Physics (Essay) Paper 2 May/June 2015

Question 9

(a) Define specific latent heat of vaporization.           
(b) (i)   What are renewable energy sources?
      (ii)  List four renewable energy sources

(c) Explain why tomatoes keep longer when kept in a moist jute bag in a clay pot.
(d) A box has a volume of 0.28 m3 and is 70% filled with iron fillings at 25oC.  Calculate the

  • (i) Total mass of the iron fillings;
  • (ii) Energy required to melt 10% of the iron fillings.
  • [ Density of iron = 8.00 x 103 kg m-3 ; specific latent heat of fusion of iron = 1.38 x 105 Jkg-1 specific heat capacity of iron = 460 Jkg -1 K-1, melting point of iron = 1500 oC]


    Part (a):  This part was a popular question among the candidates and it was fairly   attempted.  However, few candidates failed to make reference to key words such as ‘liquid’ ‘unit mass’ at its boiling point, in their definition hence lost valuable marks.

      Part (b)(i):  Candidates performed well in this part.  Few however confused the concept, they were writing about replacing the energy after use instead of stating that the energy cannot be used up, or exhausted or finished e.t.c
    (ii) Some candidates were able to mention sources of renewable energy while few     others mixed it up by listing non-renewable sources.  It was also discovered that most correct list hardly went beyond solar, wind and hydroelectric.
    Part (c):  Not a well attempted by most responding candidates.  Many scored zero for irrelevant reasoning.  Guess the word ‘jute’ or the item ‘jute bag’ was not familiar to many candidates.

    Part (d):   A good number of candidates were able to calculate the total mass of iron fillings using the given parameter.  However, some candidates were met with brick wall where 10% of the results in (i) was required and where 70% of the volume of the box should be used.  Performance was poor as candidates could not substitutes correctly.
    Part (e):  Many candidates exhibited vague knowledge of collision and linear momentum as they failed to consider the directives of the bodies in the collision while making their substitution.   Performance was poor.

    The expected answers are:                 
    9.    (a)    This is the quantity of heat required to change a unit mass/1g/1kg of a liquid at its boiling point to gas      
    (b)(i) Renewable energy sources
    They are sources of energy that cannot be exhausted.                                     
    (ii)  Renewable energy sources

    e.g  -  solar/sun
    -   wave
    -   tidal
    -   hydroelectric
    -    wind
    -     geothermal
    -     biomass (vegetable) fuels.
    (c)     Clay pot is porous    The molecules of water in the moist jute bag will extract  latent heat of vaporization from the tomatoes  to escape through the pores of the pot, thereby  causing cooling of the tomatoes.          

            (d) (i)     ρ  =      or      m  = ρ x v                                                                     
    m  =  8.00 x 103 x   x 0.28                                                               
    =  1568 kg.                                                                                 
    (ii)     Q  =  mcΔ +    ml                                                                                  
    =    x  1568  x460 x (1500 – 25) +  x 1568 x 1.38 x 105            
    =   106,388,800  + 21,638,400                                                            
    =    128,027,200J

                                   =    1.28 x 10 8J.