Brightness of bulbs in series and parallels?

Question

Okay here is what I'm facing. (I'm not a physics major or engineer major so I don't know swat about this).

There is a 3 volt source which is connected to two circuits: one is a series of two bulbs, and one is a parallel of two bulbs... which one is brighter? which one has greater power?

Answer 1

I assume that all four bulbs are the same.

If the bulbs have constant resistance regardless of filament temperature, the resistance of the series arrangement is twice the resistance of one bulb, so the current that flows through two bulbs in series is half of the current that one bulb alone draws. In addition, the voltage across each of the series bulbs is half of the source voltage. Wattage equals voltage times current, so each bulb in the series hookup will dissipate one fourth the wattage of one bulb alone, and the two series bulbs together dissipate one half of the wattage of one bulb.

The parallel bulbs each dissipate their rated wattage, giving twice the wattage of one bulb alone. Therefore, the parallel arrangement will dissipate four times the wattage of the series arrangement.

But it's not really that simple, because the resistance of a light bulb filament varies with its temperature, and a bright (hot) bulb has a higher resistance than a dim (not so hot) bulb. Because of this, the parallel bulbs will dissipate more than the series bulbs, but it will be less than four times as much.

Answer 2

If you have two bulbs in series, then they each drop 1/2 the voltage. So, each bulb is consuming 1.5 volts.

In parallel, they are each dropping 3 volts, because each bulb is independently attached to the battery.

Obviously, all else being equal, the parallel arrangement will cause the bulbs to burn brighter. This also means that more power is being consumed.