The ............... magnetism acquired by a magnetic material when it is kept near a magnet is called ............... magnetism.
- temporary, induced
- permanent, induced
- temporary, permanent
- None of the above
Answer
temporary, induced
Reason — Due to magnetic induction, a piece of magnetic material acquires the magnetic properties temporarily in presence of another magnet near it.
A magnetic pole induces ............... polarity on the near end and a ............... polarity on the farther end of the iron bar.
- opposite, opposite
- similar, similar
- opposite, similar
- similar, opposite
Answer
opposite, similar
Reason — A magnetic pole induces opposite polarity on the near end and a similar polarity on the farther end of the iron bar.
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.
- (i)
- (ii)
- both (i) and (ii)
- none of the above
Answer
both (i) and (ii)
Reason — The correct statements are :
- They are closed and continuous curves.
- Outside the magnet, they are directed from the north pole towards the south pole of the magnet.
An iron rod buried inside the earth along ............... direction becomes a magnet.
- east-west
- equatorial
- north-south
- any direction
Answer
north-south
Reason — The Earth itself acts as a giant magnet. When the iron rod is aligned in the same direction as the Earth's magnetic field lines i.e., north-south direction, it gets magnetised due to magnetic induction.
The sum of all magnetic fields adds up to zero at the :
- north pole
- south pole
- equator
- neutral points
Answer
neutral points
Reason — At each neutral points, the resultant magnetic field is zero.
The magnetic field lines in a non-uniform magnetic field are :
- either converging or diverging
- parallel and equispaced
- only converging
- only diverging
Answer
either converging or diverging
Reason — The magnetic field around a bar magnet (or horse shoe magnet) is non-uniform. The magnetic field lines in a non-uniform magnetic field are not equispaced and parallel, but they are curved either converging or diverging.
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.
- parallel, normal
- normal, parallel
- normal, normal
- parallel, parallel
Answer
normal, parallel
Reason — The magnetic field lines of the earth are normal to the earth's surface near the magnetic poles and parallel to the earth's surface near the magnetic equator.
The direction of a magnetic field at a point on a magnetic field line is given by :
- a tangent at that point
- a normal bisector at that point
- both tangent and normal bisector at that point
- none of the above
Answer
a tangent at that point
Reason — A magnetic field line is a continuous curve in magnetic field such that tangent at any point of the curve gives the direction of the magnetic field at that point.
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 :
- along north-south direction
- along east-west direction
- equally inclined to N-S and E-W direction
- in any direction
Answer
in any direction
Reason — As the soft iron bar is not magnetised, hence it will not align itself with the Earth's magnetic field and will come to rest in any direction.
The figure given below shows two iron nails attached to the end of a magnet. Choose the correct figure:
Answer
Reason — The lower ends of both the iron nails will repel each other since they have same polarities (north pole in this case).
A freely suspended bar magnet is taken to the north pole of the earth. It comes to rest :
- in any direction
- parallel to the earth’s surface
- nearly vertical with the south pole in a downward direction
- nearly vertical with the north pole in a downward direction.
Answer
nearly vertical with the south pole in a downward direction
Reason — A freely suspended bar magnet is taken to the north pole of the earth. It comes to rest nearly vertical with the south pole in a downward direction.
Which of the following is not a correct statement for a magnet placed in the earth's magnetic field ?
- Neutral points are always at an equal distance from the magnet.
- The position of neutral points depends on the direction of the magnet in the earth’s magnetic field.
- The position of neutral points does not depend on the direction of the magnet.
- The magnetic field strength is zero at the neutral points.
Answer
The position of neutral points does not depend on the direction of the magnet.
Reason — As neutral points are the points at which two magnetic fields are equal in magnitude but opposite in direction, Therefore, the position of neutral points depends on the direction of the magnet.
Hence, the statement "The position of neutral points does not depend on the direction of the magnet" is incorrect.
Fill in the blanks to complete the sentences:
(a) Two ends of a magnet are called ...............
(b) Unlike poles of a magnet ............... each other.
(c) Like poles of a magnet ............... each other.
(d) A freely suspended magnet rests in the geographic ............... direction.
Answer
(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.
Complete the following sentences:
(a) If the field lines in a magnetic field are parallel and equidistant, the magnetic field is ............... .
(b) At a neutral point, the resultant magnetic field is ............... .
(c) The neutral points of a bar magnet kept with its north pole pointing towards geographic north are located ............... .
Answer
(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 it's north pole pointing towards geographic north are located on either side of the magnet in east and west directions.
Where is the magnetic south pole of the earth located?
