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Physics Paper 2, Nov/Dec. 2013  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments

Question 12

Question 12

(a)   List three effects of heat on matter
(b)   Explain the following observations:

  1. a thick glass cup cracks when boiling water is poured into it but the same cup 

would not crack when immersed in a bath of cold water which is then heated to boiling point;
          (ii)      a cat runs with difficulty on a highly polished floor.
(c)     (i)   State the pressure law.
         (ii)   A bottle is corked when the air in it is at 20oC and 76 cm Hg.  The bottle is
                heated until the pressure is increased by 69mmHg
                Calculate the final temperature of the air in oC.


  Part(a).  It was fairly well attempted as most candidates were able to list the effects

   Part (b)(i).  Most candidates could not express their answers systematically as required.
                     It was poorly attempted.

  Most responding candidates were able to answer this question.  Many candidates considered low or insufficient friction on highly polished floor will make the can run  with no easy.  Performance was average.

   Part(c) (i)   Many candidates stated the pressure law correctly.  However, few                                       candidates did not take into consideration the importance of having “gas of                    fixed mass”;”at constant volume” and “absolute or Kelvin temperature” in                                   their definition hence they lost marks.
   (ii)     Many candidates could not get the calculation.  Most calculated using oC
            instead of Kelvin.  Few candidates could not convert the 559K to oC. 
            Performance was poor.

The expected answers are:

             (a)    Effects of heat
                       e.g.     Change in:
             -  size (expansion/contraction)
                                   -   temperature
                                   -   colour
                                   -   state/phase
                                   -   electrical resistance.
(b)(i)              Glass is a poor conductor of heat, so also water. When boiling water is poured into the glass cup, the inner layer/surface quickly expands while the outer layer/ surface remains at room temperature  . Resulting unequal expansion   between these layers /surfaces generates thermal stress in the cup ,  causing it to crack When the cup is immersed in cold water that is heated from room temperature to boiling point , the heating gradually raises the temperature of water and cup in succession and without the development of (appreciable) thermal stress in the cup . The temperature gradient across the wall of the cup is (negligibly) small at any point in time; hence cracking does not occur .
(ii)      A highly polished floor is devoid of the friction needed for effective/ easy running of the cat  
           (Sufficient) friction will enable the cat to run easily on a floor [1].  When the floor is highly polished the friction present is not enough to make the cat run easily   
 (c)(i)      The pressure law states that at constant volume, the pressure of a fixed             mass of (an ideal) gas is directly proportional to the absolute/Kelvin temperature   
           (ii)                                       =                                                                                               
                                               T2         =     =    559K                                        
                                                            = 559 – 273                                                         

                                    = 286C
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