Magnetism

Magnetism (O Level)

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Question 1
When a bar magnet is being suspended freely,
A
the bar magnet will point to the north-south direction.
B
the bar magnet will point to the east-west direction.
C
the bar magnet will not point to a fix direction.
D
either end of the bar magnet will point to the north.
Question 1 Explanation: 
When a magnet is being suspended freely, it will be affected by the magnetic field of the Earth and points in the north-south direction.
Question 2
The magnet is strongest near the
A
poles of the magnet.
B
ends of the magnet.
C
centre of the magnet.
D
one quarter point from the poles of the magnet.
Question 2 Explanation: 
One property of a magnet is that the magnetic force is stronger at the poles. “Ends” is not a proper word to use as a magnet may have many ends depending on its shape.
Question 3
When the N-pole of a magnet is placed near the S-pole of another magnet,
A
the N-pole will be attracted to the S-pole
B
the S-pole will be attracted to the N-pole
C
there will be attraction between the 2 poles.
D
there will be repulsion between the 2 poles.
Question 3 Explanation: 
Unlike poles attract and like poles repel. It is not the north pole attracting the south pole. It is a mutual pull between the 2 unlike poles.
Question 4
When the S-pole of a magnet is placed near an unknown pole of another magnet, the two magnets
A
repel each other because the unknown pole is a N-pole
B
repel each other because the unknown pole is a S-pole
C
attract each other because the unknown pole is a S-pole
D
can either attract or repel.
Question 4 Explanation: 
Unlike poles attract and like poles repel. Two N-poles or two S-poles will always repel each other.
Question 5
Why does a compass always points to the north?
A
The compass is made of a magnetic material.
B
The compass is a magnet with the pointer as the south seeking pole.
C
The compass is a magnet with the pointer as the north seeking pole.
D
The compass is an electromagnet that is charged by rubbing when the needle is turning.
Question 5 Explanation: 
The needle of a compass is a magnet. The needle is suspended on a sharp point to minimise the effect of friction such that the weak magnetic field of the Earth can cause the north-seeking pole to point to the north and the south-seeking pole to point to the south. The north seeking pole of the compass needle is at the arrow head and the south-seeking pole is at the other end.
Question 6
When one end of a magnet is placed near a compass, the pointer (N-pole) of the compass turns and points at the magnet because
A
the pointer points at any end of a magnet.
B
the end near the compass is a N-pole.
C
the end near the compass is a S-pole.
D
a magnet always attracts the pointer of a compass.
Question 6 Explanation: 
The needle of a compass is a magnet. When there is no other magnetic object around, the pointer always points towards the north because the pointer is a north-seeking pole (N-pole). The N-pole of the compass will be attracted to the S-pole of another magnet.
Question 7
When one end of an iron rod is placed near a compass,
A
it is always the N-pole of the compass that points towards it.
B
it is always the S-pole of the compass that points towards it.
C
any pole of the compass may point towards it.
D
the compass needle will not be affected by the iron rod.
Question 7 Explanation: 
The needle of a compass is a magnet. Either end of the needle can be attracted to the iron rod because iron is a magnetic material. Which end of the compass needle turns to the iron rod depends on which end of the needle is closer to the rod.
Question 8
Which of the following are possible results when a steel bar is being magnetised by stroking? 8
A
1 and 2 only
B
1 and 3 only
C
2 and 3 only
D
1, 2 and 3
Question 8 Explanation: 
As the magnet stroke across the steel bar, the dipoles of the steel bar will be attracted to the magnetic pole of the magnet and rotate with the movement of the magnet.
Question 9
Which of the following is/are possible result(s) when a steel bar is being magnetised by double stroking method? 9
A
2 only
B
1 and 2 only
C
2 and 3 only
D
1 and 3 only
Question 9 Explanation: 
As the magnet strokes across the steel bar, the dipoles of the steel bar will be attracted to the respective magnetic poles of the magnet and rotate with the movement of the magnet.
Question 10
Which of the following are possible sets of polarity when magnetising a steel bar by stroking method? 10    
  1. P = N, Q = S, X = N, Y = S
  2. P = N, Q = S, X = S, Y = N
  3. P = S, Q = N, X = S, Y = N
  4. P = S, Q = N, X = S, Y = N
A
1 and 2 only
B
3 and 4 only
C
1 and 3 only
D
2 and 4 only
Question 10 Explanation: 
At the end on the steel bar where the stroking action ends, the induced magnetic pole will be the opposite to that of the magnet’s pole.
Question 11
What can be deducted from the following diagram showing a current flowing in a straight wire surrounded by 4 compasses? 11
A
The magnetic field caused by 4 compasses put close together will always cause the compass needles to point in a circle
B
Compass needles are affected by the material of the wire.
C
Compass needles are affected by the circular electric field in the wire
D
Compass needles are affected by the circular magnetic field produced by the current in the wire
Question 11 Explanation: 
A current in a wire will always produce a circular magnetic field around the wire.
Question 12
Plotting compasses X and Y are placed below and top of a current carrying wire respectively as shown in the diagram. 12     Which of the following shows the current compass pointers’ directions? 12_2
A
A
B
B
C
C
D
D
Question 12 Explanation: 
The direction of the circular magnetic field can be determined by the right-hand grip rule. Use your right fist to grip the wire with your right thumb pointing the direction of the current. The direction of your fingers will indicate the direction of the circular magnetic field. Only choice D shows the possible orientation of the compass needles according to the right-hand grip rule. In this case the Earth’s magnetic field causes the compass needles to align not exactly perpendicular to the current-carrying wire.
Question 13
Plotting compasses X and Y are placed on the right and on the left of a current carrying wire respectively as shown in the diagram. 13     In which directions will the compass needles point? 13_2
A
A
B
B
C
C
D
D
Question 13 Explanation: 
According to the right-hand grip rule, the magnetic field caused by the current-carrying wire will only make the needles move out or into the plane of the paper and not able to rotate the needles clockwise or anti-clockwise. The compasses therefore should be both pointing to the north-south direction. They should be pointing in the same direction and not in different directions.
