Something for nothing?

Getting more: paying nothing?

Wrong Track: There's no extra battery. We're getting the second bulb's worth for nothing!

Right Lines: Energy is shifted to the surroundings by the two bulbs at twice the rate and as a result the battery goes flat more quickly. When the second bulb is added in parallel it is just as bright as the first one. Energy is shifted by the two bulbs at twice the rate and so the battery does not last as long.

More energy shifted

Thinking about the learning

The fact that adding a second bulb in parallel results in the two bulbs being of normal brightness is counter-intuitive for most pupils. Their underlying reasoning is that the single source (or battery) is now being shared between the two consumers (or bulbs) and that they should therefore be dimmer, just like the bulbs in series.

At first it seems as though adding bulbs in parallel gets you something for nothing!

The incorrect idea here is that by adding a second bulb in parallel, it is possible to get twice the energy output without further energy cost.

Thinking about the teaching

We suggest two possible ways of addressing this idea of something for nothing.

Help pupils make sense of their observations in terms of the battery going flat more quickly:

Teacher: At first it seems as though we're getting something for nothing, but the drawback is that with the two bulbs in parallel the battery flattens more quickly. We shift the energy stored in the battery more quickly.

Help pupils to picture what is going on in terms of an identical flow of charge through each independent loop. As a consequence there is an increased flow of charged particles through the battery, which results in energy being shifted by the battery at an increased rate. This detailed approach is discussed in the next challenge.