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Resistors Questions
1. Your are provided with the following apparatus: connecting wires, an ammeter, fixed resistors,
a voltmeter, a variance resistor, a switch and two dry cells in a cell holder. Draw a circuit that
can be used using the apparatus above to verify Ohm’s law 2. (a) Three resistors A, B and C where A is resistor 200 Ω C of resistance 100 Ω and B is of
unknown resistance are connected in parallel. This arrangement is then placed in a circuit
and current passing through and potential difference across it measured. The table below
shows the result.
p.d.v. | 3.0 | 6.0 | 9.0 | 12.0 | 15.0 |
Current A | 0.15 | 0.30 | 0.45 | 0.60 | 0.75 |
(i) Plot a graph of p.d. against current A
(ii) From the graph calculate the total resistance of the resistors.
(iii) What is the value of the unknown resistor?
(b) The ammeter in the figure below shows a reading of 2A. What is the reading of the
voltmeter v.?
3. a) Three resistors of resistance 2.0 and 6.0 are connected together in a circuit. Draw
a circuit diagram to show the arrangement of the resistors which gives
i) Effective resistance of 3.0
ii) Minimum resistance
b) In figure 9 the voltmeter reads 2.1V when the switch is closed, the voltmeter reads 1.8v and
the ammeter 0.1 A
Determine:
i) The e.m.f of the cell
ii) The internal resistance of the cell
iii) The resistance of the of the lamp
c) One reason why the earth pin is longer than the neutral and the live pins is to open the
shutters of the socket, state one other reason why it is longer
d) Why are lamps in a house always connected in parallel but not in series?
4. State two other factors, which would affect the resistance of a metal conductor other than
the temperature
b) The ammeter in the circuit below has negligible internal resistance. The cell has internal
resistance of 0.5 and an electromotive force of 3.0V
Determine the value of current the ammeter registers when switch S is closed
5. a) State Ohms law
6. In the configuration of resistors given below, determine the current through the 5 resistor
7. Figure 5 is a circuit diagram of three resistors connected to a 6V battery
Determine the potential difference across the 3 resistor
8. (a) Study the circuit diagram shown below.
(i) Determine the reading of the voltmeter V.
(ii) Determine the reading of the ammeter A.
(iii) Explain the effect on the reading of the voltmeter if the 3 resistor is altered to be 6
(b) A transformer is designated to work from a 240V a.c. mains and to give a supply of 8V
to ring house bells. The primary has 4800 turns.
(i) What type of transformer is this? Give a reason.
(ii) Why is the iron core laminated?
(iii) Calculate the secondary turns if the efficiency is 100%.
9. The diagram in the figure below shows a wheat stone bridge
K, L, M and N are four resistors joined as shown. The value of resistance of resistor K is
unknown. I1, I2, I3 and I4 are the amount of current passing through K, M, L and N respectively.
It is also provided that L is a variable resistor.
(i) Explain how the set-up is used to determine the value of unknown resistance of K (ii) State why wheat stone bridge is more accurate in measuring resistance than the voltmeter-
ammeter method
(b) In an experiment to determine the resistance of a nichrome wire using the metre bridge, the
balance point was found to be at 38cm mark. If the value of the resistance in the right hand
gap needed to balance the bridge was 25, Calculate the value of resistance of nichrome wire
10. Four capacitors of capacitance, 3µf, 4µf, 5µf and 3µf are arranged as shown below. Find the
11. In the circuit diagram below, X is a fixed resistor while Y can be varied between O
and 100 using a sliding jockey
Calculate:
(i) The minimum possible current in the circuit (2mks)
(ii) The maximum possible current in the circuit (2mks)
(c) The following figure shows an electric circuit in which five resistors are connected
to a battery of e.m.f 4.0V and negligible internal resistance
Determine:
(i) The total resistance of the circuit
(ii) The potential difference between Y and Q
(d) Explain two factors that affect the resistance of a metallic conductor
12. Three identical cells of e.m.f. 2.Ov and of negligible internal resistance are connected as shown
in figure below. Determine the ammeter reading.
