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.
- Use the electrical components provided to connect the circuit as shown in the diagram leaving the circuit open.
- Connect the voltmeter to the terminals of the battery and record the voltmeter reading V0.
- Close the key and adjust the rheostat so that the ammeter reads, I = 0.3 A.
Record the corresponding voltmeter reading, V.
- Evaluate and G = .
- 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.
- Tabulate the results.
- Plot a graph with on the vertical axis and G on the horizontal axis.
- Determine the slope, s, of the graph.
- Using the graph, determine the value of V for G = 2.5.
- 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)
= $0.36
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
=