:

Lizzie: Miss! The first was 0.72 ampere and the second was 0.73 ampere. The currents just aren't the same.

Avoid this by using an ammeter with a scale which reads to one-tenth of an ampere.

What Happens During this Activity

You might introduce this activity in the following way:

Teacher: OK, we have the idea that the ammeter measures the electric current and this gives us the amount of charge passing each point per second. If we measure the current here, and then here on the other side of the bulb, what would you expect to get?

Teacher: First of all, talk it through with your partner. Think about the electric circuit model with the charge moving round. Think about the rope loop. I'll give you a few minutes and then we'll talk through your ideas.

Teacher: OK, people are suggesting that the current should be the same in both places because the rope just keeps going round, or the charge just keeps moving round the circuit. As James says, none are added or lost.

Teacher: Well, collect the equipment and use an ammeter to measure the current values for yourselves. There are 3 circuits to investigate. Go to it!

For this activity, the pupils should ideally work in pairs and each pair has one ammeter, which is placed in the different positions in circuits A, B and C as shown on the support sheet. Some pupils may think that they need three ammeters for Circuit B. This is not the case!

As the pairs of pupils complete their measurements, it is a good idea to collect the current values on the board or on a large sheet of poster paper, so that the pattern of findings becomes apparent to all.

The current stays the same

The current stays the same all of the way round each of the circuits. By this stage the pupils should have a clear picture of why this makes sense.

In terms of the electric circuit model:

The number of charged particles passing per second is the same all around the circuit. In the bulbs, energy is shifted but the charged particles keep going.

In terms of the rope loop:

All parts of the loop move around at the same speed (watch the red ribbon) as the battery sets it in motion.

Be sure to allow pupils to talk through these new ideas, as they review the current values for the whole class.