Question

A brand of lightbulb has a power of 15 W and an efficiency of 8.9%. How much power is wasted as heat by this lightbulb?

Answer 1

The efficiency of the light bulb is 8.9%. This means that only 8.9% of the power of the lightbulb becomes useful power, and it corresponds to:

`P_u = (8.9)/(100) P = (8.9)/(100) (15 W)=1.34 W`

The rest of the power instead is wasted as heat, and this amount of power wasted is given by the difference between the total power and the useful power:

`P_w = P-P_u = 15 W - 1.34 W=13.66 W`

So, the power wasted as heat is 13.66 W.