Answer
The magnetic south pole of the earth is in Canada at a distance nearly 2240 km from the geographic north pole at 70.75° north latitude and 96° west longitude.
What kind of magnetic field is represented by parallel and equidistant lines ?
Answer
Uniform Magnetic Field.
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 the axis of rotation of the earth.
What is a lodestone?
Answer
Lodestone is the first known pieces of magnets, an ore of iron oxide (Fe3O4) found in large quantities in Magnesia, in Asia Minor.
What is a natural magnet? State two limitations of a natural magnet?
Answer
The pieces of lodestone found in nature were called natural magnets.
The limitations of a natural magnet are —
- They are found in quite irregular and odd shapes.
- They are not magnetically strong enough for use.
What is an artificial magnet? State two reasons why do we need artificial magnets?
Answer
Artificial magnets are prepared from iron in different convenient shapes and sizes.
Examples of artificial magnets are bar magnet, horse shoe magnet, magnetic needle and magnetic compass.
We need artificial magnets because —
- Natural magnets are of irregular and odd shapes.
- Natural magnets are not magnetically strong enough for use.
How will you test whether a given rod is made of iron or copper?
[Hint: Iron rod gets magnetized when placed near a bar magnet by magnetic induction, while copper rod does not get magnetized]
Answer
To test whether a given rod is made of iron or copper, we should bring it near (or in contact with) a magnet, if the rod is of iron, then it will become a magnet i.e., it will acquire the property of attracting iron fillings when they are brought near it's ends.
This happens because iron rod gets magnetized when placed near a bar magnet by magnetic induction, while copper rod does not get magnetized. This way, we can detect which rod is iron and which is copper.
Explain the term induced magnetism.
Answer
The temporary magnetism acquired by a magnetic material when it is placed near (or in contact with) a magnet, is called induced magnetism.
Explain the following:
When two pins are hung by their heads from the same pole of a magnet, their pointed ends move apart.
Answer
When two pins are hung by their heads from the same pole of a magnet, their pointed ends move apart because the iron nails by magnetic induction gets magnetized and develop same poles. As we know that like poles repel each other hence, the two pointed ends of the nails repel each other.
Explain the following:
Several soft iron pins can cling, one below the other, from the pole of a magnet.
Answer
Several soft iron pins can cling, one below the other, from the pole of a magnet because the bar magnet by induction magnetizes an iron nail which gets attracted to the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it.
This process continues till force of attraction of magnet on first nail is sufficient to balance the total weight of all the nails in the chain below it.
Now holding the uppermost nail in position by fingers, and if the magnet is removed, we find that all nails fall down. The reason is that on removing the magnet, the uppermost nail loses it's magnetism, so all other nails also lose their magnetism, they get separated from each other and they all fall down due to the force of gravity.
This shows that the magnetism acquired by induction is purely temporary. It lasts so long as the magnet causing induction remains in it's vicinity.
Explain the following:
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
When the north end of a freely suspended magnetic needle is placed near a piece of soft iron, the nearer end of the piece acquires an opposite polarity by magnetic induction.
Since unlike poles attract each other, therefore the iron piece is attracted towards the end of the magnet. Thus, the piece of iron first becomes a magnet by induction and then it is attracted.
'Induced magnetism is temporary'. Comment on this statement.
Answer
If one pole of a bar magnet is brought near small iron nails, they form a chain as shown in the figure below.
The magnetised nail is attracted to the magnet and clings to it. This magnetised nail, in turn, magnetises the neighbouring nail through induction and attracts it. This process continues until the force of attraction on the first nail balances the weight of all the nails in the chain below it. If the uppermost nail is held in position by fingers and the magnet is removed, all the nails fall down. This is because, without the magnet, the uppermost nail loses its magnetism. The nails no longer attract each other and separate, falling down due to gravity. This demonstrates that induced magnetism is temporary and lasts only as long as the magnet causing the induction is nearby.
'Induction precedes attraction'. Explain the statement.
Answer
When a piece of iron is brought near one end of a magnet (or one end of a magnet is brought near the piece of iron), the nearer end of the piece acquires an opposite polarity by magnetic induction.
Since unlike poles attract each other, therefore the iron piece is attracted towards the end of the magnet. Thus, the piece of iron first becomes a magnet by induction and then it is attracted. In other words, induction precedes attraction.
What do you understand by the term magnetic field lines?
Answer
A magnetic field line is a continuous curve in magnetic field such that tangent at any point of the curve gives the direction of the magnetic field at that point.
State four properties of magnetic field lines.
Answer
The magnetic field lines have the following properties —
- They are closed and continuous curves.
