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The Ultimate Collection of Physics Problems - Using Electricity
CONTENTS
PAGE
SECTION 1 - FROM THE WALL SOCKET
2
SECTION 2 - ALTERNATING AND DIRECT CURRENT
4
Current, Charge and Time
4
SECTION 3- RESISTANCE
6
Voltage, Current and Resistance
6
Power, Energy and Time
8
Power , Current and Voltage
10
Power and Resistance
13
Section 3 Miscellaneous Questions
15
SECTION 4 - USEFUL CIRCUITS
18
Series Circuits
18
Parallel Circuits
20
Resistance in Series
22
Resistance in Parallel
23
Combination Circuits
25
Section 4 Miscellaneous Questions
27
SECTION 5 - BEHIND THE WALL
29
SECTION 6 - MOVEMENT FROM ELECTRICITY
31
USING ELECTRICITY REVISION QUESTIONS
32
General Level
32
Credit Level
35
APPENDIX (I) DATA SHEET
41
APPENDIX (II) ANSWERS TO NUMERICAL PROBLEMS.
42
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The Ultimate Collection of Physics Problems - Using Electricity
Section 1 - From the Wall Socket
The wordsearch below tests your knowledge and understanding of the main
ideas covered in the section.
Work out the answers to questions 1-10 and then find them in the
wordsearch below.
N E U T R A L E H B X T Q Z D T
B A T H R O O M A Z A G C E X J
P F G P J
B E E I
R D H T M T G
L R T M T U F Y E R T A O O H N
A W X E L H D M E M L H L K G I
S B A B L C R Y N U E V B M E S
T N J
I
T N E R E S J
O W N E D V N E R H C U D I
C F O O R R I
T U F I
J
G S Z D R A
J
I
A A E I
C
O F F L
R E K V S U M K W L N A
A D L B V U T I
G H V B I
D P O K S T
Z D L B Z O U B O I
E K U A K A E V I
E
L K N T S C M
C O W O L L E Y D N A N E E R G
D O D N U O S D N A T H G I
H I
N Q T O P P I
L U
N W L U H X M
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The Ultimate Collection of Physics Problems - Using Electricity
Wordsearch Questions
1.
The mains supplies electrical energy to a television. This energy is converted into two
other forms of energy. What are they?
2.
A household appliance with power rating 12 000 W.
3.
The colour of the neutral wire in a plug.
4.
The following symbol was found on the rating plate of a hairdryer
(a) What does this symbol mean?
(b) What must the outer casing of the hairdryer be made of?
5.
The rating plate for a radio is shown below.
model no. RF 230
A.C 230 V 50 Hz
6W
(a) What size of fuse should be fitted in the plug of the radio?
(b) Which wire is missing in the flex of the radio?
6.
Which component in the plug of an appliance breaks the circuit if the current gets too
high?
7.
A washing machine has three wires in the flex connecting it to the plug.
(a) Which wire, under normal circumstances, does not carry electricity?
(b) To what part of the machine is this wire normally connected?
8.
The human body can conduct electricity. What substance can improve this
conductivity?
9.
Which room in the house has no electrical sockets?
10. To which pin in a plug is the earth wire connected?
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The Ultimate Collection of Physics Problems - Using Electricity
Section 2 - Alternating and Direct Current
Current, Charge and Time
In this section you can use the equation:
charge = current x time
also written as
Q = It
where
1.
Q = charge in coulombs (C)
I = current in amperes (A)
t = time in seconds (s).
Find the missing values in the following table.
Charge (C)
2.
Current (A)
Time (s)
(a)
5
30
(b)
0·005
3 600
(c)
3
1·5
(d)
27·6
2·3
(e)
1 800
60
(f)
94
10
A bulb draws a current of 1 A. How much charge flows through it in 60 seconds?
12V
3.
756 C of charge flow though an electric heater in 180 seconds. What is the current in
the heater?
4.
A hairdryer operates with a current of 5 A. How much time would it take for 6 000 C
to pass through the hairdryer?
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The Ultimate Collection of Physics Problems - Using Electricity
5.
A current of 2·1 A flows through an electric
shaver for 5 minutes. How much charge flows
in this time?
6.
A 60 W bulb is switched on for 30 minutes. If 450 C pass through it in this time, what
is the current flowing in the bulb?
7.
A ‘surround sound’ speaker system on a TV draws a current of 0·6 A. In an average
day 6 480 C of charge flows through the speaker system. For how long is the TV
switched on each day?
8.
An electric kettle has a label on it as shown below.
MODEL No. 5510 - 01
capacity 1·7 litres
9·2 A /220 - 240 V
2·2 kW
After the kettle is switched on it automatically switches off when the water in it has
boiled. On one occasion 1 656 C passed through the kettle before it switched off. Use
the information given to work out how long the water took to boil?
9.
An electric fire is rated at 2·875 kW, 230 V, 12·5 A. How much charge will flow
through this fire in a period of 2 hours 20 minutes?
10. One day an electric iron was switched on from 1·45 p.m. until 3·15 p.m. What current
was drawn by the iron if 2·484 x 104 C passed through it in this time?
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The Ultimate Collection of Physics Problems - Using Electricity
Section 3 - Resistance
Voltage, Current and Resistance
In this section you can use the equation:
voltage = current x resistance
also written as:
V = IR
where
V = voltage in volts (V)
I = current in amps (A)
R = resistance in ohms ().
Helpful Hint.
Many appliances run from mains voltage which is 230 V ac.
Useful units for electricity are:
1 A = 0·000 001 A = 1 x 10-6 A
1 mA = 0·001 A = 1 x 10-3 A
1.
