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.

k6-01

 

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:

P=VI=V**2/R.

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:

P=VI,

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