WASSCE SHS
Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 degree Celsius. Water's value is high, which is why a large mass of water stores a lot of energy.
The energy transferred when a substance changes temperature, without changing state, is Q = mcΔθ. Here m is mass in kilograms, c is specific heat capacity in joules per kilogram per degree Celsius, and Δθ is the temperature change in degrees Celsius. For water, c is 4200 in those units, unless the question gives another value. Use the value you are given. A temperature change is final minus initial. It is not the final temperature unless the body started at zero.
Changing state is a different calculation, using specific latent heat, and the temperature stays constant while the state changes. Do not use Q = mcΔθ across melting or boiling. The questions on this page stay inside one state, so the temperature does change and latent heat is not involved. Show the substitution. A marker can award the method when the multiplication is wrong, and cannot invent a method you did not write.
Mass in kilograms. A mass in grams left unconverted makes the energy a thousand times too big if c is in J per kg. Temperature change, not temperature. Units of energy are joules. If you use c = 4200, say that you are taking the specific heat capacity of water as 4200 J/kg°C, or use the value printed in the question.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
3 marks
Calculate the energy needed to raise the temperature of 0.50 kg of water by 10 degrees Celsius. Take the specific heat capacity of water as 4200 J/kg°C. (3 marks)
Q = mcΔθ = 0.50 × 4200 × 10 = 21000 J.
2 marks
The same energy is supplied to 0.50 kg of a metal whose specific heat capacity is much smaller than that of water. State how the temperature rise compares, and why. (2 marks)
The temperature rise is larger. A smaller specific heat capacity means less energy is needed for each degree, so the same energy produces a bigger change.
2 marks
Why is Q = mcΔθ the wrong equation while ice is melting at 0 degrees Celsius? (2 marks)
The temperature is not changing. The energy is going into changing the state, so the latent heat equation is the one to use.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Physics freeThe energy needed to raise the temperature of 1 kg of the substance by 1 degree Celsius. For water the value used here is 4200 J/kg°C. The energy to warm a mass m through a change Δθ is mcΔθ.
Because that value of c is per kilogram. 0.50 kg of water warmed by 10 degrees needs 0.50 × 4200 × 10, which is 21000 J. If you leave the mass as 500 g and still use 4200, the answer is a thousand times too big. All WASSCE Physics past questions