Find the missing values in the following table.
Voltage (V)
2.
Current (A)
Resistance ()
(a)
15
35
(b)
0·2
1 000
(c)
230
125
(d)
24
550
(e)
120
12
(f)
6
6·25 x 10-3
Look at the following circuits and calculate the supply voltage in each case:
(a)
Vs
(b)
10 A
5
(c)
Vs
Vs
2·56 A
250 
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50 mA
480 
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The Ultimate Collection of Physics Problems - Using Electricity
3.
Look at the following circuits and calculate the current in each case:
(a)
24 V
(b)
12 V
12 
I
50 
550 
Look at the following circuits and calculate the unknown resistance in each case:
(a)
24 V
(b)
12 V
R
(c)
48 V
30 A
25 mA
5.
48 V
I
I
4.
(c)
R
660 A
R
Calculate the resistance of a lamp if the current through it is 10 mA when operated by
a 24 V supply.
6. A power drill is operated at mains voltage and
has a resistance of 1·5 k. Calculate the current
through the drill.
7.
The maximum current an electric motor can safely handle is 10 mA and it has a
resistance of 360 . Calculate its safe operating voltage.
8.
A cooker draws a maximum current of 28·75 A and has a resistance of 8 . At what
voltage should it operate?
9.
Hairdryers work from the mains voltage and can have currents of up to 15 mA
flowing through them. Calculate the resistance of the hairdryer.
10. Overhead cables have resistance of 25 k. If the voltage across the cables is 4 000 V
calculate the current through them.
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The Ultimate Collection of Physics Problems - Using Electricity
Power, Energy and Time
In this section you can use the equation:
power = energy
time
also written as
P=E
t
where
1.
P = power in watts (W)
E = energy in joules (J)
t = time in seconds (s).
Find the missing values in the following table.
Power (W)
Energy (J)
Time (s)
(a)
1 500
30
(b)
180 000
36 000
(c)
100
600
(d)
1 200
2
(e)
3 000
120 000
(f)
2·5
25
2.
How long will it take for a 60 W bulb to use 720 J of electrical energy?
3.
A bulb uses 45 000 J of energy in 300 seconds.
What is its power rating?
4.
A 50 W immersion heater is switched on for 80 seconds. How much electrical energy
passes through it in this time?
5.
Calculate the power rating of an electric sewing
machine which uses 4 560 J of energy in 8 minutes.
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The Ultimate Collection of Physics Problems - Using Electricity
6.
A 1 200 W hairdryer is switched on for 20 minutes. How much electrical energy does
it use?
7.
For how many minutes must a 600 W shaver be switched on in order to use 540 000 J
of electrical energy?
8.
An electric fire uses 5·22 MJ of electrical energy in half an hour. Calculate the power
rating of the fire.(1 MJ = 1 x 106 J = 1 000 000 J )
9.
How long will it take a 1·4 kW paint stripper to use 1·68 MJ of electrical energy?
10. A microwave oven is on for twenty minutes each day. If it uses 7·98 MJ of electrical
energy in one week, what is its power rating?
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The Ultimate Collection of Physics Problems - Using Electricity
Power, Current and Voltage
In this section you can use the equation:
power = current x voltage
also written as
P = IV
where: P = power in watts (W)
I = current in amps (A)
V = voltage in volts (V).
Helpful Hint
The voltage of the mains is 230 V ac.
1.
Find the missing values in the following table.
Power (W)
Current (A)
Voltage (V)
(a)
2·5
12
(b)
0·6
9
(c)
1·5 x 103
230
(d)
36
12
(e)
0·624
2·6 x 10-3
(f)
1·5
0·25
2.
A car battery supplies a voltage of 12 V. One headlamp bulb draws a current of 3 A.
What is the power rating of this bulb?
3.
An electric shower has a power rating of
12 W and draws a current of 0·11 A. What
voltage is required to operate the shower?
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The Ultimate Collection of Physics Problems - Using Electricity
4.
What current flows through a 230 V, 60 W household lamp when it is operating at the
correct voltage?
5.
Calculate the power rating of an electric drill which draws a current of 3 A when
connected to the mains.
6.
What would be the reading on the ammeter in the circuit shown?
12 V
24 W
A
7.
The following information was found on the rating plate of a food processor:
BL300
360 W
230 V 50/60 Hz
made in the U.K
Calculate the current flowing in the food processor.
8.
A fridge has a power rating of 160 W. When it is plugged into the mains what current
will it draw?
9.
A radio has a power rating of 6 W and draws a current of 0·5 A when operating
normally.
(a) What voltage does this radio need?
(b) How many 1·5 V batteries would be needed to operate the radio?
10. The circuit shows a heating element (resistor) operating correctly.
I = 4·2 A
1 kW
What is the voltage of the supply?
11. A torch bulb draws a current of 500 mA. It has a power rating of 1·75 W. What
voltage is required to light the bulb to its correct brightness?
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The Ultimate Collection of Physics Problems - Using Electricity
12. An electric locomotive on the East Coast line gets its electricity from the overhead
cables which supply 25 kV a.c. The locomotive has a top speed of 140 mph and it
operates at 4·7 MW. Calculate the current flowing to the locomotive.
13. A helium - neon laser emits red light and has many uses in medicine. The laser uses
the 230 V mains supply and has a power rating of 5 mW. What current flows in the
laser?
14. The current flowing in an electric keyboard is measured as 800 mA. What voltage is
required to operate the keyboard if it has a power rating of 7·2 W?
15. An electric fire has three heating elements which can be switched on and off
independently. Each element has a power of 1·3 kW and is controlled by a switch on
the side of the fire.
