Physics Paper 2, Nov/Dec. 2012  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments

Question 12

 Question 12

(a)   State the principle of conservation of energy.
(b)   State three effects of heat on a substance.
(c)    Explain why when the bulb of thermometer is dipped into ice-cold water the
        mercury level first rises before falling.
(d)    Water of volume 200 cm3 in an aluminum container at 20o C is cooled to – 10oC
         when put in a freezer.

  1. Calculate the quantity of heat energy extracted from the water.
  2. If heat is extracted at a rate of 120 Js-1 by the freezer, calculate the time taken to produce this ice.
  3. State a reason why the calculated time will be less than the actual time.

(Density of water = 1000 kg m-3; specific heat capacity of water = 4200 J kg-1; specific latent heat of fusion of ice = 336000\j kg-1; specific heat capacity of ice = 2100J kg-1 K-1)


P)     This was well answered by most candidates however, few candidates omitted the
        phrase “ in a closed system or “isolated system” in their statement.

(b)   The three effects of heat on a substance fell an easy prey to most candidates.
        Performance was satisfactory.

  • Explanation of how mercury level changes when dipped in ice-cold water proved         difficult to most respondent candidates as they could not provide satisfactory explanation.
  •   The numerical section was poorly tackled by most respondent candidates.  The 

   formula m = v eluded most of the candidates.

   Also applying the formula Q = mc  appropriately was another problem due to their poor  understanding of the concepts of specific heat capacity and latent heat.

The expected answers are:

(a)       In an isolated / closed system, the total amount of energy is (always) constant
            (although energy may be transformed from one                 form to another)                   
            (b)       The effect of heat on a substance
            e.g      Change
            -           in temperature
            -           in size (expansion/contraction)
            -           of  phase/state
            -           in resistance
            -           in colour
(c)         The bulb comes in contact with the cold water before the mercury, hence the glass cools first, contracts and squeezes the mercury up the stem until (thermal) equilibrium is reached.  The mercury being a better conductor of heat cools faster than the glass; hence the fall in mercury level

(d)(i)    m         =           x V                                                                                      
                        =          1000 x       200
                        =          0.2 kg                                                                                    
            Q         =          MwCwӨw + Mwlf + Mi Ci Өi                                    
                        =          0.2 x 4200 x (20-0) + 0.2 x 3.36 x 105 + 0.2 x 2100 x [0-(-10)  
                        =          16800 + 67200 + 4200
                        =          88200J                                                                                              

   (ii)     Q      = Pt    or P = Q
                           t =            

                            =    88200
                            =  735s  or 12.25 minutes

 (iii)      Why the calculated time is less than the actual time
           e.g (No consideration is given to)                                                               

    • Heat absorbed by the aluminum container
    • Heat lost / absorbed from the surroundings
    • Effect of impurities on freezing point of liquid.

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