So why does the battery go flat more quickly?

More flow – the same in each loop

Wrong Track: The current leaves the battery and splits, some down each loop.

Right Lines: Each loop has a flow just as it would on its own. So each one flattens the battery just as it would on its own. There are two loops, so twice as much current, and so the battery is working twice as hard.

Flattening the battery more quickly

Thinking about the learning

When the second bulb is connected in parallel, the current in the battery doubles. This means that double the number of charged particles are flowing through the battery each second. With double the number of charged particles, energy must be shifted from the battery's store at twice the rate. So, the battery's store is depleted of energy in half the time.

Thinking about the teaching

In terms of the electric circuit model:

Teacher: OK, so with one battery and one bulb, we can picture the charged particles moving through the battery. When a second bulb is added in parallel, an extra loop of charged particles is provided. The number of charged particles passing through the battery each second is therefore doubled. So, with the second loop, the charged particles shift energy from the battery at twice the rate and the battery will go flat more quickly.

In terms of the rope loop:

Teacher: With one loop, I have to work to keep the rope moving and Julia feels her hand warming up as she grips the rope. If I add a second loop of rope, I now need to increase my rate of work to keep both loops going and both Julia and Anita feel their hands warming up. Of course, in this case I get tired more quickly!