Electrical Circuit
Electricity and Magnetism

How come the bulb lights straight away?

Teaching Guidance for 14-16 Supporting Physics Teaching

Revisiting the idea of remote working – it's nearly instantaneous

Wrong Track: Look! The bulb is a long way from the cell so it's obvious that it will take time for the energy to arrive and for the bulb to light. The current comes from the cell. It has to go through the cell to collect the energy and it will take time to get to the bulb.

Right Lines: No! When the circuit is completed the bulb lights immediately because the current is (almost) immediately the same in all parts of the circuit.

Avoiding unhelpful models

Thinking about the learning

The wrong track thinking shown here involves two common (mistaken) ideas:

  • The current originates in the cell.
  • The charges must collect energy from the cell before that energy can be shifted in the bulb.

Neither of these ideas is correct. The charge that constitutes the current is already present in all parts of the circuit and, as soon as the circuit is completed, the charged particles start flowing and energy is shifted by the bulb.

Thinking about the teaching

The teaching challenge here is to emphasise the immediate-all-at-once nature of the charge flow in an electric circuit. Without doubt this is best addressed with the rope loop analogy. In fact, if you think that your students might have lingering problems in this area, it would be worth going back to the big circuit and rope loop combination of activities first introduced in the SPT: Electric circuits topic.

Set up the big circuit in the lab and invite students to predict whether or not the bulb will light straight away when the circuit is completed. The bulb lights immediately. Now invite the students to use the rope loop analogy to explain why this is the case.

As one 13-year-old girl explained:

Jill: It's obvious! As soon as the battery starts the rope moving, the bulb heats up as the rope passes through. There's no delay because the rope all starts moving together.

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