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Magnetism - Questions & Answers

EXERCISE-10

(A) MULTIPLE CHOICE TYPE :

(Choose the correct answer from the options given below).

1. The ........ magnetism acquired by a magnetic material when it is kept near a magnet is called ...... magnetism.
(a) temporary, induced
(b) permanent, induced
(c) temporary, permanent
(d) None of the above
Answer: (a) temporary, induced

2. A magnetic pole induces ......... polarity on the near end and a ......... polarity on the farther end of the iron bar.
(a) opposite, opposite
(b) similar, similar
(c) opposite, similar
(d) similar, opposite
Answer: (c) opposite, similar

3. Out of the following, the incorrect statement(s) is/are :
(i) Magnetic field lines are open and continuous curves.
(ii) Outside the magnet, magnetic field lines are directed from the south pole to the north pole of the magnet.
(a) (i)
(b) (ii)
(c) both (i) and (ii)
(d) none of the above
Answer: (c) both (i) and (ii)

4. An iron rod buried inside the earth along ............. direction becomes a magnet.
(a) east-west
(b) equatorial
(c) north-south
(d) any direction
Answer: (c) north-south

5. The sum of all magnetic fields adds up to zero at the :
(a) north pole
(b) south pole
(c) equator
(d) neutral points
Answer: (d) neutral points

6. The magnetic field lines in a non-uniform magnetic field are :
(a) either converging or diverging
(b) parallel and equispaced
(c) only converging
(d) only diverging
Answer: (a) either converging or diverging

7. The magnetic field lines of the earth are .......... to the earth's surface near the magnetic poles and ....... to the earth's surface near the magnetic equator.
(a) parallel, normal
(b) normal, parallel
(c) normal, normal
(d) parallel, parallel
Answer: (b) normal, parallel

8. The direction of a magnetic field at a point on a magnetic field line is given by :
(a) a tangent at that point
(b) a normal bisector at that point
(c) both tangent and normal bisector at that point
(d) None of the above
Answer: (a) a tangent at that point

9. A soft iron bar is tied by a thread in the middle and is suspended from a rigid support such that it is free to rotate in a horizontal plane. It shall come to rest :
(a) along north-south direction
(b) along east-west direction
(c) equally inclined to N-S and E-W directions
(d) in any direction
Answer: (d) in any direction

10. The figure given below shows two iron nails attached to the end of a magnet. Choose the correct figure.
(a) (b) (c) (d)
Answer: (a) (In this figure, the pointed ends of the two nails repel and move apart because they acquire the same polarity).

11. A freely suspended bar magnet is taken to the north pole of the earth. It comes to rest :
(a) in any direction
(b) parallel to the earth's surface
(c) nearly vertical with the south pole in a downward direction
(d) nearly vertical with the north pole in a downward direction
Answer: (c) nearly vertical with the south pole in a downward direction

12. Which of the following is not a correct statement for a magnet placed in the earth's magnetic field ?
(a) Neutral points are always at an equal distance from the magnet.
(b) The position of neutral points depends on the direction of the magnet in the earth's magnetic field.
(c) The position of neutral points does not depend on the direction of the magnet.
(d) The magnetic field strength is zero at the neutral points.
Answer: (c) The position of neutral points does not depend on the direction of the magnet.

(B) VERY SHORT ANSWER TYPE :

1. Fill in the blanks to complete the sentences :
(a) Two ends of a magnet are called poles.
(b) Unlike poles of a magnet attract each other.
(c) Like poles of a magnet repel each other.
(d) A freely suspended magnet rests in the geographic north-south direction.

2. Complete the following sentences :
(a) If the field lines in a magnetic field are parallel and equidistant, the magnetic field is uniform.
(b) At a neutral point, the resultant magnetic field is zero.
(c) The neutral points of a bar magnet kept with its north pole pointing towards geographic north are located on either side of the magnet in east and west directions.

3. Where is the magnetic south pole of the earth located?
Answer: The magnetic south pole of the earth is located near the geographic north pole.

4. What kind of magnetic field is represented by parallel and equidistant lines ?
Answer: A uniform magnetic field.

5. What is the angle between the magnetic axis of the earth and the axis of rotation of the earth ?
Answer: The magnetic axis of the earth makes an angle of 17° with its axis of rotation.

