Physics Paper 2, May/June. 2014  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 Main

General Comments

Question 9

(a)   Define heat capacity and state its unit.
(b)   List two effects of heat on a substance
(c)   Explain how a tightly fitted glass stopper could be removed from a reagent bottle.
(d)   A quantity of pepper soup of mass 800g poured into a plastic container with a tight fitting lid has a
        temperature of 30oC
        The container is then placed in a microwave oven, rated 1200 W and operated for 3 minutes.

  1.   Calculate the final temperature attained by the soup.  (Assuming no heat losses)
  2.   Explain why containers with tight-fitting lids are not suitable for use in microwave cooking.
  3.   When the soup is brought out and allowed to cool, a dent was observed on the container.  Explain.

                       [ Take specific heat capacity of the soup  =  4000 Jkg-1 K-1 ]


Part (a).  This question on heat energy appeared to be very popular.  Many candidates attempted it.  Performance was above average.  However, few candidates wrongly defined specific heat capacity for heat capacity and its unit.
Part (b).  Performance was above average.  Majority of the respondent candidates scored full marks.
Part (c).  Performance was average.  Some stated that they would heat the entire bottle to remove the glass stopper rather than warming the neck of the glass bottle.
Part (d) (i)  Performance was fair.  However, some candidates failed to convert mass in grammes to kilogr   ammes while specific heat capacity in J kg-1 K-1 was used.  Others did not express time in unit of seconds.
(ii)   Performance was very low.  Most candidates failed to link the real life experience to classroom lessons on pressure and expansion.
(iii)   Performance was poor.  Some candidates lost marks by attributing the observed  dent on the container            to adhesion.
The expected answers are:
9.     (a)     Heat Capacity

                  This is the quantity of heat required to change/raise/lower the temperature of a body
                   by IK 

                  Its unit is JK-1                                                                                                    
      (b)      Effects of heat on a substance
                 e.g  change -   /increase in temperature
                                    -   /increase in( electrical) resistance
                                    -   of state
                                    -    in colour
                                     -   in size (expansion)
     (c)      Removing a tightly fitted glass cork

                The neck of the glass bottle should be warmed gradually from outside ( using warm
                cloth ).   This makes the glass to expand creating space for the cork to be removed  
(d)     (i)                    Pt      =     mc                                                                       
                   1200 x 3 x 60  =    x 4000 x                                                        

                                      =     67.5o
                    Final temp     =     =   67 . 5o  + 30o                                                                    
                                                   =  97.5oC                                                                

(ii)      The molecules of the steam from the content are confined within the container.   As the temperature increases the rate of collision of the molecules with walls of the container increases,  thus the pressure inside the container increases.  If  it is tightly fitted, the pressure can make the container burst thus releasing its hot content.

(iii)     The container will dent because the condensation of the steam in the container will lead to a decrease in pressure in the container  making the atmospheric pressure greater than the pressure in the container.





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