Question

A 2 kW, 40 litres water heater is switched on for 20 minutes. The heat capacity $c_p$ for water is 4.2 kJ/kgK. Assuming all the electr5ical enrgy has gone into heating the water, increase of the water temperature in degree centigrade is

• A. 2.7
• B. 4.0
• C. 14.3
• D. 25.25

Answer 1

The correct option is C 14.3

(c)
Given data:
$P=2kW$
$V=40litres=\frac{40}{1000}=0.04m^3$
$t=20minutes=20\times 60s$
$=1200s$
$c_p=4.2kJ/kgK$
Heat supplied:
$Q=Power\times Time$
$=2\times 1200=2400kJ$
also $Q=m{c}_p\Delta T=\rho V{c}_p\Delta T$ $\because$ $m=\rho V$
$\therefore$ $2400=1000\times 0.04\times 4.2\times \Delta T$
or $\Delta T={14.28}^{o}C={14.3}^{o}C$