(C) SHORT ANSWER TYPE :

1. What is a lodestone ?
Answer: Lodestone is an ore of iron oxide found in nature which behaves as a natural magnet.

2. What is a natural magnet ? State two limitations of a natural magnet.
Answer: A natural magnet is a material found in nature that possesses magnetic properties. Two limitations are: (i) They are found in quite irregular and odd shapes, and (ii) They are not magnetically strong enough for practical use.

3. What is an artificial magnet ? State two reasons why do we need artificial magnets.
Answer: An artificial magnet is a manufactured magnet made from iron or steel into convenient sizes and shapes. We need artificial magnets because natural magnets are weak and have irregular shapes, whereas artificial magnets can be made in any desired shape and with strong magnetic strength for specific uses.

4. How will you test whether a given rod is made of iron or copper ?
[Hint : Iron rod gets magnetised when placed near a bar magnet by magnetic induction, while copper rod does not get magnetised]
Answer: Bring a strong magnet near the given rod. If the rod gets attracted towards the magnet, it is made of iron because iron gets magnetised by magnetic induction. If it shows no attraction, it is made of copper, as copper does not get magnetised.

5. Explain the term induced magnetism.
Answer: The temporary magnetism acquired by a magnetic material when it is kept near or in contact with a magnet is called induced magnetism.

6. Explain the following :
(a) When two pins are hung by their heads from the same pole of a magnet, their pointed ends move apart.
(b) Several soft iron pins can cling, one below the other, from the pole of a magnet.
(c) The north end of a freely suspended magnetic needle gets attracted towards a piece of soft iron placed a little distance away from the needle.
Answer: (a) The heads of both pins acquire opposite polarity to the magnet's pole, while their pointed ends acquire the same polarity. Since like poles repel, their pointed ends move apart.
(b) The magnet induces opposite polarity at the near end of the first pin and similar polarity at its farther end. The farther end acts as a magnetic pole for the next pin, inducing magnetism in it, and so on, forming a chain.
(c) The north end of the magnetic needle induces a south pole on the nearer end of the soft iron piece by magnetic induction. Since unlike poles attract each other, the north end of the needle is pulled towards the soft iron.

7. 'Induced magnetism is temporary'. Comment on this statement. (HOTS)
Answer: The statement is correct because the magnetic material acquires magnetic properties only in the presence of another magnet. As soon as the magnet causing induction is removed, the material loses its acquired magnetism completely.

8. 'Induction precedes attraction'. Explain the statement. (HOTS)
Answer: When a piece of unmagnetised magnetic material is brought near a magnet, its nearer end first acquires an opposite polarity due to magnetic induction. Because unlike poles attract, this induced opposite pole is then pulled towards the magnet. Therefore, the piece of iron first becomes a magnet by induction and is then attracted.

9. What do you understand by the term magnetic field lines ?
Answer: A magnetic field line is a continuous curve in a magnetic field such that the tangent at any point of the curve gives the direction of the magnetic field at that point.

10. State four properties of magnetic field lines.
Answer: (1) They are closed and continuous curves. (2) Outside the magnet, they are directed from the north pole towards the south pole. (3) They never intersect one another. (4) They are crowded near the poles where the magnetic field is strong and far away near the middle where the field is weak.

11. Explain why iron filings which are sprinkled on a sheet of cardboard placed over a bar magnet, take up a definite pattern when cardboard is slightly tapped. (EL)
Answer: Due to the magnet, each piece of iron filing gets magnetised by magnetic induction and experiences a force. Therefore, they arrange themselves along the curved lines of the magnetic field, forming a definite pattern.

12. Can two magnetic field lines intersect each other ? Give reason to your answer.
Answer: No, two magnetic field lines never intersect. If they did intersect, it would mean there are two different directions of the magnetic field at the single point of intersection, which is impossible.

13. In Fig. 10.12, draw at least two magnetic field lines between the two magnets.
(a) S N ... N S
(b) N S ... N S
Answer: (a) Since the facing poles are both North, they will repel. The magnetic field lines will emerge from both North poles and curve back towards their respective South poles without intersecting.
(b) Since the facing poles are North and South, they will attract. The magnetic field lines will emerge from the North pole of the left magnet and travel straight into the South pole of the right magnet.

