Physics Paper 3 WASSCE (SC), 2023

Alternative A

 

Question 3A

 

(a)

You are provided with a rheostat, an ammeter, a voltmeter, 1 Ω standard resistor, a key, a source of electricity of emf, E and connecting wires.

  1.    Use the electrical components provided to connect the circuit as shown in the diagram leaving the circuit open.
  2. Connect the voltmeter to the terminals of the battery and record the voltmeter reading V0.
  3. Close the key and adjust the rheostat so that the ammeter reads, I = 0.3 A.

Record the corresponding voltmeter reading, V.

  1. Evaluate  and G = .
  2. Repeat the experiment for four other values of I = 0.5 A, 0.7 A, 0.9 A and 1.1 A. In each case, record the corresponding value of V, and evaluate  and G.
  3. Tabulate the results.
  4. Plot a graph with  on the vertical axis and G on the horizontal axis.
  5. Determine the slope, s, of the graph.
  6. Using the graph, determine the value of V for G = 2.5.
  7. State two precautions taken to ensure good results

b)        (i)      A battery of emf 4.5 V has a voltmeter connected to its terminals. If the combination is connected in series with a standard resistor and a key, it is observed that the voltmeter reading is less than 4.5 V when the key is closed. Explain the observation
(ii)    An electric device draws 0.50 A in a 120 V circuit. Calculate the cost of using the device for 24 hours if the rate is $ 0.25 per KWh

Observation

 

 

EXPECTED RESPONSE:

 (a)    OBSERVATIONS [08]
(i)           Value of Vo correctly measured and recorded to at least 1 d.p. in volts.     
(ii)           Five values of I correctly recorded in Amperes.                                          
(Deduct ½ mark for each wrong or missing value)
(iii)         Five values of V correctly recorded to at least 1 d.p. in volts and in trend.
Trend: As I increases, V decreases.
                         (Deduct 1 mark for each wrong or missing value)
(iv)          Five values of  correctly evaluated to at least 2 d.p.                        
                         (Deduct ½  mark for each wrong or missing value)
(v)          Five values of I-1 correctly evaluated to at least 3 s.f.                                 
(Deduct ½ mark for each wrong or missing value)

         GRAPH [06]
(i)            Both axes correctly distinguished              (Award ½ mark each)               
(ii)           Reasonable scales                                       (Award ½ mark each)                
(iii)          Five points correctly plotted.                                                                       
(Deduct 1 mark for each wrong or missing point)
(iv)          Line of best fit                                                                                               

SLOPE [02]
(i)            Large right-angled triangle                                                                            
(ii)           D I-1   correctly determined                                                                          
(iii)          DG correctly determined                                                                              
(iv)          correctly evaluated                                                                               

         DETERMINATION OF V [02]
(i)            Value of I-1 corresponding to G = 2.5 correctly shown.                               
(ii)           I correctly evaluated                                                                                     
(ii)            correctly calculated                                                                          

         ACCURACY [01]
Based on s-1 = V0 ± 10%

         PRECAUTIONS [02]
Award 1 mark each for any 2 correct precautions stated in acceptable tense.
Eg

    • Ensured clean terminals.
    • Ensured tight terminals/connections.
    • Noted/ corrected zero error on ammeter.
    • Avoided parallax error when taking readings from ammeter/voltmeter
    • Repeated readings shown on table.
    • Opened key when readings are not being taken.

      (b)        (i)        
      When the key is closed, the battery supplies current in the circuits. The observed drop in the voltmeter reading is due to the voltage drop/lost across the internal resistance of the battery.
      OR
      Part of the emf is used up within the cell itself.        

      (ii)      
      Power of device = IV                                                                                      
      = 0.5 × 120
      = 60 W    = 0.06 kW                                                             
      Energy consumed in 24 hours, E = 0.06 × 24 kWh = 1.44 kWh                     
      Cost of consumption = 1.44 × $0.25
      = $0.36                                                                              
      OR

      =                                                                   

                                                     = $0.36