Circuits: how do you know which power equation to use in what situation?

P=IV

P=I^2R

P=V^2/R

My teacher said something about looking for which variable is constant...

Here%26#039;s a question I%26#039;m trying to solve...

3 lamps are connected in a series, all rated 120 V with power ratings of 60 W, 75 W, and 200 W, respectively. Why do the intensities of the lamps differ? Which lamp has the greatest resistance? How would their intensities differ if they were connected in parallel?

then there%26#039;s a picture showing the 3 bulbs, the 60 W is the brightest, the 75 W a little less bright and the 200 W is pretty dim.

Circuits: how do you know which power equation to use in what situation?

since they are hooked in series the current is constant, the voltage drop across each lamp will be different for each lamp. the sum of the voltages will equal 120v. If they were connected in parallel the 200w would be the brightest then the 75, then 60. The lamps in series differ in intensity because of differences in resistance. the resistance of each bulb can be calulated using p=v^2/r since the wattage is based on a 120v rating. r=v^2/p . the 60 w has the greatest resistance.