14. State two evidences of the existence of earth's magnetic field.
Answer: (i) A freely suspended magnetic needle always rests in the geographic north-south direction.
(ii) Neutral points are obtained on plotting the field lines of a magnet where the earth's magnetic field completely neutralizes the magnet's field.

15. Sketch four magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone.
Answer: The sketch would consist of four straight, parallel, and equidistant lines pointing in the same direction, from geographic South to geographic North.

16. What conclusion is drawn regarding the magnetic field at a point if a compass needle at that point rests in any direction ? Give reason for your answer.
Answer: The net magnetic field at that point is zero. Reason: The earth's magnetic field at that point is perfectly neutralised by the magnetic field of some other magnetised material, creating a neutral point.

17. What is a neutral point ? How is the position of a neutral point located with the use of a compass needle ?
Answer: Neutral points are the points at which two magnetic fields are equal in magnitude but opposite in direction, making the net magnetic field zero. To locate it, a compass needle is placed in the magnetic field; at a neutral point, the needle remains completely unaffected and will rest pointing in any arbitrary direction.

18. State the positions of neutral points when a magnet is placed with its axis in the magnetic meridian and with its north pole (i) pointing towards the geographic north, (ii) pointing towards the geographic south. (SI)
Answer: (i) When the north pole points towards the geographic north, the two neutral points are located in the east and west directions, on either side of the magnet.
(ii) When the north pole points towards the geographic south, the two neutral points are located in the north and south directions, along the axis of the magnet.

(D) LONG ANSWER TYPE :

1. (a) Draw the pattern of magnetic field lines near a bar magnet placed with its north pole pointing towards the geographic north. Indicate the position of neutral points by marking x.
(b) State whether the magnetic field lines in part (a) represent a uniform magnetic field or non-uniform magnetic field ?
Answer: (a) The field lines curve from the North pole to the South pole. The neutral points (marked 'x') will lie on the equatorial line in the east and west directions, equidistant from the center.
(b) The magnetic field lines represent a non-uniform magnetic field, as the lines are curved and not parallel or equispaced.

2. Fig. 10.13 shows a bar magnet placed on the table top with its north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic material nearby. (EL)
(a) Insert two magnetic field lines on either side of the magnet using arrow head to show the direction of each field line.
(b) Indicate by crosses, the likely positions of the neutral points.
(c) What is the magnitude of the magnetic field at each neutral point? Give a reason for your answer.
Answer: (a) The magnetic field lines will emerge from the North pole (which is pointing south) and curve towards the South pole on both sides of the magnet.
(b) The neutral points will be situated along the longitudinal axis of the magnet, in the geographic North and geographic South directions.
(c) The magnitude of the net magnetic field at each neutral point is zero. Reason: The magnetic field produced by the bar magnet is exactly neutralised by the horizontal component of the earth's magnetic field at those points.

3. You are provided with two similar bars, one is a magnet and the other is a soft iron bar. How will you distinguish between them without the use of any other magnet or bar?
[Hint : A magnet when suspended freely will rest only in north-south direction, but soft iron bar will rest in any direction]
Answer: Tie a silk thread to the middle of each bar one by one and suspend them so they can rotate freely in a horizontal plane. The bar which always aligns itself and comes to rest in the geographic north-south direction is the permanent magnet. The other bar, which can rest in any random direction, is the soft iron bar.

4. A small magnet is suspended by a silk thread from a rigid support such that the magnet can freely swing. How will it rest ? Draw a diagram to show it.
Answer: It will rest in the geographic north-south direction with its north pole pointing towards the geographic north, making some angle with the horizontal due to the inclination of the earth's magnetic field.

5. Explain what do you understand by magnetic induction. What role does it play in attraction of a piece of iron by a magnet ?
Answer: Magnetic induction is the process in which a piece of magnetic material acquires magnetic properties temporarily in the presence of another magnet near it. When a piece of iron is brought near a magnet, the magnet induces an opposite polarity on the nearer end of the iron piece by magnetic induction. Because opposite poles attract each other, this causes the iron piece to be attracted towards the magnet.

