Question

If an electric kettle marked 500 W, 220 V takes 15 min to boil 1 kg of water from 10ŗC then the efficiency of kettle is –

• A. 79%
• B. 84%
• C. 72%
• D. 69%

Answer 1

Answer: (B)

84%

Answer 2

P × t = mSDT

P = $\frac{mS\Delta T}{t}$= $\frac{1 \times 4200 \times 90}{15 \times 60}$

= 420 watt (power given by kettle)

\ n% = $\frac{P_{output}}{P_{input}}$× 100 = $\frac{420}{500}$× 100 = 84%