RI Prelims 2004 P2 Q9a, b
(a) Water has a specific heat capacity of 4200 J/(kg °C).
What is meant by specific heat capacity of 4200 J/(kg °C)?
(b) A student conducted an experiment to determine the specific heat capacity of a liquid X. He immersed a heater in a beaker of liquid X at room temperature as shown in the figure below. The heater was switched on and a graph was obtained as shown.
(i) Describe briefly how thermal energy is transferred from the heating element to all the liquid in the beaker.
(ii) Another student suggested that liquid X should first be cooled to about 10 °C below room temperature, then heated to about 10 °C above room temperature.
Explain, in terms of heat transfer with the surroundings, why this suggested procedure will give a more accurate result.
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Answer:
(a) The amount of energy needed to raise the temperature of 1 kg of water by 1 °C is 4200 J.
(b)
(i) The heating element heats up the layer of liquid molecules nearest to it by conduction. This heater layer expands upon heating, causing its density to decrease. The heated less dense liquid rises, and is replaced by the colder denser liquid above. The bulk movement of liquid of different densities set up a convection current, causing the liquid to be heated up.
(ii) When the temperature of the liquid is lowered by 10 °C below room temerature, it gains heat from the surroundings. This compensates for the heat loss to the surroundings when it is heated to a temperature 10 °C higher than its surroundings, and hence, there is no net heat transfer to its surroundings.
