Physics (Essay) Paper 2 WASSCE (SC), 2018

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