(a) Calculate the current drawn from the socket when one heating element is
switched on.
(b) What is the maximum current that the fire could draw?
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The Ultimate Collection of Physics Problems - Using Electricity
Power and Resistance
Helpful Hint
By combining the formulae:
P = I V and V = I R
we have alternative forms of the formula for power.
i.e.
P = I (IR)
P = I2 R
and
P = (V/R) V
P = V2/R
Use these alternative forms of ‘power formulae’ in the following questions.
1.
Calculate the power rating of a lawn mower
which has a resistance of 70·5  and draws a
current of 2·5 amps.
2.
What power is dissipated in the element of a toaster if it has a resistance of 64  and
draws a current of 3·75 amps?
3.
Find the power rating of a television given that it operates at mains voltage and has a
resistance of 480 .
4.
What would be the power rating of an iron if it operates at mains voltage and has a
resistance of 45 ?
5.
Calculate the power rating of a hedge trimmer given that it draws a current of 1·67 A
and has a resistance of 144 .
6.
Calculate the power rating of a Karaoke machine if it has a resistance of 5 760  and
operates at mains voltage.
7.
How much power is dissipated in curling tongs if they draw a current of 1·05 A and
have a resistance of 230  while operating from the mains?
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The Ultimate Collection of Physics Problems - Using Electricity
8.
A microwave oven has a resistance of 68  and works properly from a mains supply.
What is the power rating for such an oven?
9.
Find the power rating of a Hoover
designed with a resistance of 110 
when operating at mains voltage.
10. What current is drawn from the supply by a transistor radio if it has a power rating of
3 W and a resistance of 12 ?
11. Find the resistance of a table lamp which has a power rating of 115 W and draws a
current of 0·5 A.
12. An alarm clock is driven by a domestic supply voltage of 230 V. What is its resistance
if it has a power rating of 2 W?
13. Calculate the total resistance of an electric fire if it operates safely at 230 V and uses
electrical energy at a rate of 750 W.
14. Calculate the voltage of a car alarm which has a power rating of 3 W and resistance of
50 .
15. Find the power rating of an electric food
mixer if it operates at mains voltage and
has a resistance of 400 .
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The Ultimate Collection of Physics Problems - Using Electricity
Section 3 Miscellaneous Questions
1.
The following circuit is set up in a laboratory.
3A
12 V
The bulb is switched on for 3 minutes and the voltmeter and ammeter readings remain
as shown in the diagram.
(a) What is the power rating of the bulb?
(b) Calculate the resistance of the bulb.
(c) How much electrical energy is supplied to the bulb in the 3 minutes?
(d) How much charge passes through the bulb in the 3 minutes?
2.
The resistor in the following circuit has a power rating of 0·09 W.
A
10 k
V
(a) What reading would you expect to find on the ammeter?
(b) What reading would you expect to find on the voltmeter?
(c) How long would it take for 5·4 joules of electrical energy to be supplied to the
resistor?
(d) How much charge passes through the resistor in 1 minute?
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The Ultimate Collection of Physics Problems - Using Electricity
3.
Some students decide to measure the resistance of a torch bulb and each set up a
circuit to do this. The circuits are shown below.
V
A
A
V
Circuit 1
Circuit 2
(a) Which circuit has been set up correctly? Explain your answer.
(b) Use the following results to plot a graph of voltage against current and
hence or otherwise find the resistance of the torch bulb.
4.
Current (A)
Voltage (V)
0·5
5
0·8
8
1·1
11
1·4
14
1·7
17
The rating plate on an electric kettle is shown below.
Type KE
230 V
180
Serie 2
50 - 60 Hz
2·2 kW
(a) Calculate the current flowing in the kettle.
(b) What is the resistance of the heating element in the kettle?
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The Ultimate Collection of Physics Problems - Using Electricity
5.
A vacuum cleaner connected to the mains draws a current of 3·1 A.
(a) What is the power rating of the vacuum cleaner?
(b) What is the resistance of this appliance?
6.
An iron rated at 11 kW draws a current of 4·6 A. One day 8 280 C of charge passed
through the iron.
(a) How long was the iron in use?
(b) How much electrical energy was supplied in this time?
7.
A bulb in a laboratory operates with a current of 2 A. During an experiment 1 200 C
of charge flow through the bulb, supplying it with 14 400 J of electrical energy.
(a) How long did this experiment last?
(b) What was the power rating of the bulb?
8.
The power rating of an electric fire is 3 kW and it draws a current of 12·5 A. One
afternoon the fire used up 8·1 MJ of electrical energy.
(a) For how long was the fire switched on?
(b) How much charge passed through the fire in this time?
9.
An electrical appliance has a power rating of 2·4 kW. If 432 000 J of electrical energy
are supplied to the appliance by 1 800 C of charge, calculate the current drawn by the
appliance.
10. The following label is attached to a hairdryer.
Type 4533
1·44 kW
230 V / 6 A
Calculate the amount of electrical energy supplied to the hairdryer by 4 320 C of
charge.
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The Ultimate Collection of Physics Problems - Using Electricity
Section 4 - Useful Circuits
Series Circuits
Helpful Hint
The rules for series circuits are:
1.
2.
1.
the current is the same at all points in the circuit
the voltage of the source is shared amongst the components in the circuit.
Two identical 2·5 V bulbs are connected to a supply as shown. What is the voltage of
the supply?
2·5 V
2.
2·5 V
Four identical resistors are connected across a 12 V supply as shown in the diagram.
What is the voltage across each of the resistors?
12 V
A
3.
B
C
D
A simple circuit with a bulb and resistor in series is shown below.
