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More bulbs, more batteries

Electricity and magnetism · level 2, lesson 5 of 6

A question to start

One battery lights one bulb. What happens if you add a second bulb in the loop? What if you add a second battery instead?

The idea

In a single loop:

  • More bulbs in the loop make each bulb dimmer. The same battery has to push the electricity through more of them.
  • More cells, joined + to −, make the bulbs brighter. They push harder together.
  • Too many cells for a small bulb can make it so bright it burns out, and then the whole loop goes dark.

If one bulb in a single loop breaks, there is a gap, and every bulb in that loop goes out.

1 cell, 1 bulb1 cell, 2 bulbs: dimmer2 cells: brighter

Worked examples

1. Fairy lights. Some old strings of lights were one long loop. One broken bulb made the whole string go dark.

2. A torch. A torch with two cells in a line is brighter than it would be with one.

3. A flat battery. As a battery runs down it pushes less hard, and the bulb slowly gets dimmer.

Try it

  1. What happens to the bulbs when you add another bulb to a single loop?
  2. What happens when you add another cell, the right way round?
  3. In a single loop of three bulbs, one breaks. What happens to the others?
  4. Why can too many cells be a problem?
Answers
  1. They all get dimmer.
  2. The bulbs get brighter.
  3. They go out, because the loop now has a gap.
  4. They can push so hard that the bulb burns out.

Common mistake

"The first bulb in the loop is brightest." In a single loop, identical bulbs glow equally. The electricity is not used up on the way round.

Where it leads