A kilogram of water (c = 4.186 kJ/kg $^{\circ}$C)is stirred using a 400 watt device for 5 minutes.At the end of this time the temperature of the water has increased by 15$^{\circ}$C.How much heat was lost to the surroundings?

Question

Quizplus · Accepted Answer

We can use the formula Q = mcΔT to solve for the heat lost to the surroundings. 
m = 1 kg (mass of water) 
c = 4.186 kJ/kg°C (specific heat capacity of water) 
ΔT = 15°C (change in temperature)

Q = (1 kg)(4.186 kJ/kg°C)(15°C)
Q = 62.79 kJ (amount of heat absorbed by water)

Since the device used 400 watts for 5 minutes, the total energy used is: 
E = (400 watts)(5 minutes)(60 seconds/minute)
E = 120000 J = 120 kJ

Therefore, the heat lost to the surroundings is:

Heat lost = Energy used - Heat absorbed by water 
Heat lost = 120 kJ - 62.79 kJ 
Heat lost = 57.2 kJ

Rounding to one decimal place, the answer is B) 57.3 kJ.

A Kilogram of Water (C = 4 ∘^{\circ}∘ C)is Stirred Using a 400 Watt Device for 5 Minutes

A Kilogram of Water (C = 4 $^{\circ}$ C)is Stirred Using a 400 Watt Device for 5 Minutes