36 V
R
12 V, 36 W
(a) If the bulb is operating at its correct voltage and power rating what is the voltage
across the resistor R?
(b) The current in the bulb is 3 A. What current flows in the resistor?
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The Ultimate Collection of Physics Problems - Using Electricity
4.
Two resistors are connected in series to a supply as shown in the diagram.
15 V
200 
100 
(a) The current in the 200  resistor is 0·05 A. What is the current in the other
resistor?
(b) The voltage across the 100  resistor is 5 V. What is the voltage across the
200  resistor?
5.
A rheostat is used as a dimmer switch in a simple series circuit as shown.
14 V
The rheostat is adjusted until the bulb is shining brightly. The voltage across the bulb
is 13·8 V and the current through the rheostat at this setting is 1·7 A.
(a) Calculate the voltage across the rheostat.
(b) What is the current flowing in the bulb?
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The Ultimate Collection of Physics Problems - Using Electricity
Parallel Circuits
Helpful Hint
The rules for parallel circuits are:
1.
2.
1.
the voltage is the same across all the components in parallel.
the current from the supply is shared amongst the different branches of the
circuit.
Two resistors are connected in parallel to a 12 V battery.
12 V
0·2 A
0·3 A
(a) What is the voltage across R1?
(b) What is the voltage across R2?
R1
(c) What size of current is drawn from
the battery?
R2
6V
4A
2.
L1
Two identical bulbs and a resistor are connected
in parallel to a 6 V supply.
L2
(a) What is the voltage across L2 ?
(b) A current of 1·8 A flows through each of the bulbs.
What is the current flowing through the resistor?
3.
R1
An electric fire has three elements which can be switched on and off independently.
The elements are connected in parallel to the mains supply. Each element draws a
current of 0·3 A when switched on.
230 V
(a)
What is the voltage across the middle
element?
(b)
What is the total current flowing from the
supply when two of the elements
are switched on?
(c)
What is the maximum current drawn
from the mains by the fire?
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The Ultimate Collection of Physics Problems - Using Electricity
4.
The headlamps and side lights in a car are connected in parallel. The diagram below
shows how they are connected. The side lights (L1 & L2) may be switched on by
themselves using switch S1. The headlights (H1 & H2) are switched on by switch S2
and only come on if the sidelights are already on.
S2
S1
12 V
L2
L1
H2
H1
(a) What is the voltage across the sidelight L1?
(b) What is the voltage across the headlight H2?
(c) Each sidelight draws a current of 3 A from the car battery. What is the total
current drawn from the battery when S1 only is closed?
(d) Each headlight draws a current of 5 A from the car battery. What is the total
current drawn from the battery when S1 and S2 are closed?
5.
A hairdryer contains a motor and heating elements (resistors). The hairdryer shown
below has three heat settings- cold, warm and hot. The circuit diagram shows how
these settings are achieved using switches A, B and C.
A
230 V
B
M
C
H1
H2
The motor draws a current of 3 A from the mains and the heating elements draw a
current of 2 A each from the mains.
(a) Which switches must be closed to make the hairdryer blow warm air?
(b) What current is drawn from the mains when the hairdryer blows warm
air?
(c) Which switches must be closed to make the hairdryer blow hot air?
(d) What current is drawn from the mains when the hairdryer blows hot air?
(e) What is the minimum current drawn from the mains when the hairdryer is on?
(f) What is the voltage across the motor?
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The Ultimate Collection of Physics Problems - Using Electricity
Resistance in Series
In this section you can use the equation:
Rs = R1 + R2 + R3 +......
where
1.
Rs = total resistance of a series circuit or series section of a circuit ().
Three resistors R1, R2 and R3 are arranged in series as shown in the diagram below.
R2
R1
R3
Find the missing values in the table.
R1 ()
R2 ()
R3 ()
(a)
5 000
490
85
(b)
80
300
25
(c)
800
2 000
200
(d)
700
300
(e)
(f)
2.
1 400
140
225
100
550
85
390
Calculate the total resistance of the following circuit.
800 
3.
Rs ()
5 k
3·2
k
The resistance of the following circuit is 8·8 k. Calculate the resistance of R.
950 
R
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6·3
k
22
The Ultimate Collection of Physics Problems - Using Electricity
Resistance in Parallel
In this section you can use the equation
1
1
1
1
+
+ ....
+
=
Rp
R1 R2 R3
where
1.
Rp = total resistance of a parallel circuit or parallel section of a circuit ().
Calculate the total resistance of each of the following circuits:
(a)
(b)
(c)
80 
20 
120 
80 
20 
120 
(d)
(e)
(f)
80 
100 
400 
120 
150 
1 600 
(g)
(h)
(i)
300 
150 
800 
600 
300 
2 400 
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The Ultimate Collection of Physics Problems - Using Electricity
2.
Calculate the total resistance of each of the following circuits:
(a)
(b)
600 
3 k
1·2
k
600 
3 k
600 
600 
3 k
600 
(d)
3.
(c)
(e)
(f)
280 k
300 
3 k
560 k
600 
900 
560 k
200 
600 
The total resistance of the circuit below is 80 . Calculate the resistance of R.
R
220 
1 232 
4.
The total resistance of the following circuit is 112·5 . Calculate the resistance of
resistor A.
300 
A
900 
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The Ultimate Collection of Physics Problems - Using Electricity
5.
The total resistance of the following circuit is 240 . Calculate the resistance of
resistor X
1 200 
X
1 800 
Combination Circuits
Helpful Hint
Rs = R1 + R2 + R3 +......
1 =
Rp
1.
1 + 1 + 1 +......
