# Light Bulbs in Series vs. Parallel

## Homework Statement

You have two 120V light bulbs, one 25W and one 100W, that are normally connected in parallel with 120V across each. Your friend had what seemed like a good idea which was to connect them in series across a 240V power line. However, one of them burned out immediately. Which one burned out and why?

P=IV
P=I^2*R

## The Attempt at a Solution

If light bulbs are in series, the same current flows through both of them. Should I treat this like a resistor problem and calculate the resistor of the light bulbs in both cases? I'm not sure how to figure out which one burns out.

OmCheeto
Gold Member

## Homework Statement

You have two 120V light bulbs, one 25W and one 100W, that are normally connected in parallel with 120V across each. Your friend had what seemed like a good idea which was to connect them in series across a 240V power line. However, one of them burned out immediately. Which one burned out and why?

P=IV
P=I^2*R

## The Attempt at a Solution

If light bulbs are in series, the same current flows through both of them. Should I treat this like a resistor problem and calculate the resistor of the light bulbs in both cases?
Yes.
I'm not sure how to figure out which one burns out.
As you stated, first calculate the resistance for each bulb in the 120 volt circuit. The resistances of your light bulbs are fixed values. Then putting them in series with 240 volt source, you will find the total system current. From that you can determine the power consumed by each bulb. Comparing the calculated power of each bulb with it's rating will make it obvious which one would burn out.

Yes.

As you stated, first calculate the resistance for each bulb in the 120 volt circuit. The resistances of your light bulbs are fixed values. Then putting them in series with 240 volt source, you will find the total system current. From that you can determine the power consumed by each bulb. Comparing the calculated power of each bulb with it's rating will make it obvious which one would burn out.

Okay great, thanks for your help!