The Constant Current Misconception

If you have a simple battery and bulb circuit and you add another bulb in parallel, would you say the current now 'splits' at the junction?

Or imagine starting with the same simple circuit and adding a bulb in series. Could you explain the fact that the bulbs are both dimmer by saying that the battery’s energy is now shared between two bulbs?


If you think either of these explanations seem pretty reasonable then you may hold the constant current misconception.

The constant current misconception is the implicit belief that batteries are constant current providers.

In the parallel example there isn't a 'the current' to split. When you add the extra bulb in parallel then the current drawn from the battery doubles, it doesn't just split differently.

In the series example the assumption is extended to imply that batteries provide energy at a constant rate. They don't. When the extra bulb is added in series then the battery provides energy at half the rate. It doesn't provide energy at the same rate and then share it out differently.

Batteries are constant voltage providers (as long as you don't make them work too hard) and the current they provide depends on the circuit they are connected in. Any change to the circuit will always change the current.