R1 R2 R3
( for a series section of a circuit)
(for a parallel section of a circuit)
Calculate the total resistance in each of the following networks:
(a)
(b)
20 
10 
60 
40 
60 
20 
(c)
(d)
10 
10 
10 
10 
(e)
10 
100 
100 
10 
15 
15 
30 
30 
(f)
5
10 
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10 
5
25
The Ultimate Collection of Physics Problems - Using Electricity
2.
The following circuit shows part of a car lighting system.
250 
X
250 
560 
Z
Y
560 
Calculate the resistance between points:
(a) X and Y
(b) Y and Z
(c) X and Z.
3.
Calculate the resistance of the network of resistors shown below.
40 
80 
80 
120 
60 
4.
Which network of resistors has the lowest total resistance?
Network A
Network B
90 
90 
90 
15 
15 
90 
5.
A school technician has different resistors to use in building house wiring models.
10 
10 
50 
50 
100 
100 
He has two 10  resistors, two 50  resistors and two 100  resistors.
How can these resistors be combined to produce a total resistance of:
(a) 260 
(b) 300 
(c) 30 
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(d) 35 ?
26
The Ultimate Collection of Physics Problems - Using Electricity
Section 4 Miscellaneous Questions
1.
Bulbs B1, B2 and B3 are all identical. What is the voltage across bulb B2 and bulb B3?
12 V

B1
8V

B2
B3
2.
Look at the circuit shown and find:
24 V
(a) the voltage across R2
I=2A
(b) the current through R1.
R1
16 V 
3.
R2
R3
In the following circuit the bulbs are identical. Find:
(a) the voltage across each bulb
(b) the current through each bulb.
36 V
I = 0·9 A
 12 V 
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27
The Ultimate Collection of Physics Problems - Using Electricity
4.
In the following circuit all the resistors are identical. Find:
(a) the voltage across each resistor
(b) the current through each resistor.
24 V
3A
5.
A
B
C
D
Daral has a selection of resistors and has to combine them in some way to obtain a
total resistance of 60 .
He has two 5 , two 20  and two 90  resistors.
He sets up three different networks as shown below. Which one is correct?
20 
20 
20 
90 
5
5
90 
Network 1
Network 2
20 
90 
5
20 
90 
Network 3
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28
The Ultimate Collection of Physics Problems - Using Electricity
Section 5 - Behind the Wall
In this section you can use the equation:
energy = power x time
also written as
E = Pt
where
E = energy in kilowatt hours (kWh)
P = power in kilowatts (kW)
t = time in hours (h).
Helpful Hint
Since the joule is such a small unit of energy electricity boards use the unit
kilowatt hour to measure how much electricity has been used.
1 kWh = 1 000 W x 3 600 s
1 kWh = 3 600 000 J
(1 kWh is often called a ‘unit’ of electricity )
1.
How many kilowatt hours of energy does a 3 kW electric fire use when it is switched
on for 4 hours?
2.
A 4 kW immersion heater is left on for 2 hours to heat water for a bath.
(a) How many kilowatt hours of energy does it use?
(b) How many joules of energy does it use?
3.
How many units of energy will a 100 W security light use if it is left on overnight for
8 hours?
4.
A 35 kW electric fire is left on full power for 4 hours.
(a) How many units of electricity did the fire use?
(b) If each unit of electricity costs 7·4 p calculate the total cost of running the fire for
4 hours.
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29
The Ultimate Collection of Physics Problems - Using Electricity
5.
How long would it take a 1·6 kW fridge to use 8 kWh of energy?
6.
A 360 W food processor used 0·072 kWh of energy. For how many minutes was it
working?
7.
An electric iron uses 0·3 kWh of electricity when left on for 15 minutes. What is the
power rating of the iron?
8.
A 2 kW hot water urn is left on full power for 30 minutes.
(a) How many kilowatt hours of energy does it use in this time?
(b) What did this electricity cost if each unit costs 7·4 p?
9.
How much more expensive would it be to run a 4·5 kW heater for 15 minutes than a
60 W bulb for 10 hours if each unit of electricity costs 7·4 p?
10. The electricity board issue a bill every three months. The bill shows the electricity
meter reading at the beginning and end of this period and the total number of units
used. The cost per unit of electricity is given along with the standing charge for use of
the electricity board’s facilities. An exert from an electricity bill is given below.
Meter Reading
Present
Previous
49211
48347
Detail of charges
domestic cost per unit 7·4 p
standing charge
£10·73
(a) How many units of electricity were used?
(b) What was the total cost of this bill?
(c) How many joules of electricity were used?
(d) Why do you think that electricity companies measure the amount of electricity
used in kilowatt hours rather than joules?
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The Ultimate Collection of Physics Problems - Using Electricity
Section 6 - Movement from Electricity
The questions and wordsearch below revise the main ideas in this section.
Copy and complete the passage then find the missing words in the
wordsearch.
An...............is created when current flows in a wire wrapped around an iron bar.
The...............and...............are two devices which use electromagnets.
When a current carrying wire is placed in a magnetic...............it experiences a ................
The electric.................. uses this principle to rotate a coil of wire in a magnetic field.
Current flows into this coil of wire through
...............which make contact with a rotating commutator.
The ...............is a split ring of metal which changes the direction of current in the wire at
every half turn.
Large commercial motors use ............... for the brushes because it is soft enough to prevent
wearing of the commutator. They also use electromagnets rather than ....................
magnets as this is a cheaper, lighter and more compact way of producing a magnetic field.