Question 14
What is the main reason for using a solenoid instead of a straight wire to produce magnetic field?
A
To concentrate the magnetic field produced throughout the wire to flow through the core of the solenoid.
B
To reduce the space taken up by the long wire.
C
To ensure that the magnetic field strength is constant.
D
To reduce the amount of wires used to produce the same magnetic field strength as a straight wire.
Question 14 Explanation: 
When the current-carrying wire is being coiled up, the circular magnetic field produced by portion on the straight wire is being concentrated in the core of the coil. This will produce a stronger magnetic field strength.
Question 15
Which coil produces the strongest electromagnet for a given flow of current?
A
A 10 cm coil with 100 turns
B
A 5 cm coil with 200 turns
C
A 10 cm coil with 200 turns
D
A 20 cm coil with 200 turns
Question 15 Explanation: 
One of the factors that determine the strength of the electromagnet is the number of coil per unit length. The more concentrated the coils are being packed, the stronger will be the magnetic field strength. B has an equivalent of 400 turns in a coil length of 10 cm.
Question 16
Which coil produces the strongest electromagnet for a given flow of current? 16
A
A
B
B
C
C
D
D
Question 16 Explanation: 
One of the factors that will increase the strength of the electromagnet is the presence of a magnetic core. The core becomes an induced magnet when it is placed inside the coil and increases the overall magnetic field strength of the electromagnet. Iron is the only magnetic material, the rest of the materials given are not magnetic material.
Question 17
Which is the strongest electromagnet? 17
A
A
B
B
C
C
D
D
Question 17 Explanation: 
One of the factors that determine the strength of the electromagnet is the magnitude of the current flowing in the solenoid. Having more cells connected in series can increase the voltage across the solenoid and hence increases the current flowing through the solenoid. Another factor is the materials. Iron is a soft magnetic material. It will make a stronger electromagnet than steel which is a hard magnetic material.
Question 18
Which of the following is the correct sequence to effectively demagnetise a steel bar using electrical method?
  1. Insert the magnetised steel bar into a solenoid
  2. connect direct current to a solenoid
  3. connect alternating current to a solenoid
  4. pull the steel bar slowly out of the solenoid
  5. switch off the current supply
A
1 → 2 → 4 → 5
B
3 → 1 → 4 → 5
C
2 → 1 → 4 → 5
D
1 → 3 → 5 → 4
Question 18 Explanation: 
The procedure to demagnetise a steel bar is: 1. connect alternating current to a solenoid. 2. insert the magnetised steel bar into a solenoid. 3. pull the steel bar slowly out of the solenoid. 4. switch off the current supply (not important)
Question 19
In which diagram(s) is/are the correct poles for the electromagnet indicated? 19
A
2 only
B
1 and 2 only
C
2 and 3 only
D
1, 2 and 3
Question 19 Explanation: 
Using the right-hand grip rule, it can be verified that only diagram 2 shows correctly the magnetic poles.
Question 20
Which of the following diagrams shows the correct magnetic field pattern between the poles of two bar magnet? 20
A
A
B
B
C
C
D
D
Question 20 Explanation: 
For unlike poles, the field lines will come out from the N-pole and enter the S-pole. The field lines will concentrate at the centre of 2 unlike poles and gets weaker (shown by the widening of field lines) as they are further away from the poles.
Question 21
Which diagram below indicates the correct magnetic field pattern between the poles of two bar magnet? 21
A
A
B
B
C
C
D
D
Question 21 Explanation: 
For like poles, the field will come out from the N-pole and pushes each other away. The answer is not D as the direction of the field is wrong.
Question 22
A plotting compass is placed beside a bar magnet as shown below. 22     Which of the drawings shows the correct direction of the compass needle? 22_2
A
A
B
B
C
C
D
D
Question 22 Explanation: 
The magnetic field of a bar magnet is shown below. The compass needle will point towards the direction of the magnetic field.
Question 23
A light steel bar and a light iron bar are attached to a magnet as shown. What will possibly happen when the magnet is removed? 23
A
The steel and iron bars repel each other
B
Both steel and iron bars lose their magnetism.
C
Steel bar retains its magnetism and iron bar loses its magnetism.
D
Iron bar retains its magnetism and steel bar loses its magnetism.
Question 23 Explanation: 
Steel is a hard magnetic material. It retains some magnetism after the magnet is being removed. Steel will continue to attract the iron based on induced magnetism.
Question 24
A magnet is inserted into the centre of a coil and an e.m.f. is induced across the coil. The magnitude of the induced current depends on
  1. the diameter of the coil.
  2. the strength of a magnet
  3. the thickness of the wire of which the coil is made
A
1 and 2 only
B
2 and 3 only
C
1 and 3 only
D
1, 2 and 3
Question 24 Explanation: 
When the diameter of the coil is too big, the magnetic field cutting the conducting coil will be weaker because the magnet is further away from the coil. The induced e.m.f. will be small and thus the induced current also weakened. The induced e.m.f. is dependent on the rate of change of magnetic flux cutting the coil. Therefore the stronger the magnet, the higher will be the induced e.m.f. and thus the higher will be the induced current.
Question 25
Two magnets P and Q attract four soft iron bars, F, G, H and I at their poles as shown below. What will happen to the iron bars when the two magnets move towards each other and attract each other? 25
A
Nothing will happen.
B
G and H will drop off.
C
G and H will repel away each other.
D
F and I will attract G and H respectively.
Question 25 Explanation: 
When the two magnets are attracted to each other, iron bars G and H are at the centre of the combined magnets. The centre of a bar magnet has a very weak magnetic strength. The weight of the iron bars G and H will cause the bars to drop off.
Question 26
A magnet is suspended freely at the North Pole of the Earth. Which of the following shows the correct direction of the magnet? 26
A
A
B
B
C
C
D
D
Question 26 Explanation: 
The N-pole of the magnet is known as the North seeking pole. It is actually the S-pole relative to the Earth’s magnetic field. The magnet’s north-seeking pole will be attracted and hence pointing to the North Pole of the Earth.
Question 27
The diagram below shows the circuit of an electric bell. 27     What happens when the bell button is switched on and switched off?

