Just how big can the current get?

A good teaching question

Good teaching questions that challenge pupils' understanding and encourage discussion can often be generated by considering extreme cases.

So:

Teacher: Is there any limit to how large the current can become as more batteries are added to the circuit?

In (simple) theory as more batteries are added, the charged particles in the circuit are simply pushed round more quickly, so the size of the electric current increases. In practice, there is a limit to the size of electric current through the bulb – the filament melts.

Teacher Tip: Explore edge cases to investigate what a model predicts about a situation.

But do take care: when moving from one to two batteries in a circuit, pupils often anticipate (sensibly) that the current will double in strength. What happens in practice is that the current certainly increases, but not to the extent of doubling. The reason for this is that as the current in the bulb increases, the temperature of the filament is higher and its resistance increases. The increased thermal agitation of the atoms of the filament makes it harder for the charged particles to pass.