B
W
P
C
O
M
S
H
L
E
X
L
E
R
Y
C
C
M
U
K
R
O
T
L
L
R
O
T
O
M
I
A
E
L
E
C
T
R
I
C
B
E
L
L
N
R
R
L
P
E
R
R
T
I
U
N
C
R
O
F
M
B
E
T
R
I
I
B
O
G
Z
I
N
E
N
Y
O
F
O
R
E
L
A
A
R
C
E
T
R
Y
J
N
O
C
O
R
B
M
O
B
N
O
S
F
Y
O
I
R
O
T
O
O
D
E
A
R
S
I
I
S
E
E
M
N
A
R
M
I
M
T
T
E
P
E
H
L
N
A
O
T
I
A
R
O
O
L
H
L
D
E
E
O
G
C
U
L
E
C
R
D
T
S
I
A
S
N
P
E
R
M
A
N
E
N
T
O
U
N
A
Y
T
L
A
F
M
E
A
R
T
C
B
R
U
S
T
E
E
C
R
O
F
L
I
D
E
O
B
E
M
I
R
O
T
I
C
O
E
Y
A
L
U
M
M
O
C
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The Ultimate Collection of Physics Problems - Using Electricity
Using Electricity Revision Questions
General Level
1.
Clare has set up a circuit, as shown below, in order to switch on a bulb.
She wants to measure the voltage across the bulb and the current through it.
(a) What should Clare use in order to measure the voltage across the bulb?
(b) What should be used to measure the current in the bulb?
(c) Redraw the circuit to show the correct positions of these meters.
2.
Look at the following circuits:
0·3 A
12 
2V
100 
6V
0·015 A
R
(a) Calculate the voltage across the 12  resistor.
(b) Calculate the current through the 100  resistor.
(c) What is the value of resistor R?
3.
An electric kettle operates at mains voltage drawing a current of 9 A.
(a) What is the value of mains voltage?
(b) What is the power rating of the kettle?
(c) How much electrical energy is supplied to this kettle each second?
(d) How much electrical energy would the kettle use if it was switched on for
3 minutes?
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The Ultimate Collection of Physics Problems - Using Electricity
4.
For each of the following circuits what should be the reading on the ammeter and
voltmeter?
3·6 V
2·5 V
(a)
(b)
A
A
05 A
10 
20 
1·2 V
V
05 A
05 A
V
1·5 V
(c)
A
0·2 A
V
5.
Calculate the power rating of each of the following electrical appliances.
(a) An electric shaver which draws a current of 2 A from the mains supply.
(b) An electric fire which uses 5 400 000 J of energy in 30 minutes.
(c) A lamp which operates on the mains voltage drawing a current of 0·26 A.
(d) A hairdryer which uses 1 080 000 J of electrical energy in 15 minutes.
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33
The Ultimate Collection of Physics Problems - Using Electricity
6.
The labels for two electrical appliances are shown below but one important piece of
information is missing from each label.
Kettle
Model No. 5510
capacity 17 litres
..............V, 50 Hz
2 000 W
BEEB approved
Model No. 50736
Lamp
230 V a.c
50 Hz
.....................W
(a) Suggest suitable values for the operating voltage of a kettle and the power rating
of a lamp.
The flexes for these appliances are shown below:
Flex A
blue
green & yellow
brown
blue
brown
Flex B
(b) Complete the following table for flex A.
Colour of
insulation
Name of wire
brown
green and yellow
blue
(c) What is the purpose of the green and yellow wire in flex A?
(d) Which flex belongs to the lamp? Explain your answer.
(e) Both the kettle and the lamp are fitted with a 3 A fuse. Which appliance fails to
operate? Explain your answer and suggest a solution.
7.
Explain how each of the following situations could result in an accident.
(a) Eight appliances are connected to one socket using adapters.
(b) An electricity socket is fitted in the bathroom.
(c) A 13 A fuse is fitted to an alarm clock/radio.
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The Ultimate Collection of Physics Problems - Using Electricity
Credit Level
1.
A lamp is connected in parallel with a resistor as shown below. The bulb is a 12 V, 36
W bulb.
(a) Calculate the current through the bulb
when it is operating at its correct
voltage.
(b) What is the resistance of the bulb?
12 V
6
(c) Calculate the current through the
resistor.
(d) Calculate the total current drawn from
the supply.
2.
The rating plate from a hand blender is shown below.
Electronic model no. 3456
230 V 50 Hz
1 500 W
(a) Calculate the current drawn by the blender.
(b) Calculate the resistance of the blender.
(c) Explain the significance of the symbol on the rating plate.
The blender has to be charged up before use. It is charged for 5 hours and the
average charging current is 10 mA.
(d) Calculate the charge stored.
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35
The Ultimate Collection of Physics Problems - Using Electricity
3.
Paul is always being accused of wasting electricity because he has nearly every
electrical appliance switched on at once in his room.
His stereo, CD player, television, lamp and computer are all connected to the mains
via an adapter as shown below.
mains
voltage
hairdryer
R = 500 
TV
R = 480 
lamp
R = 400 
computer
R = 2 k
stereo
R = 2·2
k
(a) What kind of circuit is this?
(b) What simple test could you carry out to prove this?
(c) Calculate the total resistance of the circuit when all appliances are on.
(d) Calculate the current drawn from the mains when all the appliances are on.
(e) What is the power rating of the computer?
(f) How many units of energy are used when the computer is on for 6 hours?
(g) If a unit of electricity is 7·4p how much does it cost to run the computer
for this length of time?
(h) The lamp is usually on for 5 hours a day. How much money is saved:
(i) per day?
(ii) per week?
if the lamp is switched off.
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36
The Ultimate Collection of Physics Problems - Using Electricity
4.
