Question 129: Current Electricity

Question

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 $\mathrm{cal} / \mathrm{s}$ is

Accepted Answer

The motor operates at 100 V and draws 1 A, so the total electrical power input is:
$$P_{	ext{in}} = IV = 100 \, 	ext{V} 	imes 1 \, 	ext{A} = 100 \, 	ext{W}.$$
With an efficiency of 91.6%, only 91.6% of the input power is used for the motor's work, while the rest is lost as heat. The output power is:
$$P_{	ext{out}} = 0.916 	imes 100 \, 	ext{W} = 91.6 \, 	ext{W}.$$
The power lost is:
$$P_{	ext{loss}} = P_{	ext{in}} - P_{	ext{out}} = 100 \, 	ext{W} - 91.6 \, 	ext{W} = 8.4 \, 	ext{W}.$$
Since $1 \, 	ext{W} = 1 \, 	ext{J/s}$ and $1 \, 	ext{cal} \approx 4.2 \, 	ext{J}$, we convert the lost power into calories per second:
$$P_{	ext{loss}} (	ext{in cal/s}) = \frac{8.4 \, 	ext{J/s}}{4.2 \, 	ext{J/cal}} = 2 \, 	ext{cal/s}.$$
Thus, the correct answer is:
Option B: 2

Answer

6.2

Answer

2

Answer

8.4

Answer

4

Current Electricity

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 \mathrm{cal} / \mathrm{s} is

Choose the correct answer: