EXAMPLES – ELECTRICITY
LONG QUESTIONS
In the circuit shown, the identical cells each
have an emf of 3 V and internal resistance of
0,25 . The voltmeters have a very high
resistance, while that of the ammeter is
negligible.
1.1
Switch S is open. What is the reading on
1.1.1 voltmeter V1?
1.1.2 Voltmeter V2?
The switch is then closed.
1.2
Calculate the reading on
1.2.1 the ammeter.
1.2.2 voltmeter V2
1.3
Calculate the power dissipated inside the
battery.
V1
10 
1.
A
S
6
(SC Northern Province 2000)
V1
2.
3,5 
The accompanying circuit consists of five cells connected in
series, each with an emf of 2 V and an internal resistance of
0,1 , a switch S and an arrangement of resistors. The two
voltmeters each have an extremely high resistance.
2.1
What is the reading on V1 when the switch is open?
2.2
The switch is now closed. Calculate the
2.2.1 reading on V1;
2.2.2 reading on V2;
2.2.3 potential difference between points X and Y.
3
V2
S
2
6
(
S
X
Y
10 
6
V2
(WC Western Cape 2000)
3.
3.1
3.2
3.3
3.4
3.5
The following results were obtained during an experiment to determine the relationship
between V and I across a resistor:
V (V)
I (A)
10,0
5,0
8,0
4,0
5,0
2,5
3,0
1,5
1,6
0,8
Plot a graph to illustrate the relationship between V and I using the scales:
horizontal axis:
2 cm = 1 ampere
vertical axis:
2 cm = 2 volts.
State the relationship between voltage and current strength.
Name the law demonstrated in this experiment.
Calculate the resistance of the resistor by measuring the slope of your graph. Indicate how
you obtained your values on the graph.
Mention two precautions that you would take while performing this experiment.
4.
In the circuit diagram the battery has an emf of 12 V and unknown internal resistance. The
reading on voltmeter V1 is 11,4 V
4.1
4.2
4.3
4.4
State Ohm’s Law.
What will be the reading on V2?
Calculate the reading on ammeter A.
Calculate the internal resistance of the
battery.
Calculate the potential difference across
the 15  resistor.
Calculate the rate of energy dissipation in
the 90  resistor.
4.5
4.6
10 
V1
15 
90 
A
V2
12 V
20 
(S.C. Northern Cape 2000)
12 V
5.
V1
S
E, 1
R2
5.1
5.2
In the circuit shown, the battery has an emf , E,
and an internal resistance of 1 . When the
switch is closed, the reading on ammeter A1 is
three times that on ammeter A2, and the readings
of the voltmeters V1 and V2 are 12 V and 4 V
respectively.
Calculate:
V2
4V
A1
the resistance of R1 and R2.
the emf, E, of the battery.
R1
A2
5
7
(S.C. North West Province 2000)
6.
Three identical bulbs, A, B and C, with a resistance of
2  each, are connected to a 6 V source as shown in
the sketch. The internal resistance of the source and
connecting wires is negligible.
6V
S
6.1
6.2
6.3
6.4
7.
B
L
Calculate the effective resistance of the parallel
combination when switch S is closed.
L
Calculate the current through bulb B when S is
L
A
closed.
C
Calculate the energy conversion in bulb C in 2
minutes when S is closed.
Explain what will happen to the brightness of bulbs A and C respectively if the switch is
opened. Refer to current strength, resistance and power. No calculations are required.
(S.C. Free State 1999)
The battery in the circuit below has an emf of 12
V and an internal resistance of 0,3 .
V
S1
7.1
Switch S1 and S2 are now both open. What is
7.1.1 the reading on the ammeter?
7.1.2 the reading on the voltmeter?
Switch S1 is now closed, while switch S2 remains
open. Calculate:
7.2.1 the reading on the ammeter.
7.2.2 the reading on the voltmeter.
7.3
Both S1 and S2 are now closed. Calculate:
7.3.1 the reading on the ammeter.
7.3.2 the reading on the voltmeter.
7.3.3 the amount of power used up inside the battery.
2
7.2
12 V; 0,3 
A
4
3
S2
(S.C. Northern Province 1999)
8.
8.1
A circuit is connected as shown in the
accompanying sketch.
A
4
2
S
What effect does it have in the circuit if
the internal resistance of a cell is not
zero?
6
3
B
L
A
0,4 
8.2
If the switch S is open, determine:
8.2.1 the effective resistance of the parallel
combination.
