Physics Paper 3 WASSCE (SC), 2017

Alternative A

 

 

 

Question 3A

 

 

    You are provided with cells, a potentiometer, an ammeter, a voltmeter, a bulb, a key, a jockey and other necessary materials.

    1. Measure and record the e.m.f E of the battery
    2. Set up a circuit as shown in the diagram above.
    3. Close the key K and use the jockey to make a firm contact at J on the potentiometer wire such that PJ = x = 10 cm.
    4. Take and record the voltmeter reading V and the corresponding ammeter reading I.
    5. Evaluate log V and log I.
    6. Repeat the procedure for five other values of x = 20cm, 30cm, 40cm, 50cm and 60cm.
    7. Tabulate your readings.
    8. Plot a graph with log I on the vertical axis and log V on the horizontal axis.
    9. Determine the slope, s, of the graph.
    10. Determine the intercept, c, on the vertical axis.
    11. State two precautions taken to ensure accurate results.

     

    (b)  (i)  How is the brightness of the bulb affected as x increases?

    (ii)  List two electrical devices whose actions do not obey Ohm’s law.


Observation

 

 

Part (a) This was a popular question among the candidates. Many candidates attempted the question and performed well. However, many recorded ridiculous values of V and I. where emf of battery is 3.0, candidates recorded Vi = 3.5V to 5.2V. Many candidates could not evaluate log I and log L to the required number of d.p.
Part (b) Candidates responded satisfactorily in this part.
Candidates were expected to:

  • measure and record the value of emf E to at least 1 d.p in volt
  • read and record six values of x to at least 1 d.p in cm
  • read and record six values of V to at least 1 d.p in volt and in trend
  • read and record six values of I to at least 1 d.p in Ampere and in trend
  • evaluate six values of log V to at least 3 d.p
  • evaluate six values of log I to at least 3 d.p
  • show composite table containing at least x, V, I, log V and log I                 
  • Both axes correctly distinguished   
  • plot a graph using reasonable scales
  • draw line of best fit
  • determine the slope and intercept of the graph
  • state any two of the following precautions.

e.g.

  • Ensured clean terminals
  • Ensured tight terminals
  • Opened key when reading was not taken
  • Avoided parallax error on Voltmeter/Ammeter
  • Avoided sliding of jockey on the potentiometer wire
  • Zero error noted/corrected/avoided on Voltmeter/Ammeter

b(i)       -    Brightness of the bulb increases                                                                              
-    Voltage across/current through the bulb increases                                                 

 

(ii)     Devices that do not obey Ohm’s law
Any correct 2 x 1 mark  = 2 marks
e.g   -     Diode
-     Transistor
-     Thyristor
-     Capacitor
-     Thermistor
-     Filament bulb
-     Vacuum tubes
-     (Crystal) rectifier

  -     Inductor