Electromagnetic Force

Electromagnetic Force (O Level)

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Question 1
Two current-carrying wires are arranged in parallel as shown below. What is the direction of the electromagnetic force on each wire? 1
A
Wire X Wire Y to the right to the right
B
to the left to the left
C
to the left to the right
D
to the right to the left
Question 1 Explanation: 
By the right hand grip rule, it can be deduced that both circular fields are in anticlockwise direction and that the two fields are in opposite direction between the wires. This implies that there is an attraction force between the two wires caused by the interaction of the field between the wires and reinforcement of the field outside the wires.
Question 2
Two current-carrying wires are arranged in parallel as shown below. What is the direction of the electromagnetic force on each wire? 2
A
Wire X Wire Y to the right to the right
B
to the left to the left
C
to the left to the right
D
to the right to the left
Question 2 Explanation: 
By the right hand grip rule, it can be deduced that both circular fields are in anticlockwise direction and that the two fields are in same direction between the wires. This implies that there is a repulsive force between the two wires caused by the interaction of the fields. (Stronger field between the wires and weaker field outside them).
Question 3
A current-carrying wire is placed between two magnets as shown below. What is the direction of force acting on the wire? 3
A
Into the plane of paper
B
Out of the plane of paper
C
To the right
D
To the left
Question 3 Explanation: 
By using Fleming’s left hand rule, it can be verified that the force is pointing into the paper.
Question 4
A current-carrying wire is placed between two magnets as shown below. What is the direction of force acting on the wire? 4
A
upwards
B
downwards
C
To the right
D
To the left
Question 4 Explanation: 
By using Fleming’s left hand rule, it can be verified that the force is pointing downwards.
Question 5
Four current-carrying wires are placed between two magnets as shown in the plan view below. Which wire would experience the smallest electromagnetic force? 5
A
A
B
B
C
C
D
D
Question 5 Explanation: 
For the same current and the same magnetic field, the current that is flowing perpendicularly to the magnetic field will feel the strongest electromagnetic force and the current that is flowing in parallel to the magnetic field will not feel any electromagnetic force.
Question 6
Two current-carrying wires (P and Q) are placed between two magnets and their currents are equal but in opposite directions as shown below. What are the directions of forces acting on the wires? 6
A
Force on P Force on Q upwards upwards
B
downwards downwards
C
upwards downwards
D
downwards upwards
Question 6 Explanation: 
By using Fleming’s left hand rule, it can be verified that the force acting on P is acting downwards and the force acting on Q is acting upwards.
Question 7
A proton enters a region of magnetic field as shown below. The proton will 7
A
not be affected and continue to travel straight through.
B
experience an upwards force.
C
experience a downwards force.
D
experience a force into the paper.
Question 7 Explanation: 
The direction of motion of proton is the direction of the current. According to Fleming’s left-hand rule, the force on the proton will be upwards.
Question 8
An electron enters a region of magnetic field as shown below. The electron will 8
A
not be affected and continue to travel straight through.
B
experience an upwards force.
C
experience a downwards force.
D
experience a force into the paper.
Question 8 Explanation: 
The direction of motion of electron is opposite the direction of the current. According to Fleming’s left-hand rule, the force on the electron will be upwards.
Question 9
The core of a d.c. motor is usually made of soft iron because soft iron
  1. is easily magnetized and demagnetized.
  2. is light and will not slow sown the rotating parts.
  3. conducts electricity.
A
1 only
B
1 and 2 only
C
1 and 3 only
D
2 and 3 only
Question 9 Explanation: 
The main reason for using soft iron is that it can be easily magnetised and easily demagnetised. It can also induce a stronger magnetic field as compared to other common magnetic materials such as steel, nickel and cobalt.
Question 10
In the diagram below, what are the directions of the electromagnetic forces acting on wire X and wire Y? 10
A
Force on X Force on Y ↑ ↑
B
↓ ↓
C
↑ ↓
D
↓ ↑
Question 10 Explanation: 
The current flowing through the parallel circuit are in the same direction. Using the Fleming’s left hand rule, it can be shown that there is an attraction between wire X and wire Y.
Question 11
The figure below shows a current-carrying conductor between two magnets. Which of the arrows correctly indicates direction of the force acting on the conductor? 11
A
A
B
B
C
C
D
D
Question 11 Explanation: 
Use Fleming’s left hand rule
Question 12
The figure below shows an upwards force acting on a current-carrying wire. What is the direction of the magnetic field? 12
A
B
C
D
Question 12 Explanation: 
Use Fleming’s left hand rule
Question 13
The diagram below shows a U-shaped wire hanging freely in rods X and Y. Rods X and Y are connected to a d.c. supply. Which of the following is true? 13
A
U-shaped wire starts to oscillate to and fro, moving in between the two bar magnets and away from the magnets.
B
U-shaped wire swings away from the magnets and stays at a position away from the magnets.
C
U-shaped wire swings inwards, towards the magnets and stays at a position in between the magnets.
D
Nothing happens to the U-shaped wire.
Question 13 Explanation: 
Current flows from B to C (from + to -) and magnetic field flow direction is from top to bottom (from N-pole to S-pole). Using Fleming’s left hand rule, the U-shaped wire will swing away from the magnet.
Question 14
The figure below shows a current-carrying wire being acted upon by a downward force. What is the direction of the current to cause this force? 14
A
Into the plane of the paper
B
Out of the plane of the paper
C
Towards the N-pole
D
Towards the S-pole
Question 14 Explanation: 
Using Fleming’s left hand rule, the current should be moving out of the plane of the paper to achieve a downward force.
Question 15
A plotting compass is placed beside a current-carrying wire as shown below. Which of the following shows the correct direction of the plotting compass? 15
A
A
B
B
C
C
D
D
Question 15 Explanation: 
By using the right hand grip rule, it can be seen that the circular magnetic field is turning in the clockwise direction. The compass at the right of the current-carrying wire will thus be affected by the magnetic field and points the N-pole downwards.
Question 16
A plotting compass is placed beside a current-carrying wire as shown below. Which of the following shows the correct direction of the plotting compass? 16
A
A
B
B
C
C
D
D
Question 16 Explanation: 
By using the right hand grip rule, it can be seen that the circular magnetic field is turning in the anticlockwise direction. The compass at the left of the current-carrying wire will thus be affected by the magnetic field and points the N-pole downwards.
Question 17
A plotting compass is placed beside a current-carrying wire as shown below. Given that the current flowing through both wires has the same magnitude, which of the following shows the correct direction of the plotting compass? 17
A
A
B
B
C
C
D
D
Question 17 Explanation: 
By using the right hand grip rule, it can be seen that the two circular magnetic fields are turning in opposite direction. The compass at the centre of the two current-carrying wires will thus be affected by the magnetic field and points the N-pole downwards.
Question 18
A plotting compass is placed in between two current-carrying wires as shown below. Given that the current flowing through both wires has the same magnitude, which of the following shows the correct direction of the plotting compass? 18
A
A
B
B
C
C
D
D
Question 18 Explanation: 
By using the right hand grip rule, it can be seen that the two circular magnetic fields are turning in same direction (clockwise direction). The two magnetic fields cancel their effects and create a null point at the centre of the two wires. The compass which is situated at the centre will thus not be affected by the two opposite magnetic fields. It will then be affected by the weak magnetic field of the Earth and points to the magnetic north.
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There are 18 questions to complete.

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