A metal block of heat capacity placed in a room at is heated electrically

PHYSICS · Single correct · CBSE · Physics · Thermal Properties of Matter · Thermal Properties of Matter

Problem

A metal block of heat capacity

80\,\text{J}/^{\circ}\text{C}

placed in a room at

20^{\circ}\text{C}

is heated electrically. The heater is switched off when the temperature reaches

30^{\circ}\text{C}

. The temperature of the block rises at the rate of

2\,^{\circ}\text{C/s}

just after the heater is switched on and falls at the rate of

0.2\,^{\circ}\text{C/s}

just after the heater is switched off. Assume average Newton's law of cooling to be valid. Assuming that the power radiated at

25^{\circ}\text{C}

represents the average value in the heating process, find the time for which the heater was kept on. (A) 2 sec. (B) 5.2 sec. (C) 10.2 sec. (D) 8 sec.