6. Explain the mechanism of attraction of iron nails by a magnet when brought near them.
Answer: When a magnet is brought near iron nails, its pole induces an opposite polarity at the head of the first nail and a similar polarity at its tip via magnetic induction. Due to the attraction of opposite poles, the first nail sticks to the magnet. The tip of this first nail then acts as a magnetic pole and induces opposite polarity in the next nail, causing it to attract as well. This process continues to form a chain of nails.

7. A small iron bar is kept near the north pole of a bar magnet. How does the iron bar acquire magnetism ? Draw a diagram to show the polarity on the iron bar. What will happen if the magnet is removed ?
Answer: The iron bar acquires magnetism temporarily through a process called magnetic induction. The end of the iron bar nearer to the north pole of the magnet becomes a south pole, and its farther end becomes a north pole. If the original magnet is removed, the iron bar will immediately lose its induced magnetism.

8. Explain the method of plotting magnetic field lines by using a small compass needle.
Answer: Fix a white sheet of paper on a drawing board and place a bar magnet in the middle, marking its boundary. Place a small compass needle near the north pole of the magnet and mark two dots exactly at the two ends of the needle. Then, move the compass so that one end coincides with the second dot and mark the new position of the other end with a third dot. Keep repeating this process until you reach the south pole. Joining all these dots smoothly gives a continuous curve representing one magnetic field line.

(E) ASSERTION - REASON TYPE QUESTIONS :

(Choose the answer based on the codes given below.)
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are True and R is not the correct explanation of A
(c) Assertion is false but reason is true
(d) Assertion is true but reason is false

(i) Assertion (A) : Two bar magnets attract when they are brought near to each other with the same pole.
Reason (R) : Unlike poles attract each other.
Answer: (c) Assertion is false but reason is true

(ii) Assertion (A) : Magnetic field lines never intersect each other.
Reason (R) : At a particular point, magnetic field has only one direction.
Answer: (a) Both A and R are true and R is the correct explanation of A

(iii) Assertion (A) : Neutral points are the points at which two magnetic fields are equal in magnitude and in the same direction.
Reason (R) : The net magnetic field at a neutral point is zero.
Answer: (c) Assertion is false but reason is true

(F) CASE STUDY BASED QUESTIONS :

1. In a physics laboratory, a student places a bar magnet near a compass. At certain positions, the compass needle shows no deflection. (EL)
(a) Why does the compass needle show no deflection at certain points? What are these points called?
Answer: The compass needle shows no deflection because the magnetic field of the bar magnet is perfectly equal and opposite to the horizontal component of the earth's magnetic field at that location, resulting in a net magnetic field of zero. These points are called neutral points.

(b) What happens to the magnetic field lines at these points?
Answer: Magnetic field lines do not pass through these points because the resultant magnetic field at these points is zero.

(c) Why are these points useful in studying magnetic fields?
Answer: They are useful because they demonstrate the interaction between two independent magnetic fields (the magnet's field and the Earth's field) and can be used to compare the strength of a magnet with the Earth's magnetic field.

(d) Are these points fixed, or do they change their positions?
Answer: They change their positions depending upon the magnetic strength of the specific magnet used and how the magnet is aligned relative to the geographic north-south direction.

2. Ravi observed that an electromagnet could lift iron pieces only when electric current was flowing through it. When the current was switched off, the iron pieces fell down.
(a) Why does an electromagnet lose its magnetism when the current stops?
Answer: An electromagnet produces a magnetic field temporarily due to the flow of electric current. When the current stops, the source of the magnetic field is removed, and it completely loses its induced magnetism.

(b) How is an electromagnet different from a permanent magnet?
Answer: An electromagnet is temporary, its poles can be reversed by reversing the current, and its strength can be easily changed. A permanent magnet holds its magnetism continuously, has fixed poles, and maintains a constant magnetic strength.

(c) How can the strength of an electromagnet be increased?
Answer: The strength can be increased by increasing the magnitude of the electric current flowing through it and by increasing the number of turns in its coil.

(d) Why are electromagnets preferred in cranes?
Answer: They are preferred because they can be made extremely powerful to lift heavy magnetic loads, and their magnetism can be conveniently switched on to pick up objects and switched off to instantly drop them.

(e) Which material is preferred for making the core of an electromagnet?
Answer: Soft iron is heavily preferred because it gets rapidly magnetised when current flows and completely loses its magnetism as soon as the current is switched off.