Junior Spark tries his hand at some DIY electrics in and around his new home.
He designs two circuits. Circuit 1 is a model for a room light and Circuit 2 is a model
for toy car lights.
His designs are shown below.
24 V
Circuit 1
R = 900
Circuit 2
S1
S2
L1
S3
L2
H1
Sidelights 12 V , 24 W
H2
Headlights 12 V , 36 W
Look at the circuits and answer the following questions.
(a) Describe the function of the variable resistor in circuit 1.
(b) The variable resistor can be set at any resistance between 10  and 10 k. Use
this range to calculate the minimum and maximum current that could flow
through the bulb.
(c) In circuit 2, which switches must be closed to switch on the head lights?
(d) What size of current is drawn from the battery when all the lights in circuit 2 are
on?
(e) What is the advantage of having the sidelights and the headlights connected in
parallel instead of in series with one another?
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37
The Ultimate Collection of Physics Problems - Using Electricity
5.
A typical toaster design consists of two heating elements arranged in parallel as shown
below.
80 
230 V
80 
Calculate the power rating of the toaster.
6.
Look at the circuit below and answer the following questions:
A
10 
S
6V
20 
V
60 
(a) Calculate the resistance of the circuit when switch S is open.
(b) What is the reading on the ammeter when switch S is open?
(c) What is the reading on the voltmeter when switch S is open?
(d) Calculate the resistance of the circuit when switch S is closed.
7.
Consider the following circuit.
9V
A
40 
20 
90 
(a) What is the total resistance of this circuit?
(b) Calculate the reading on the ammeter.
(c) What is the voltage across the 90  resistor?
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38
The Ultimate Collection of Physics Problems - Using Electricity
8.
The following circuit was set up in a classroom.
Vs
0·04A
300 
V
A
100 
P
Q
0·03A
0·25 W
(a) What is the current through the bulb?
(b) Calculate the voltmeter reading.
(c) What should the reading on the ammeter be?
(d) Calculate the voltage across the 100  resistor.
(e) What is the voltage across the 300  resistor?
(f) What is the voltage across PQ?
(g) Calculate the supply voltage, Vs.
9.
A basic design for an electric bell is shown below.
Explain how the bell operates.
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39
The Ultimate Collection of Physics Problems - Using Electricity
10. A d.c. electric motor was built using the following design:
(a) Name the parts of the motor labelled X, Y and Z.
(b) Explain how the d.c. motor operates.
(c) Why is carbon used in commercial motors as the material for component X?
(d) Explain how the magnetic field is produced in larger commercial motors.
(e) How could component Y be improved, from this simple design, for use in large
scale motors.
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40
The Ultimate Collection of Physics Problems - Using Electricity
Appendix (i)
Data Sheet
Speed of light in materials
Material
Speed in m/s
Air
3 x 108
Carbon dioxide
3 x 108
Diamond
12 x 108
Glass
20 x 108
Glycerol
2.1 x 108
Water
23 x 108
Gravitational field strengths
Gravitational field strength
on the surface in N/kg
Earth
10
Jupiter
26
Mars
4
Mercury
4
Moon
16
Neptune
12
Saturn
11
Sun
270
Venus
9
Uranus
117
Pluto
42
Specific latent heat of fusion of materials
Material
Specific latent heat of
fusion in J/kg
Alcohol
099 x 105
Aluminium
395 x 105
Carbon dioxide
180 x 105
Copper
205 x 105
Glycerol
181 x 105
Lead
025 x 105
Water
334 x 105
Specific latent heat of vaporisation
of materials
|Material
Sp.l.ht vap(J/kg)
Alcohol
112 x 105
Carbon dioxide
377 x 105
Glycerol
830 x 105
Turpentine
290 x 105
Water
226 x 105
Speed of sound in materials
Material
Speed in m/s
Aluminium
5 200
Air
340
Bone
4 100
Carbon dioxide
270
Glycerol
1 900
Muscle
1 600
Steel
5 200
Tissue
1 500
Water
1 500
Specific heat capacity of materials
Material
Specific heat
capacity in J/kgoC
Alcohol
2 350
Aluminium
902
Copper
386
Glass
500
Glycerol
2 400
Ice
2 100
Lead
128
Silica
1 033
Water
4 180
Steel
500
Melting and boiling points of materials
Material
Melting
Boiling
point in oC point in oC
Alcohol
-98
65
Aluminium
660
2470
Copper
1 077
2 567
Glycerol
18
290
Lead
328
1 737
Turpentine
-10
156
SI Prefixes and Multiplication Factors
Prefix
Symbol
Factor
giga
mega
kilo
milli
micro
nano
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G
M
k
m

n
1 000 000 000=109
1 000 000
=106
1 000
=103
0001
=10-3
0000 001
=10-6
0000 000 001=10-9
41
The Ultimate Collection of Physics Problems - Using Electricity
Appendix (ii) Answers to Numerical Problems.
Section 2-A.C.& D.C.
Current, Charge &
Time (p.4)
1.
(a) 150 C
(b) 18 C
(c) 2 s
(d) 12 s
(e) 30 A
(f) 94 A
2. 60 C
3. 42 A
4. 1 200 s
5. 630 C
6. 025 A
7.
(a) 10 800 s
8. 180 s
9. 105 000 C
10. 46 A
Section 3 Resistance
Voltage, Current
Resistance(p.6)
1.
(a) 525 V
(b) 200 V
(c) 184 A
(d) 004 A
(e) 10 
(f) 960 
2.
(a) 50 V
(b) 640 V
(c) 24 V
3.
