Alternative A
Question 3A
- Measure and record the e.m.f E of the battery
- Set up a circuit as shown in the diagram above.
- 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.
- Take and record the voltmeter reading V and the corresponding ammeter reading I.
- Evaluate log V and log I.
- Repeat the procedure for five other values of x = 20cm, 30cm, 40cm, 50cm and 60cm.
- Tabulate your readings.
- Plot a graph with log I on the vertical axis and log V on the horizontal axis.
- Determine the slope, s, of the graph.
- Determine the intercept, c, on the vertical axis.
- State two precautions taken to ensure accurate results.
You are provided with cells, a potentiometer, an ammeter, a voltmeter, a bulb, a key, a jockey and other necessary materials.
(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