Quick Navigation:
Quick Review Flashcards - Click to flip and test your knowledge!
Question
What is the chemical name and formula for the ore known as lodestone?
Answer
It is an ore of iron oxide with the formula $Fe_{3}O_{4}$.
Question
Where was the first known natural magnet, lodestone, found in large quantities?
Answer
It was found in Magnesia, in Asia Minor.
Question
What are the two primary properties observed in lodestone?
Answer
It attracts small pieces of iron and sets itself in a definite direction when suspended freely.
Question
Which civilization is credited with using lodestone to guide boats earlier than 2500 B.C.?
Answer
The Chinese.
Question
Why are artificial magnets prepared despite the existence of natural magnets?
Answer
Natural magnets are irregularly shaped and are not magnetically strong enough for practical use.
Question
In which geographic direction does a freely suspended magnet always point?
Answer
It always points in the geographic north-south direction.
Question
How are the two poles of a magnet named based on their resting direction?
Answer
The end pointing north is the north pole, and the end pointing south is the south pole.
Question
What is the fundamental law of magnetic poles regarding attraction and repulsion?
Answer
Like poles repel each other, while unlike poles attract each other.
Question
Define the term 'induced magnetism'.
Answer
It is the temporary magnetism acquired by a magnetic material when it is kept near or in contact with a magnet.
Question
Which type of polarity is developed at the 'near end' of a magnetic material during induction?
Answer
An opposite polarity to that of the magnet pole near it.
Question
Which type of polarity is developed at the 'farther end' of a magnetic material during induction?
Answer
A similar polarity to that of the inducing magnet pole near the other end.
Question
What happens to the magnetism of a soft iron bar once the inducing magnet is removed?
Answer
The bar loses its magnetism, meaning the induced magnetism is purely temporary.
Question
Explain why an ordinary piece of iron is attracted towards a magnet.
Answer
The iron first becomes a magnet by induction, and then the opposite poles attract.
Question
The phenomenon where induction must occur before a magnet can pull an object is summarised by the phrase _____.
Answer
Induction precedes attraction.
Question
How is a 'chain of nails' formed when one nail is attached to a magnet?
Answer
The magnet magnetises the first nail by induction, which then magnetises the next nail in the same manner.
Question
What is the 'magnetic field' of a magnet?
Answer
The region around a magnet in which a magnetic needle or compass gets influenced.
Question
How does the strength of a magnetic field change as the distance from the magnet increases?
Answer
The effect of the magnetic field decreases as the distance increases.
Question
Is magnetic field a scalar or a vector quantity?
Answer
It is a vector quantity.
Question
How is the magnitude of a magnetic field at a point measured?
Answer
By the force experienced by a magnetic pole of unit strength placed at that point.
Question
What determines the direction of the magnetic field at any given point?
Answer
The direction in which the north pole of a magnetic compass needle rests when placed at that point.
Question
Define 'magnetic field lines'.
Answer
Continuous curves in a magnetic field such that the tangent at any point gives the direction of the magnetic field.
Question
In what direction do magnetic field lines travel 'outside' the magnet?
Answer
They are directed from the north pole towards the south pole.
Question
In what direction do magnetic field lines travel 'inside' the magnet?
Answer
They are directed from the south pole towards the north pole.
Question
What is indicated by the tangent at any point on a magnetic field line?
Answer
The direction of the magnetic field at that specific point.
Question
Why can two magnetic field lines never intersect?
Answer
Intersection would imply two different directions of the magnetic field at the same point, which is impossible.
Question
What does the density or 'crowding' of magnetic field lines represent?
Answer
It represents the strength of the magnetic field; crowded lines indicate a strong field.
Question
Where are magnetic field lines most crowded in a bar magnet?
Answer
Near the poles of the magnet.
Question
What type of magnetic field is represented by parallel and equidistant field lines?
Answer
A uniform magnetic field.
Question
What physical behaviour do magnetic field lines mimic according to their properties?
Answer
They behave like stretched elastic rubber strings.
Question
What happens to an iron rod buried deep inside the Earth in a north-south direction?
Answer
It eventually becomes a weak magnet.
Question
What are 'neutral points' in the context of magnetic field plotting?
Answer