- Outside the magnet, they are directed from the north pole towards the south pole of the magnet.
- The tangent at any point on a field line gives the direction of magnetic field at that point.
- They are crowded near the poles of the magnet where the magnetic field is strong and far away near the middle of the magnet and far from the magnet where the magnetic field is weak.
Explain why iron fillings which are sprinkled on a sheet of cardboard placed over a bar magnet, take up a definite pattern when cardboard is slightly tapped.
Answer
The iron fillings which are sprinkled on a sheet of cardboard placed over a bar magnet, take up a definite pattern when cardboard is slightly tapped because each piece of iron fillings gets magnetized by magnetic induction and experiences a force due to the magnet. Therefore, they arrange themselves along curved lines and these curved lines are called magnetic field lines.
Can two magnetic field lines intersect each other? Give reason to your answer.
Answer
Two magnetic field lines never intersect each other.
If they would intersect, this would mean that there are two directions of the field at the specific point which is not possible.
In below figure, draw at least two magnetic field lines between the two magnets.
Answer
(a) As the north pole of both the magnets are facing each other so they will repel. The magnetic field lines between the two magnets are shown below:
(b) As opposite poles of both the magnets are facing each other so they will attract. The magnetic field lines between the two magnets are shown below:
State two evidences of the existence of earth's magnetic field.
Answer
The evidences of the existence of earth's magnetic field are —
- A freely suspended magnetic needle always rests in geographic north-south direction.
- An iron rod buried inside the earth along north-south direction becomes a magnet.
Sketch four magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone.
Answer
Magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone are shown below:
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
If a compass needle at a point rests in any direction then we can say that the magnitude of the magnetic field at that particular point is zero.
The reason for this is that the earth's magnetic field at that point is neutralized by the magnetic field of some other magnetized material.
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. The net magnetic field at a neutral point is zero.
A compass needle if placed at the neutral point, will rest in any direction. Hence, we can find the position of neutral points with the help of compass needle.
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.
Answer
(i) When a magnet is placed with it's axis in the magnetic meridian and with it's north pole pointing towards the geographic north then the neutral point will be in east-west direction.
(ii) When a magnet is placed with it's axis in the magnetic meridian and with it's north pole pointing towards the geographic south then the neutral point will be in north-south direction.
(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) Below diagram shows the magnetic field lines near a bar magnet placed with it's north pole pointing towards the geographic north. The position of neutral points is marked with x:
(b) The magnetic field lines as shown in part (a) are non-uniform.
Below figure 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.
(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 for the magnet are as shown below:
(b) The likely position of the neutral points is indicated by crosses in the diagram below:
(c) The magnitude of magnetic field at each neutral point is zero because at these points the magnetic field of the magnet and the magnitude of the earth's horizontal magnetic field are equal in magnitude but in opposite directions such that the two fields neutralize each other.
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
If we suspend the given bars with a silk thread such that it is free to rotate in a horizontal plane, then we will observe that one bar always sets itself in geographical north south direction as shown in the picture given below, while the other bar sets itself in any direction.
This happens because a magnet when suspended freely will rest only in north-south direction, but soft iron bar will rest in any direction. Hence, by suspending the two given bars freely and observing the position in which they rest we can distinguish between a magnet and a soft iron bar.
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
When a magnet is suspended with a silk thread such that it is free to rotate in a horizontal plane, it will rest in the geographical north-south direction with north pole towards the geographic north, making some angle with the horizontal.
Explain what do you understand by magnetic induction. What role does it play in attraction of a piece of iron by a magnet?
Answer
The process in which a piece of magnetic material acquires the magnetic properties temporarily in presence of another magnet near it is called magnetic induction.
Induction precedes attraction — When a piece of iron is brought near one end of a magnet (or one end of a magnet is brought near the piece of iron), the nearer end of the piece acquires an opposite polarity by magnetic induction.
Since unlike poles attract each other, therefore the iron piece is attracted towards the end of the magnet. Thus, the piece of iron first becomes a magnet by induction and then it is attracted. In other words, induction precedes attraction.
Explain the mechanism of attraction of iron nails by a magnet when brought near them.
Answer
If one pole of a bar magnet is brought near small iron nails, they form a chain of nails as shown below, because the bar magnet by induction magnetizes an iron nail which gets attracted to the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it.
This process continues till force of attraction of magnet on first nail is sufficient to balance the total weight of all the nails in the chain below it.
Now holding the uppermost nail in position by fingers, and if the magnet is removed, we find that all nails fall down. The reason is that on removing the magnet, the uppermost nail loses it's magnetism, so all other nails also lose their magnetism, they get separated from each other and they all fall down due to the force of gravity.
