Question 9
(a) List two factors each that affect heat loss by:
(i) radiation;
(ii) convection
(b) State two factors that determine the quantity of heat in a body
(c) Explain the statement: The specific latent heat of vaporization of mercury is 2.72 x 105 J kg -1
(d) A jug of heat capacity 250 JK-1 contains water at 28oC. An electric heater of resistance 35 Ω connected to a 220 V source is used to raise the temperature f the water until it boils at 100oC in 4 minutes. After another 5 minutes, 300 g of water has evaporated. Assuming no heat is lost to the surroundings, calculate the:
(i) mass of water in the jug before heating;
(ii) specific latent heat of vaporization of steam.
[Specific heat capacity of water = 4200 J kg-1 K-1]
Observation
Part (a): This question was well attempted. In the case of radiation, some candidates mentioned ‘change in temperature’ instead of ‘temperature’ to lost valuable mark.
Part (b): Performance was fair. Many candidates were able to score full mark but some candidates lost marks for listing ‘change in temperature’.
Part (c): Many candidates scored full mark in this part. Some mistook ‘Specific latent heat’ for ‘Specific heat capacity’. Some candidates lost some marks because they gave just the definition. Explaining a statement involves definition and additional information.
Part(d): Majority of candidates who attempted this questions either avoided this part or got confused right from the onset. Performance was low.
.
The expected answer is:
(a) (i) Radiation
- Surface area
- Temperature
- Nature of the surface
(ii) Convection
- Nature/type of fluid
- Density/viscosity of fluid
- Thermal conductivity of fluid
- Specific heat capacity of fluid
- Exposed surface area
(b) Factors that Determine the Quantity of Heat in a Body
- Heat/thermal capacity
- Temperature (Not Temperature change)
NOTE: Accept any two of:
- Mass
- Temperature (Not Temperature change)
- Specific heat capacity
(c) It means that 2.72 x 105 J of heat energy is required /needed to change 1 kg of mercury at its boiling point to vapour (without temperature change)
(d)(i) Heat supplied by heater = Heat gained by water + Heat gained by jug
