Electrical Power in Series and Parallel circuits; Bulbs in series and Parallel

There is quite confusing question most of the time asked in interview is that if there are three bulbs having ratings as 60W, 30W and 20W and these bulbs are connected in parallel then which of these bulb will be brightest??

Answer to that question is that in Parallel

  1               =  1  + 1  +  1

R(eq)             R1   R2    R3

Multiply both sides by Rated voltage square i.e. (V _Rated) ²

We get

(V _Rated) ²   = (V _Rated) ² + (V _Rated) ²  + (V _Rated) ²

R(eq)              R1                R2               R3

This means

P(eq)= (P1)+(P2)+(P3)

This means in Parallel Power will be additive

In series we get

R(eq)= R1 + R2 + R3

Now divide both sides by (V _Rated) ²

We get

R(eq)      = R1 + R2 + R3

(V _Rated) ²           (V _Rated) ²

Thus we get

   1         =    1      +     1     +    1

P(eq)          P1           P2        P3

Thus we see that when all bulbs are connected in Parallel then 60W i.e. highest rating will be brightest and when all bulbs are connected in series then bulb with lowest rating i.e. 20W will be brightest.