What does the electric current measure?
Teaching Guidance for 11-14
Current measures flow of charge
Wrong Track: The current measures how fast the charges are moving.
Right Lines: The current measures how much charge passes each second.
Current is more than just speed of charged particles
Thinking about the learning
The idea that the electric current is a measure of how much charge passes per unit time can be quite challenging for pupils. It is common for pupils to mix up how much charge passes with how fast the charged particles are moving. Some pupils refer to the electric current being faster or slower when changes are made to a circuit.
Mixing up these two ideas is understandable. For example, if we add a second cell to a circuit with one bulb, the current increases because the charged particles in the circuit move round more quickly (this circuit is considered in detail in episode 02). Nevertheless, we must be clear in stating that the electric current is measured in terms of the amount of charge passing.
With a big electric current, many charged particles pass each second.
With a small electric current, fewer charged particles pass each second.
Thinking about the teaching
A useful approach to getting over the idea of measuring electric current is to encourage the pupils to picture what is happening in the wires of the circuit – so starting with a teaching model.
You might start with the rope loop referring to the amount of rope that passes by each second. Then move on to the idea of the number of charged particles passing per second. You might well explicitly connect the length of rope passing each second to the quantity of charge passing in each second.
In introducing the ammeter, emphasise that its job is to measure how many charged particles pass through that point in the circuit each second. In such a way, talking about the ammeter helps to clarify and reinforce the concept of electric current.
You could almost imagine that there is a little helper inside the ammeter. The little helper is armed with a stop watch in one hand and a counting stick in the other. As the charged particles pass through the meter the little helper has the job of counting the amount of charge passing each second, and this is the current reading.
When the pupils start measuring electric currents in ampere, take every opportunity to talk through what is meant by the readings that they take.
Teacher: So, the current here is 5 ampere and the current there is also 5 ampere. Anybody – what's going on with the charged particles in the circuit?
Paula: The amount of charge passing through both points each second is the same.
Teacher: Excellent! Same charge passing everywhere, per second.