First slide

Specific heat

Question

An electric kettle (rated accurately at 2.5kw) is used to heat 3 kg of water from 15°C to boiling point. It takes 9.5 minutes. Then the amount of heat that has been lost is

Moderate

A

$10 \cdot 70 \times 10^{5} J$
B

7 X 10⁶J
C

3.5x 10⁴J
D

7 x 10⁸J

Solution

Energy consumed in 9.5 min is given by
$= (2 \cdot 5 \times 1000) \times (9 \cdot 5 \times 60) = 14 \cdot 25 \times 10^{5} joule$
Energy consumed in heating
$= 3 \times 1000 \times (100 - 15) joule = 2 \cdot 55 \times 10^{5} joule$
Loss $= (14 \cdot 25 - 2 \cdot 55) 10^{5} \approx 10 \cdot 70 \times 10^{5} joule$

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