Tutorial 3
Variable resistors, resistance, current and voltage
1
2
Draw the symbols for the following components
a) ammeter
b)
voltmeter
c) ohmmeter
d)
resistor
d) variable resistor
f)
battery
Copy each of the following circuits and add meters are required.
a) Measure the current through L1
and the voltage across R1.
b) Measure the current through L1 and the
voltage across R1.
L1
L1
R1
R1
c) Measure the current through L2
d) Measure the current through L2, the
and the voltage across R1.
voltage across R2 and the voltage
across M1.
L1
L1
M1
R1
M
R1
M
L2
3
R2
L2
M2
R2
a) Draw a circuit diagram to show how the resistance of a lamp should be measured.
b) Which of the following is the most likely resistance for a lamp 0.5 , 50  or 5 000 ?
Tutorial 3
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Unit 1
4
Shown is a series circuit.
6V
The meter readings are shown in the circles.
Calculate the following meter readings
a) Ammeter A1.
A2
2A
b) Ammeter A2.
A1
c) Voltmeter V2.
1V
5
V1
Shown is a parallel circuit.
V1
The meter readings are shown in the circles.
9A
a) Calculate the following meter readings
(i)
Ammeter A1.
(ii)
Ammeter A2.
(iii)
Voltmeter V1.
(iii)
Voltmeter V2.
A2
V2
A1
6V
b) Calculate the resistance of the lamp.
c) Calculate the resistance of the resistor.
6
3A
Two resistors are connected in series to a supply as shown in the diagram.
a) The current in the 100  resistor is 0.05 A.
15 Volt
State the current in the other resistor, R.
b) The voltage across the 100  resistor in 5 V.
Calculate the voltage across the other resistor, R.
100 
R
c) Calculate the value of resistor R.
7
Three identical 6 V, 0.6 A lamps are connected in a series circuit to a supply.
a) Draw a circuit diagram of this arrangement.
b) Calculate the resistance of one lamp.
c) The lamps operate are their stated values.
State the current in the circuit.
d) Calculate the supply voltage required to operate the lamps are their stated value.
Tutorial 3
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Unit 1
8 The circuit below shows a d.c. supply, lamp, ammeter and variable resistor.
A
The setting of the variable resistor is changed so that the resistance is increased.
Explain how the brightness of the lamp changes as the resistance is increased.
9 A variable resistor is used in the speed controller for a model railway engine.
The variable resistor controls the current in the motor of the model engine.
The motor is connected in series with the variable resistor and supply as shown below.
(a)
At one particular speed, the voltage across the variable resistor is 4 volts and the
current in it is 0.5 ampere.
Calculate the resistance of the variable resistor
(b) (i)
The resistance of the variable resistor in the speed controller is increased.
State what happens to the speed of the train. Give a reason for your answer.
(ii)
Tutorial 3
Give one other practical use of a variable resistor.
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Unit 1
10 Shown below is a circuit with a d.c. supply, motor, 100 Ω resistor and 2,000 Ω resistor
and a two way switch.
100 Ω
A
M
B
2,000 Ω
The two way switch can be connected to either point A or point B, in the circuit.
(a) Which connection A, or B, will cause the motor to go fastest?
(b) Explain your answer.
11 The circuit shown opposite is used to
investigate how the voltage across a
resistor is changed as the current
through the resistor is changed.
A
V
(a) Making reference to the above circuit, explain how
the current is changed.
The table shown opposite contains the results obtained in
this investigation.
(b) On graph paper, plot a graph of voltage (y axis)
against current (x axis).
(c) Using data from the line, not data from the table,
calculate the resistance of the resistor
(d) Sketch the graph that would be obtained if the
resistor was replaced by a lamp.
Current
Voltage
(amps)
(volts)
0.0
0
0.2
7
0.4
23
0.6
33
0.8
37
1.0
53
(e) Explain why the two graphs are different.
Tutorial 3
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Unit 1