Resistors Answers
1. – The ammeter should be in series
– Voltmeter in parallel
– Variable resistor in series
– The apparatus must be workable
2. a) ii) gradient = 10v1 mk
0.5A
= 20
1 mk
iii) 1 + 1 + 1 = 1
1 mk
200 100 R 20
1 = 1 – 1 – 1
R 20 200 100
1 = 9 1 mk
9 200
= 20
1 mk
b) Current through 10 resistor = 2A
p.d across 10 resistor = 2 x 10
= 20v
p.d. across 5 = 20v
current = 20 = 4A 1 mk
5
Total current 4 + 2 = 6A
Current through 2 = 6A
p.d = 6 x 2 = 12V
Total voltage = 12 + 20
= 32V
3. (i) (ii)
b) i) 2.1 V
ii) 2.1 v – 1.8V = Ir = 0.1V
r = 0.3 = 3
0.1
iii) 0.1 X R = 1.8 V
R = 18R
4. – Length of conductor
– Type /nature of material
– Diameter/thickness of material
b) E = IR + Ir
3.0 = I(3.5 + 0.5) = I (4.0)
I = 0.75A
5. a) – The current passing through a conductor is directly proportional to the
potential difference across its ends provided temperature and other
physical conditions are kept constant
6. I = 1 + 1 = 7
R 5 2 10
R = 10/7
RT = 10 + 3 = 31 (Effective resistance)
7 1 7
but I = 12.4 x 7 = 2.8A
31
p.d across the 3 resistor = 2.8 x 3 = 8.4V (p.d across the 3 resistor)
p.d across the 5 and 2 = 12.4 – 8.4 = 6.0V
Current across the 5 resistor = 6/5 = 1.2A (answer 3mks)
7. Rs = 3 + 4
Rp = 7 x 5 = 35
7 + 5 12
II = 6 x 12 = 2.057A
35
I through 3 resistor = I1
I1 x 5 = I2
5(2.05 –I2).25 – 5I2
10.25 -5I2 = I2
6I2 = 10.25 – UI2= 10.25 = 1.708A
6
8. a) (ii)
(ii) Effective resistance:
RE = 4 + 6 x 3
3 + 6
= 4 + 18
9
= 6
V = IR
I = 12 = 2A
6
V = 2 x 4 = 8V
(ii) V = 12 – 8 = 4V
V = IR
I = 4 = 2 = 0.667A
6 3
(iii) Effective resistance in parallel
RE = 6 x 6 = 36 = 3
6 + 6 12
The potential drop will increase; hence the reading of V will decrease
(c) (i) Step-down- The voltage is reduced from 240V to 8V
(ii) To reduce loss of energy due to eddy currents
(iii) VP = Np
Vs Vs
740 = 4800
8 Vs
Vs = 4800 x 8
240
= 20 x8 = 160turns
9 (i) – Set Galvanometer to zero balance by adjusting the variable resistor L.
– P.d across BD is therefore zero
– P.d across AB = P.d across AD
P.d across BC = p.d across DC
I1 flows through K & L (I1 = I3)
I2 flows through M and N (I2 = I4)
I1K = I2M
I3L = I4N
I1K = I2M
I1L I2N
K = M
L N
(ii) The method does not depend on the accuracy of the current measuring instrument
(b) R = 0.38
25 0.62
R = 25 x 0.38
0.62
= 15.32
10. C11 = 4f + 5 f = 9 f
I = 1 + 1 +
CT 3 9 3
3 + 1 + 3 = 7
9 9
CT = 9 = 1.29F
11. (i) Minimum current is when y is at max resistance , i.e 100 (x and Y parallel)
current I = 220V
100
= 2.2A
(ii) Maximum current is when R = 500 at y (when X and Y are parallel)
I = 220V
50 = 4.4A
(c) (i) For the upper resistors in series
R = 1 + 4= 5
for the lower resistors in series
R = 2 + 3 =5
For the combined resistance of the parallel sets
I = 1 + 1 = 2
R 5 5 5
R = 2.50
Total resistance = 2.5 + 5.50 = 8.00
(ii) Current 1y = 0.5A
Vy = 40 x 0.25A = 1.0V
V2 = 2 x 0.25A = 0.5V
Vy2 = 0.5V *
(d) – Thickness/x-sectional area – Resistance is inversely proportional to the thickness of a
conductor
– Length : Resistance is directly proportional to length of a conductor