Points where the resultant magnetic field is zero because the magnet's field is neutralised by Earth's field.
Question
How does a magnetic compass behave when placed at a neutral point?
Answer
It can rest in any direction because the net magnetic force is zero.
Question
Define 'earth's magnetic poles'.
Answer
The two places on the Earth's surface where a magnetic needle becomes vertical.
Question
Define the 'magnetic equator'.
Answer
The imaginary line joining places where a magnetic needle remains horizontal.
Question
What is the approximate angle between the Earth's magnetic axis and its axis of rotation?
Answer
Approximately $17^{\circ}$.
Question
Where is the Earth's magnetic south pole currently located geographically?
Answer
In Canada, at approximately $70$-$75^{\circ}$ North latitude and $96^{\circ}$ West longitude.
Question
Where is the Earth's magnetic north pole currently located geographically?
Answer
Near the geographic south pole at $73^{\circ}$ South latitude and $155^{\circ}$ East longitude.
Question
Do the Earth's magnetic poles remain in a stationary position over long periods?
Answer
No, they gradually change their positions over time.
Question
In a limited space, how are the magnetic field lines of the Earth described?
Answer
They are parallel and equidistant to each other.
Question
What is the direction of Earth's uniform magnetic field lines relative to geographic directions?
Answer
They are directed from geographic south to geographic north.
Question
When plotting field lines for a bar magnet with its north pole facing geographic north, where are the neutral points located?
Answer
On either side of the magnet in the east-west direction (broad side-on position).
Question
When plotting field lines for a bar magnet with its south pole facing geographic north, where are the neutral points located?
Answer
On either side of the magnet in the north-south direction (end-on position).
Question
What two magnetic fields are equal and opposite at a neutral point?
Answer
The magnetic field produced by the magnet and the horizontal component of the Earth's magnetic field.
Question
Describe the magnetic field lines produced by a horse shoe magnet.
Answer
They are curved, non-equispaced, and non-parallel (non-uniform).
Question
How can you test if a rod is made of iron or copper using a magnet?
Answer
An iron rod will get magnetised by induction and be attracted, whereas a copper rod will not.
Question
In the experiment to plot Earth's magnetic field, why are brass pins used to fix the paper?
Answer
Brass is a non-magnetic material and will not interfere with the magnetic compass needle.
Question
What is the net magnetic field magnitude at a neutral point?
Answer
Zero.
Question
At which geographic location is the Earth's magnetic field directed vertically upwards?
Answer
At the Earth's magnetic north pole (located near the geographic south pole).
Question
At which geographic location is the Earth's magnetic field directed vertically downwards?
Answer
At the Earth's magnetic south pole (located near the geographic north pole).
Question
Why do iron filings arrange themselves in curved lines when sprinkled over a bar magnet?
Answer
Each filing becomes magnetised by induction and experiences a force that aligns it with the magnetic field lines.
Question
What is the horizontal component direction of Earth's magnetic field?
Answer
From geographic south to geographic north.
Question
If two magnets are placed with like poles facing each other, where is the neutral point located?
Answer
At a point exactly midway between the two magnets.
Question
True or False: The magnetic field lines of Earth are normal to the Earth's surface near the magnetic poles.
Answer
True.
Question
What is the orientation of Earth's magnetic field lines relative to the surface at the magnetic equator?
Answer
They are parallel to the Earth's surface.
Question
How can you mark the polarity of an unmarked magnet using a magnetic compass?
Answer
The end that repels the north pole of the compass is the north pole; the end that attracts it is the south pole.
Question
Why do two iron pins hung from the same pole of a magnet move apart at their pointed ends?
Answer
The pointed ends acquire similar polarities by induction and repel each other.
Question
Under what condition will the neutral point be on the vertical line away from the north pole?
Answer
When a magnet is standing vertical with its north pole up.
Question
What defines the 'end-on position' for neutral points?
Answer
Points located on the axis of the magnet in the north-south direction.
Question
What defines the 'broad side-on position' for neutral points?
Answer
Points located on the equatorial line of the magnet in the east-west direction.
Question
Why does a compass needle point in any direction at a neutral point?
Answer
Because the magnetic field of the magnet is exactly balanced by the Earth's magnetic field, leaving no resultant force.