 

Switch On

Switch Off

A

1 ting is heard silence

B

1 ting is heard 1 ting is heard

C

continuous ringing silence

D

silence continuous ringing
A
A
B
B
C
C
D
D
Question 27 Explanation: 
When the circuit is closed, current flows through the coil and it becomes an electromagnet. The electromagnet attracts the soft iron towards it and caused the hammer to strike gong 1 once. As long as the switch is closed, the soft iron would be attracted to the electromagnet and therefore no further sound would be heard. When the circuit is opened, current stops flowing through the coil and the electromagnet is deactivated. The soft iron is pulled back by the springy metal strip and caused the hammer to strike gong 2 once. As long as the switch is opened, the soft iron would be stationary and therefore no further sound would be heard.
Question 28
The diagram below shows the magnetic field of 2 magnets placed close together. Three plotting compasses are placed at the locations shown. 28       Which is the set of compasses showing the correct direction of the needles?

 

1

2

3

A

B

C

D

A
A
B
B
C
C
D
D
Question 29
The diagram shows the magnetic field of 2 magnets placed close together with 3 plotting compasses placed at the locations shown. 29       Which is the set of compasses showing the correct direction of the needles?

 

1

2

3

A

B

C

D

A
A
B
B
C
C
D
D
Question 30
The diagram below shows the magnetic field of 2 current carrying wires going into the plane of the paper. 30     Which is the set of compasses showing the correct direction of the pointers?

 

1

2

3

A

B

C

D

A
A
B
B
C
C
D
D
Question 31
The diagram below shows the magnetic field of 2 current carrying wires perpendicular to the plane of the paper. 31     Which is the set of compasses shows the correct direction of the pointers?

