Question

A 76.0 W, 110-V bulb is connected in parallel with a 45.0 W, 110-V bulb. What is the net resistance?

Answer 1

When two bulbs are in parallel connection they both get same voltage diiference. In this case the voltage difference is 110v. But as their power ratings are different they draw different amount of current. Now we know from joule heating effect that Power(P), voltage difference(V), and current(I) are related as,

P=VI

So the 1st bulb(i.e 76 w bulb) draws current

And the 2nd bulb(i.e 45w bulb) draws current

.

Now from Ohm's law, Resistance of the 1st bulb is

and of 2nd bulb is

.

So we get the resistences. Now as they are in parallel connection we can write the net resistence as

Hence equivalent resistence is . This is the answer.