# Which bulb glows brighter?

1. The problem, statement, all variables and given/known data
Say there are 3 bulbs of power 100W, 500W and 1000W connected in series. Which bulb will glow brighter and why? How would it be different for a parallel combination of these bulbs?

## Homework Equations

P=I^2R
P=V^2/R

I think in series combination, the 1000W bulb will glow more because power and resistance are directly related in this case, and resistance is directly related to voltage. So, the one with higher power will glow more. For parallel combination, since voltage is constant across each bulb, I think we will take current into account. Here, resistance and power are inversely related, so the one with higher power (1000W) will have less resistance and more current will flow through it so it will glow more. Is it certain that P=I2R should be used for series circuit and P=V2/R should be used for parallel circuit?

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The power ratings of bulbs come along with a voltage value. For example a bulb with rating 1000W,40V means that when 40V is supplied to the bulb, its power consumption will be 1000W.
Now ponder over this and you should arrive at the answer.

Mark44
Mentor
P=RI
This should be P = IE, not ##P = R^I##.

gneill
Mentor
This should be P = RI or IR, not ##R^I##.
How about ##P = I^2 R## ?

Mark44
Mentor
How about ##P = I^2 R## ?
My mistake. I was thinking P = IE but mistakenly wrote IR. I've fixed my earlier post.

The power ratings of bulbs come along with a voltage value. For example a bulb with rating 1000W,40V means that when 40V is supplied to the bulb, its power consumption will be 1000W.
Now ponder over this and you should arrive at the answer.

Yes, and just to clarify for the purpose of this problem you must assume that all the bulbs are rated at the SAME voltage.

For problems like these they usually don’t explain that because they want you to realize that almost all household bulbs are (well, used to be) rated at the same voltage. 110V AC is the standard in the US. However, I think this problem is a bit of a cheat in that regard. I’m not familiar with many 500 and 1000W bulbs for the home, so if they aren’t household 110V bulbs how can you make the leap of faith that these are rated at the same voltage? Anyhow, that is just careless problem writing. Trust me, for this problem you have to assume the ratings are at the same voltage.