Thermal Physics – Power

A 100-watt electric heater element is placed in a pan containing one liter of water. Although the heating element is on for a long time, the water, though close to boiling does not boil. When the heating element is removed, approximately how long will it take the water to cool by 1 degree Celsius? (Assume that the specific heat for water is 4.2 kJ/kgoC)

A. 20 s
B. 40 s
C. 60 s
D. 130 s
E. 200 s

(GR9677 #13)

Solution:

P = 100 W
V = 1 L = 1 m3 → m = 1 kg (STP)
Δ1oC
c = 4.2 kJ/kgoC

Q = mcΔT = Pt
1 × 4200 × 1 = 100t
t = 42 s

Answer: B

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