The energy involved in change of state
3 marks · NormalAn electric heater supplies 200 W to a block of ice at -10°C. The ice has a mass of 0.5 kg. The specific heat capacity of ice is 2100 J/kg°C. Calculate the time needed to raise the temperature of the ice to 0°C.
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12.5 min (±0.2)
Must show: Correct formula and substitution, Conversion of seconds to minutes
Also accept: 750 s as a final answer
Common mistakes: Forgetting to convert seconds to minutes; Not using correct mass or temperature change
Show mark scheme and worked solution
Marking points
M1 · 1 mark · States or uses Q = m x c x Δθ (follow-through)
M2 · 1 mark · Calculates correct energy (Q) for ice from -10°C to 0°C as 0.5 × 2100 × 10 = 10,500 J (follow-through)
A1 · 1 mark · Finds t = Q / P, gets 52.5 s and converts to 12.5 min (or gives 750 s) (follow-through)
Worked solution
Energy needed to raise temperature:
Q = 0.5 \times 2100 \times 10 = 10,500~\text{J}
Power = energy/time so time = energy/power:
52.5 s = 0.88 min (or students may leave answer as 52.5 s if not asked to give in minutes)