Electrical Circuit
Electricity and Magnetism

The resistance sets the current for the whole circuit

Teaching Guidance for 5-11 11-14 Supporting Physics Teaching

One current set by the resistance

Wrong Track: The current is smaller in the filament of the bulb because of the very high resistance, but then speeds up and gets bigger through the conducting wires.

Right Lines: The extra resistance at a single place in the circuit has the effect of reducing the current around the whole circuit. The same amount of charge passes per second through each and every part of the circuit.

The current is reduced in the whole circuit

Thinking about the learning

A further important point to emphasise is that when additional resistance is introduced to a circuit in one place, the current is reduced everywhere in the whole circuit. This can be the source of confusion for pupils.

Thinking about the teaching

It may be helpful to think of the resistance as providing the rate-determining step for the whole circuit. The rope loop is useful in establising this. As further resistance (more pupils gripping the rope) is added to the circuit, the movement of the whole rope loop is slowed down. All parts of the rope loop move around at the same speed.

It is worth exploring the effect on the current of a full range of resistance.

Teacher: What would happen to the current if more and more bulbs were added to the circuit?

Quite simply, as more bulbs are added, the resistance in the circuit increases, the charge moves around more slowly and the current gets smaller. With an infinite resistance, the current would fall to zero. We can demonstrate this simply by making a gap in the circuit.

Teacher: What would happen to the current if all of the bulbs were removed from the circuit and we were left with a connecting lead from one side of the battery to the other?

This is called a short circuit and is to be avoided! Because there is very little resistance in the circuit (apart from that provided by the battery itself), a very large current results with the charged particles moving around very quickly. It is likely that the battery will be damaged, or the wire become so hot that it will burn you or even melt!

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