Seeing parallel circuits as two loops

A parallel circuit is two loops

Thinking about the learning

If pupils are asked to build a circuit so that…

one battery lights two bulbs to normal brightness

… more often than not, they use the single pair of battery terminals to make two circuit loops.

When the circuit is constructed and drawn in this way as two separate loops, which share a single battery, it makes good sense to pupils that:

• Each bulb is of normal brightness.
• Each bulb has a current in it which is equal to that produced in a simple one battery and one bulb circuit.
• The battery flattens more quickly because it is providing energy for both circuits and bulbs.

These pupil insights provide a very helpful starting point for teaching about parallel circuits.

Thinking about the teaching

To help pupils understand parallel circuits as consisting of two loops, and to relate them to these conventional formats, we have found the following sequence to be very useful.

• Build a circuit with a single loop to the right of the battery, using short wires of one colour and a single bulb.
• Detach that loop from the battery, and put it to the right hand side.
• Build a circuit with a single loop to the left of the battery, using long wires of a different single colour and a single bulb.
• Detach that loop from the battery, and put it to the left hand side.
• Now attach both loops, keeping them on the same side.
• Now lift the longer loop over to the right, without breaking any connections.

By following this sequence through, the point can be made that in the parallel circuit, each bulb has the same current in it and that therefore there is a double current (the sum of the currents in each loop) in the battery. The double current shifts energy at double the rate.