I understand that a circuit with just one bulb will be brighter than a circuit with two, because Ohm's law states that current is inversely proportional to distance, and therefore the greater resistance presented by two bulbs will cause a lower current to pass through the circuit.

The thing that I am unsure about is why all of the bulbs in a series circuit will have the current shared equally and therefore be equally bright, as my intuition would say the first bulb would receive more energy than the second, and that more than the third etc. At the risk of asking a silly question, how do the electrons 'know' how much energy to give the first bulb without having any obvious way of 'knowing' the number of bulbs / total resistance of the system?

My teacher said he thought it might have something to do with the idea that the electrons pass round the circuit repeatedly, so the charge averages in some sense, but if this is true would the first light seem brighter if the circuit was only activated for long enough for the electrons to complete a single circuit?

  • $\begingroup$ Possible duplicate: physics.stackexchange.com/q/7936/50583 $\endgroup$ – ACuriousMind Oct 27 '16 at 14:56
  • $\begingroup$ You should note that questions about brightness are based on the behaviour of old-fashioned incandescent filament light bulbs of the sort that are no longer legal to sell in Europe and some other regions. CFL or LED bulbs for example will not display the same sort of difference in brightness in the same range of circumstances. $\endgroup$ – RedGrittyBrick Oct 27 '16 at 15:09
  • $\begingroup$ @RedGrittyBrick While that's true for domestic lighting, I am pretty sure filament type incandescent auto bulbs will be available for a long time as they're still fitted to new cars. $\endgroup$ – JustABitOfCode Oct 28 '16 at 12:16