Alternative B
Question 3
- You are provided with a cell of e.m.f. E, key K, resistance box R, standard resistor
Ro of resistance 2, ammeter, voltmeter and other necessary materials.
- Set up a circuit as shown in the diagram above, leaving the key open.
- Set the resistance in the resistance box to R = 2.
- Close the key, read and record the current I ammeter and its corresponding potential difference, V, on the voltmeter.
- Evaluate P = and R-1.
- Repeat the procedures for four other values of R = 5Ω, 7Ω, 10Ω and 12 Ω. In each case, evaluate P and R-1.
- Tabulate the results.
- Plot a graph with P on the vertical axis and R-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.
- Evaluate c-1
- State two precautions taken to ensure accurate results.
-
- Why is it important to earth electrical appliances?
- A wire carries a current of 100 mA. Calculate the time taken by 5.0 x 1020 electrons to pass a given cross-section of the wire. [Magnitude of electronic charge = 1.6 x 10-19C].
Observation
Part (a) Candidates performance was poor. Many candidates did not start the graph from the origin
(0,0) and results were not recorded in the appropriate d.p. and s.f.
Part (b) Candidates response was unsatisfactory.
The expected response:
3. (a) OBSERVATIONS [07]
(i) Five values of R recorded in ohms
(Deduct ½ mark for each wrong or missing value)
(ii) Five values of I recorded to at least 1d.p in amperes and in trend
Trend: As R increases, I decreases
(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 R increases, V increases
(Deduct ½ mark for each wrong or missing value)
(iv) Five values of P = I/V correctly evaluated to at least 2 d.p.
(Deduct ½ mark for each wrong or missing value)
v) Five values of R-1 correctly evaluated to at least 3 d.p.
(Deduct ½ mark for each wrong or missing value)
(vi) Composite table containing at least R, I, V, P and R-1.
GRAPH [06]
(i) Both axes correctly distinguished (½ mark each)
(ii) Reasonable scales (½ mark each)
(iii) Five points correctly plotted
(Deduct 1 mark for each wrong or missing point)
(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) ∆P correctly determined
(iii) ∆R-1 correctly determined
(iv) ∆P/(∆R-1 ) correctly evaluated
STRAIGHT LINE [01]
Straight line making 45° with horizontal axis and intersecting the curve.
INTERCEPT [01]
Intercept, c, on the vertical axis
Correctly shown
Correctly recorded
EVALUATION [01]
Evaluation of c-1
correct substitution
correct arithmetic
ACCURACY [01]
Based on slope, s = 1 ± 10%
OR
c-1 = Ro = 2 Ω ± 10% or teacher’s value of Ro
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
- Ensured tight connections
- Avoided parallax error when taking reading on ammeter/voltmeter
- Ensured clean terminals
- Repeated readings shown on table
- Noted/Corrected zero error on ammeter/voltmeter
(b)(i) Electrical appliances are earthed to prevent electric shock.
OR
To prevent damages or fire incidence.
OR
To ensure safety
(ii) t = (amount of charge (Q))/(current (I))
t = (5 x 1020)x(1.6 x 10-19)/(100 x 10-3)
= 800.0 s