 

1

2

3

A

B

C

D

A
A
B
B
C
C
D
D
Question 32
The diagram shows a bar magnet being placed in a magnetic field. What will happen to the bar magnet? 32
A
The bar magnet does not react to the magnetic field.
B
The bar magnet moves in the direction of the field.
C
The bar magnet moves in the opposite direction of the field.
D
The bar magnet rotates clockwise by 90 deg.
Question 32 Explanation: 
The magnetic field always comes out from N-pole. The N-pole of the bar magnet will be repelled and the S-pole of the bar magnet will be attracted. This would cause a 90 deg clockwise rotation immediately.
Question 33
Which of the following observations confirms that an object is a magnet?
A
The object is attracted by a strong bar magnet.
B
The object causes a compass needle to move.
C
The object rotates before being attracted to a bar magnet.
D
The object is attracted to both poles of a horseshoe magnet.
Question 33 Explanation: 
The true test for a magnet is repulsion. Given that the object rotated before being attracted to a bar magnet showed that the object’s like pole is being repelled away and the unlike pole turned towards the bar magnet before the attraction took place.
Question 34
A bar magnet is broken as shown below. What are the polarities at X and Y? 34    

 

Polarity at X

Polarity at Y

A

N-pole S-pole  

B

S-pole N-pole  

C

N-pole N-pole  

D

S-pole S-pole  
A
A
B
B
C
C
D
D
Question 34 Explanation: 
A magnet is made up of many dipoles pointing in one direction. Once the magnet ois broken, the two broken pieces would become two independent magnets as shown below.
Question 35
Two physically identical metal bars X and Y are positioned in the 2 diagrams shown below. If there is a strong attraction in figure 1 and a very weak attraction in figure 2, what are the possible materials for X and Y? 35      

 

Bar X

Bar Y

A

steel bar iron bar

B

steel bar bar magnet

C

bar magnet steel bar

D

bar magnet bar magnet
A
A
B
B
C
C
D
D
Question 35 Explanation: 
The centre portion of a bar magnet has a very weak magnetic strength. When a steel bar is placed on it, there is almost no attraction force.
Question 36
Two long iron pins are placed at the ends of a bar magnet as shown. Which of the following diagrams, A, B, C, D shows the situation when a shorter magnet is used? 36
A
A
B
B
C
C
D
D
Question 36 Explanation: 
The tow pins are induced as magnets and their polarities are shown below. Due to the opposite poles induced at the free ends of the two pins, the pins attract each other when they are near enough.
Question 37
A current-carrying wire is coiled around an iron horseshoe as shown. 37     What are the polarities of X and Y?

 

At X

At Y

A

N-pole N-pole

B

S-pole S-pole

C

N-pole S-pole

D

S-pole N-pole
A
A
B
B
C
C
D
D
Question 37 Explanation: 
Based on right-hand grip pule, both A and Y are S-poles.
Question 38
A light bar magnet is suspended near a current-carrying wire as shown below. What will happen to the light bar magnet? 38
A
It will rotate clockwise
B
It will rotate anticlockwise
C
It will move to the right
D
It will move to the left
Question 38 Explanation: 
Based on right-hand grip rules, the magnetic field caused by the current carrying wire will be as shown. This circular magnetic field will cause the magnet to rotate clockwise.
Question 39
A compass is placed near a copper bar as shown in the diagram. In which direction will the compass needle point? 39
A
A
B
B
C
C
D
D
Question 39 Explanation: 
Copper is not a magnetic material. It will not affect the compass needle. The needle will therefore only be affected by the Earth’s magnetic field.
Question 40
A compass is placed between two magnets of equal magnetic strength as shown in the diagram. In which direction will the compass needle point? 40
A
A
B
B
C
C
D
D
Question 40 Explanation: 
Unlike poles attract.
Question 41
A compass is placed between two magnets as shown in the diagram. Magnet X is stronger than magnet Y. In which direction will the compass needle point? 41
A
A
B
B
C
C
D
D
Question 41 Explanation: 
Unlike poles attract. And since magnet X is stronger, its ability to attract N-pole and repel S-pole is stronger, therefore the N-pole of the compass needle will point slightly to magnet X.
Question 42
Equal amount of current is flowing in two insulated wires perpendicular to each other as shown. Which segment, A, B, C or D has the strongest magnetic field flowing out of the paper? 42
A
A
B
B
C
C
D
D
Question 42 Explanation: 
Based on right-hand grip rule, the strength of magnetic field going in and coming out of the paper based on the contribution of the two current carrying wires are shown below. From the diagram, segment A has more magnetic field flux or magnetic field lines coming out of paper.
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