Two light bulbs with different power (wattage) levels are attached in parallel (the usual way for household power distribution) across a 110 volt AC source. The higher power bulb (the larger bulb physically) glows more brightly, as seen in the photograph below.



Now suppose that the two bulbs are wired in series across the same 110 volt AC source. Which of the three possibilities given below will result?

  • (a) The larger bulb will glow more brightly.
  • (b) The smaller bulb will glow more brightly.
  • (c) The bulbs will be equally bright.

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The answer is (b): the smaller bulb will glow more brightly, as seen in the figure at the right below.

 k6-01  k6-01a

A small light bulb has more resistance than a large one, so that it will pass less current and consume less power:


However, when the two bulbs are connected in series, the larger resistance has the greater voltage drop across it, while the currents in the two bulbs are the same:


so the smaller bulb consumes more power and therefore glows more brightly.