8.2.2 the effective resistance of the circuit with
all components set as shown.
8.2.3 the reading on the ammeter.
8.2.4 the energy dissipated in the light bulb in
one hour.
0
20 V
1
8.3
The properties of the circuit may change if the switch is closed.
8.3.1 If the switch is still open, determine the potential difference between A and B. Show all your
calculations, or state a clear explanation.
8.3.2 Switch S is now closed. What will be the effect on the brightness of the light bulb? Explain.
8.4
The resistance of the rheostat is now increased. What will be the effect on the brightness of
the light bulb? Explain.
(S.C. Eastern Cape 1999)
9.
In the accompanying circuit
diagram, the battery has
negligible internal resistance
and the ammeters and
voltmeter are such they do not
affect the current in the circuit.
With the switch open, calculate
the readings on
9.1.1 ammeter A1 and
9.1.2 voltmeter V.
R
V
A1
6
9.1
12 V
A2
2
S
9.2
The switch is now closed.
9.2.1 Does the voltmeter reading decrease, increase or remain the same? Explain without doing a
calculation.
9.2.2 If the reading on A1 is now three times that of A2, what is the resistance of resistor R?
(S.C. Western Cape 1998)
10.1
10.2
the total resistance of the circuit.
the readings on A, V1 and V2.
20 V
2
A
V2
6
Calculate:
V1
12
The accompanying circuit diagram shows a battery of emf
20 V and internal resistance of 2  connected in series
with a closed switch, ammeter, resistor of 2  and two
parallel resistors of 6  and 12  respectively. Two
voltmeters are connected across the battery and 2 
resistor. The resistances of the ammeter and voltmeters
can be ignored.
2
10.
MULTIPLE CHOICE
1.
Which electrical circuit is used to determine the resistance of R?
V
A
A
A
A
V
V
R
A
2.
R
V
B
C
D
The accompanying diagram shows an electrical circuit with two identical resistors. What is the
effect on the readings of the voltmeter and ammeter if switch S is closed?
A
B
C
D
E
3.
R
R
Ammeter reading
Voltmeter reading
V
decreases
increases
stays constant
decreases
increases
increases
increases
decreases
decreases
stays constant
R
R
A
The five combinations shown below all consist of light bulbs which have the same resistance
when in operation. Each combination has the same potential difference connected across it.
X
L
L
X
X
1
2
L
X
L
L
L
L
L
L
X
L
L
3
4
5
In which of the combinations will bulb X burn with the same brightness?
A
B
C
D
E
4.
Consider two resistors, X and Y, connected in parallel with the
resistance of X greater than the resistance of Y. The effective
resistance of the combination is
A
B
C
D
5.
3 and 5
2 and 4
4 and 5
1, 3 and 4
None of these.
X
y
greater than X.
less than Y.
equal to X + Y.
1 1
equal to

X Y
All the bulbs in the circuit are identical. Which bulbs will grow with
the greatest intensity?
3
2
A
B
C
D
1 and 2
1,2 and 3
3 and 4
2, 3 and 4
L 1
L
L
4
L
6.
A battery of negligible internal resistance is connected to two
resistors, an ammeter and a voltmeter as shown. One resistor
burns out resulting in the following changes in the meters:
A
B
C
D
7.
Voltmeter
increase
decrease
increase
decrease
unchanged
unchanged
decrease
increase.
V
2V
Identical ammeters are used in the circuit shown. The
ammeter which will register the lowest reading is
A
B
C
D
8.
Ammeter
A
A1
A1
A2
A3
A4.
A2
2
A4
A3
4
The mathematical relationship that represents Ohm’s Law is
A
B
C
D
V = IR
VR
VI
IR
Questions 9 to14 refer to the following graphs which can be used more than once.
.
A
B
C
D
Which of the above graphs represents the mathematical relationship between
9.
energy dissipated in a resistor during a given period of time and current at constant
temperature?
10.
potential difference between the ends of a resistor and current at constant temperature?
11.
power and potential difference across a fixed resistor at constant temperature?
12.
power and current for a given resistance at constant temperature?
13.
potential difference across the ends of a resistor and current if the power remains constant?
14.
power and current through a resistor if power remains constant while potential difference
varied.
15.
Which of the following pairs of units represents potential difference?
A
A and C.s-1
B
V and C.s-1
C
V and J.C-1
D
A and V.-1