(a) 2 A
(b) 024 A
(c) 009 A
4.
(a) 960 
(b) 400 000 
(c) 72 727 
5. 2 400 
6. 015 A
7. 36 V
8. 230 V
9. 15 333 
10. 016 A
&
1.(f) 10 s
2. 12 s
3. 150 W
4. 4 000 J
5. 95 W
6. 1 440 000 J
7. 15 minutes
8. 2 900 W
9. 1 200 s
10. 950 W
Power, Current
Voltage(p.10)
1.
(a) 30 W
(b) 54 W
(c) 652 A
(d) 3 A
(e) 240 V
(f) 6 V
2. 36 W
3. 10909 V
4. 026 A
5. 690 W
6. 2 A
7. 157 A
8. 070 A
9.
(a) 12 V
(b) 8
10. 23810 V
11. 35 V
12. 188 A
13. 217 x 10-5 A
14. 9 V
15.
(a) 565 A
(b) 1696 A
Power and
Resistance(p.13)
1. 44063 W
2. 900 W
3. 11021 W
4. 1 17556 W
5. 40160 W
6. 918 W
7. 25358 W
8. 77794 W
9. 48091 W
10. 05 A
11. 460 
12. 26 450 
13. 7053 
14. 1225 V
15. 13225 W
&
Power, Energy & Time
(p.8)
1.
(a) 50 W
(b) 5 W
(c) 60 000 J
(d) 2 400 J
(e) 40 s
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Section 3
Misc.
Questions(p.15)
1.
(a) 36 W
(b) 4 
(c) 6 480 J
(d) 540 C
2.
(a) 3 mA
(b) 30 V
(c) 60 s
(d) 018 C
3.
(a) circuit 2
(b) 10 
4.
(a) 957 A
(b) 24 
5.
(a) 713 W
(b) 7419 
6.
(a) 1 800 s
(b) 198 x 106 J
7.
(a) 600 s
(b) 24 W
8.
(a) 2 700 s
(b) 33 750 C
9. 10 A
10. 1 036 800 J
Section 4 - Useful
Circuits
Series Circuits(p18)
1. 5 V
2. 3 V
3.
(a) 24 V
(b) 3 A
4.
(a) 005 A
(b) 10 V
5.
(a) 02 V
(b) 17 A
Parallel Circuits
(p.20)
1.
(a) 12 V
(b) 12 V
(c) 05 A
2.
(a) 6 V
(b) 04 A
3.
(a) 230 V
(b) 06 A
(c) 09 A
42
The Ultimate Collection of Physics Problems - Using Electricity
4.
(a)
(b)
(c)
(d)
5.
(a)
(b)
(c)
(d)
(e)
(f)
12 V
12 V
6 A
16 A
A & B
5 A
A, B & C
7 A
3 A
230 V
Resistance in
Series
Circuits(p.22)
1.
(a) 5575 
(b) 405 
(c) 3000 
(d) 400 
(e) 310 
(f) 80 
2. 9 000 
3. 1 550 
Resistance in
Parallel Circuits
(p.23)
1.
(a) 40 
(b) 10 
(c) 60 
(d) 48 
(e) 60 
(f) 320 
(g) 200 
(h) 100 
(i) 600 
2.
(a) 200 
(b) 1 k
(c) 240 
(d) 140 k
(e) 100 
(f) 32143 
3. 140 
4. 225 
5. 360 
Combination Circuits
(p.25)
1.
(a) 20 
(b) 70 
(c) 25 
(d) 20 
(e) 55 
(f) 225 
2.
(a) 125 
(b) 280 
(c) 405 
3. 27429 
4. network B
5.
(a) 100
100
(b) 100
(c)
50
100
10
50 50
50
10
50
10
(d)
50
10
50
Section 4
Misc.
Questions(p.27)
1. 4 V
2.
(a) 8 V
(b) 2 A
3.
(a) 24 V
(b) 03 A
4.
(a) 12 V
(b) 15 A
5. network 3
Section 5 - Behind
the Wall
Kilowatt hour(p.29)
1. 12 kWh
2.
(a) 8 kWh
(b) 288 x 107 J
3. 08 units
4.
(a) 14 units
(b) £104
5. 5 hours
6. 12 minutes
7. 12 kW
8.
(a) 1 kWh
(b) 74 p
9. 39 p
10.
(a) 864 units
(b) £7467
(c) 3.11 x 109 J
Revision Questions
General Level(p32)
2.
(a) 36 V
(b) 002 A
(c) 400 
©GMV Science. Photocopiable only by the purchasing institution.
3.
(a)
(b)
(c)
(d)
4.
(a)
(b)
(c)
5.
(a)
(b)
(c)
(d)
6.
(a)
230 V
2 070 W
2 070 J
372 600 J
15 A 25 V
012 A 24 V
02 A 15 V
460 W
3 000 W
598 W
1 200 W
kettle - 230 V
lamp ~ 60 W
Credit Level(p.36)
1.
(a) 3 A
(b) 4 
(c) 2 A
(d) 5 A
2.
(a) 652 A
(b) 3527 
(d) 180 C
3.
(c) 13266 
(d) 173 A
(e) 2645 W
(f) 016 units
(g) 1 p
(h)(i) 489 p
(ii) 3425 p
4.
(b) 22x10-3 A, 003 A
(d) 10 A
5. 1 3225 W
6.
(a) 30 
(b) 02 A
(c) 4 V
(d) 25 
7.
(a) 36 
(b) 025 A
(c) 9 V
8.
(a) 004 A
(b) 625 A
(c) 001 A
(d) 3 V
(e) 3 V
(f) 3 V
(g) 925 V
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