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Physics for IGCSE & O level - Magnets And Currents (Section 9)

1
In a transformer, the ratio of output voltage to input voltage is equal to the ratio of :
Answer
(B)
Number of turns in secondary to primary coil
2
Which of the following is the best description of a magnetic field?
Answer
(B)
The region around a magnet where magnetic forces can be detected.
3
Which device is used to reverse the current in a motor?
Answer
(B)
A commutator
4
What type of core is typically used in transformers?
Answer
(B)
Iron core
5
What is the formula used to calculate the turns ratio in a transformer?
Answer
(C)
Vp/Vs = Ns/Np
6
What is the relationship between the input and output currents of an ideal transformer?
Answer
B
C
7
How can the strength of the turning effect in an electric motor be increased?
Answer
A
B
D
8
In a series circuit, what happens to the current?
Answer
(C)
The current is the same throughout.
9
What is the main reason for using slip rings in an A.C. generator?
Answer
(B)
To maintain a consistent connection to the coil.
10
In a power grid, where would you typically find a step-up transformer?
Answer
(B)
At the power generation station
11
Which of the following increases the turning effect on the coil in an electric motor?
Answer
A
B
C
12
Which of the following is essential for the operation of an electric motor?
Answer
(D)
All of the above.
13
What is the effect of inserting an iron core inside a coil of wire (solenoid) carrying a current?
Answer
(A)
The magnetic field strength increases.
14
Which component is used to rectify the current in a car alternator?
Answer
(C)
A rectifier
15
What is a key feature of an electromagnet that makes it useful?
Answer
(B)
The magnetic field can be switched on and off.
16
What is the relationship between electricity and magnetism?
Answer
(D)
Both B and C
17
Which of the following statements is correct about the core material of a transformer?
Answer
B
C
D
18
What does the number of turns in the primary and secondary coils determine in a transformer?
Answer
(B)
The voltage ratio.
19
What happens to the current flowing in a wire when it is moved through a magnetic field?
Answer
(C)
The current is induced.
20
The core of a transformer is often made of:
Answer
(A)
Iron
21
What is the purpose of a step-down transformer in the distribution network?
Answer
(B)
To reduce voltage for safe consumer use.
22
A step-up transformer has more turns on the ____ coil than the ____ coil.
Answer
(B)
output, input
23
Which of the following is an example of a use for electromagnets?
Answer
(C)
A doorbell.
24
Which of the following materials would make the best core for an electromagnet?
Answer
(C)
Iron
25
What is an essential component of an electric motor?
Answer
(A)
A commutator
26
Like poles of magnets will ______.
Answer
(B)
Repel
27
If the primary voltage is 240 V and the secondary voltage is also 240 V, what is the turns ratio?
Answer
(C)
1:1
28
What could be the main reason for the use of underground cables in a power transmission system?
Answer
(C)
To avoid overhead wires
29
In a transformer, how is energy transferred between the primary and secondary coils?
Answer
(B)
Through a magnetic field
30
What happens to a magnet when pushing a magnet in a coil, which way does the induced current in the coil flow?
Answer
(A)
The current increases with the speed of the magnet's movement and the direction of induced current depends on the pole pushed into the coil.
31
What is a generator designed to produce?
Answer
(A)
Alternating Current (a.c.)
32
What causes the sound from the moving-coil loudspeaker?
Answer
(C)
The vibration of the cone.
33
In an ideal transformer, if the input power is 100W and the output voltage is doubled, what will be the approximate output power?
Answer
(B)
100W
34
Which of the following is NOT a part of a basic electromagnet?
Answer
(D)
A permanent magnet.
35
In a power transmission system, transformers are used at the:
Answer
A
B
C
36
If the input voltage of a transformer is 12 V and the output voltage is 24 V, is it a step-up or a step-down transformer?
Answer
(A)
step-up
37
Which of the following is NOT a method to increase the e.m.f. induced in a coil?
Answer
(D)
Decreasing the area of the coil.
38
What does a simple a.c. generator contain?
Answer
(D)
All of the above
39
If a transformer has a turns ratio of 1:2 and the input voltage is 10 V, what is the output voltage?
Answer
(C)
20 V
40
What does the diagram of a simple generator show?
Answer
(B)
A rotating coil, and cutting field lines as a vertical rate.
41
The core of a transformer is made of laminated sheets to:
Answer
(B)
Reduce eddy current losses
42
What is the name given to the effect when a current is induced in a wire when it moves through a magnetic field?
Answer
(A)
Electromagnetic induction
43
What is the unit for measuring the frequency of AC current?
Answer
(B)
Hertz
44
What happens to the strength of the magnetic field of an electromagnet if the number of turns of wire is reduced?
Answer
(B)
The magnetic field gets weaker.
45
Which of the following describes the relationship between the induced e.m.f. and the rate of change of magnetic flux?
Answer
(C)
The induced e.m.f. is directly proportional to the rate of change of magnetic flux.
46
What is the force between two like magnetic poles?
Answer
(B)
Repulsion
47
In a circuit breaker, what causes the switch to open and break the circuit when the current is too high?
Answer
(B)
An electromagnet.
48
Which of the following factors is directly related to the strength of a magnetic field produced by a solenoid?
Answer
A
B
49
What causes the coil in a motor to rotate?
Answer
(C)
The interaction between the magnetic field and the current in the coil.
50
Which of the following statements is TRUE about the magnetic force?
Answer
(A)
Like poles attract each other.