A motor operating on 100 V draws a current of 1 A . If the efficiency of the motor is 91.6 %, then the loss of power in units of cal / s is

Solution

Physics sir · 3 steps

Whatever the motor does not turn into work is lost as heat, so find that 8.4% and change watts into calories.

1. Input power. The motor takes in P = VI = 100 × 1 = 100 W from the supply.
2. Power lost. An efficiency of 91.6% means 91.6 W becomes useful work. The remaining 100 - 91.6 = 8.4 W is lost, mostly as heat.

   Pₗₒₛₛ = (8.4 J s⁻¹)/(4.2 J cal⁻¹) = 2 cal s⁻¹

3. So. Since 1 W = 1 J s⁻¹ and 1 cal = 4.2 J, the loss is 2 cal/s, option (b).

The idea to keep. Power lost = (1 minus efficiency) times input power VI. If the answer is asked in cal/s, divide the watts by 4.2 at the end.

Answer from the NTA final answer key. Solution by PrepWiser.