Alternative A
Question 3
- You are provided with a battery of e.m.f. E, a key K, a voltmeter, a standard
resistor Ro = 2, a resistance box R and some connecting wires.
- Measure and record the e.m.f. E of the battery.
- Set up a circuit as shown in the diagram above with the key open.
- Set the resistance on the resistance box to R = 2.
- Close the key, read and record the potential difference V on the voltmeter.
- Evaluate V-1.
- Repeat the procedures for five other values of R = 5 Ω, 10 Ω, 12 Ω, 15 Ω and 20 Ω. In each case, record V and evaluate V-1.
- Tabulate the results.
- Plot a graph with R on the vertical axis and V-1 on the horizontal axis, starting both axes from the origin (0, 0)
- Determine the slope, s, of the graph and the intercept c on the vertical axis.
- Calculate α and β from the equations s = Ro α and c = - (Ro + B).
- State two precautions taken to ensure accurate results.
-
In the circuit diagram above, the battery has negligible internal
resistance. Calculate the power dissipated as heat in the section AB of the
circuit.
- An electric equipment of power rating 3.6 kW is to be connected to a 240 V line whose circuit breaker is rated 20 A. Justify whether or not the breaker would open when the equipment is switched on. [4 marks]
Observation
Part (a) Performance was poor. Many candidates who attempted this question failed short in so
many ways. Candidates were unable to record E to at least 1 d.p in v, Ro was not recorded.
Candidates failed to start the graph from the origin (0,0).
Part (b) Candidates responded unsatisfactorily.
The expected response:
3. (a) OBSERVATIONS [07]
(i) Ro = 2 Ω
(ii) Value of E correctly measured and recorded to at least 1 d.p. in volts
(iii) Six values of R correctly recorded in ohms.
(Deduct ½ mark for each wrong or missing value)
(iv) Six values of V correctly measured and recorded to at least 1 d.p. in volts.
Trend; As R increases, V decreases
(Award ½ mark each)
(v) Six values of V-1 correctly evaluated to at least 3 s.f.
(Deduct ½ mark for each wrong or missing value)
(vi) Composite table showing at least R, V and V-1.
GRAPH [06]
(i) Both axes correctly distinguished (½ mark each)
(ii) Reasonable scales (½ mark each)
(iii) Six points correctly plotted (Award ½ mark each)
(iv) Line of best fit
NOTE: Deduct 1 mark for d.i. if axes do not start from origin (0,0)
SLOPE [02]
(i) Large right-angled triangle
(ii) ∆R correctly determined
(iii) ∆V-1 correctly determined
(iv) ∆R/(∆V)^(-1) correctly evaluated
INTERCEPT [01]
Intercept, c, on the horizontal axis
Correctly shown
Correctly read
CALCULATION OF α [01]
α= S/R_o
correct substitution
correct arithmetic
CALCULATION OF β [01]
β= -(R0 + c)
correct substitution
correct arithmetic
ACCURACY [01]
Based on α = E (candidate’s value) 10%
PRECAUTIONS [02]
Award 1 mark each for any 2 correct precautions stated in acceptable tense.
e.g
Key opened in between readings/key opened when readings were not taken
Tight connections ensured
Avoided parallax error when taking reading on voltmeter
Repeated readings shown on table
Noted/corrected zero error on voltmeter
Clean terminals ensured