This shows that the magnetism acquired by induction is purely temporary. It lasts so long as the magnet causing induction remains in it's vicinity.
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
When a small iron bar is kept near the north pole of a bar magnet as shown below, the bar becomes a magnet due to magnetic induction i.e., it acquires the property of attracting iron fillings when they are brought near it's ends.
Polarity developed at the end A of the bar AB is north (opposite to the polarity of the magnet near end A) and the polarity at end B is south (i.e. similar to the polarity at the end of the magnet near end A).
If the magnet is now removed, the bar will lose it's magnetism.
Thus, the bar of a magnetic material behaves like a magnet so long it is kept near or in contact with a magnet.
The magnetism so produced is called induced magnetism.
Explain the method of plotting magnetic field lines by using a small compass needle.
Answer
Earth's magnetic field is uniform in a limited space. Experimentally we can plot uniform magnetic field lines of the earth as follows —
Experiment — Fix a sheet of paper on a drawing board (or a table top) by means of brass pins. Place a small compass needle at position 1 (as shown in fig.) and looking from top of the needle, mark two pencil dots exactly in front of the two ends of the needle. Then move the compass needle to position 2 in such a way that one end of the needle coincides with the second pencil dot.
Mark the position of the other end of the needle with a dot. Repeat the process of moving the compass needle to position 3,4,.. to obtain several dots. On joining the different dots, you will get a straight line. Thus, one magnetic field line of the earth is traced.
Repeat the process starting from a different point and trace out another magnetic field line. In this manner, draw several magnetic field lines starting from different points. Label each line with an arrow from the south pole of needle towards the north pole to indicate the direction of the magnetic field. Fig. above shows several magnetic field lines so obtained.
It is noticed that these lines do not intersect each other. They are parallel and equidistant. They are directed from geographic south to geographic north (i.e., the direction in which a magnetic needle, suspended freely in a horizontal plane rests).
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.
- Both A and R are true and R is the correct explanation of A
- Both A and R are true and R is not the correct explanation of A
- Assertion is false but reason is true
- Assertion is true but reason is false
Answer
Assertion is false but reason is true
Explanation
Assertion (A) is false because like poles (e.g., north-north or south-south) repel each other and only unlike poles (north-south) attract so two bar magnets will always repel each other when brought near with the same pole.
Reason (R) is true because it is a correct observed behavior of magnets where only unlike poles (N-S) attract each other while similar poles like N-N or S-S repel each other.
Assertion (A) : Magnetic field lines never intersect each other.
Reason (R) : At a particular point, magnetic field has only one direction.
- Both A and R are true and R is the correct explanation of A
- Both A and R are true and R is not the correct explanation of A
- Assertion is false but reason is true
- Assertion is true but reason is false
Answer
Both A and R are true and R is the correct explanation of A
Explanation
Assertion (A) is true because if magnetic field lines intersect each other then it would imply that there are two directions of the magnetic field at a single point, which is not possible.
Reason (R) is true because if magnetic field has more than one direction at any point, it would mean that if a compass needle is placed at that point in the magnetic field then north pole of its needle will point in multiple directions simultaneously which is not possible.
This is the correct reason why magnetic field lines do not intersect and hence, reason justifies the assertion.
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.
- Both A and R are true and R is the correct explanation of A
- Both A and R are true and R is not the correct explanation of A
- Assertion is false but reason is true
- Assertion is true but reason is false
Answer
Assertion is false but reason is true
Explanation
Assertion (A) is false because neutral points are locations where the magnetic fields from two sources (e.g., a bar magnet and Earth's field) are equal in magnitude but opposite in direction, so they cancel out.
Reason (R) is true because that's the definition of a neutral point i.e., the point at which two magnetic fields are equal in magnitude, but opposite in direction so that net magnetic field is zero and a compass needle if placed at the neutral point then it will rest in any direction.
In a physics laboratory, a student places a bar magnet near a compass. At certain positions, the compass needle shows no deflection.
(a) Why does the compass needle show no deflection at certain points? What are these points called?
(b) What happens to the magnetic field lines at these points?
(c) Why are these points useful in studying magnetic fields?
(d) Are these points fixed, or do they change their positions?
Answer
(a) The compass needle shows no deflection at certain points because the magnetic field due to the bar magnet is equal and opposite to the horizontal component of Earth's magnetic field at those points. Hence, the resultant magnetic field becomes zero. These points are called neutral points.
(b) At neutral points, the magnetic field due to the magnet and the horizontal component of Earth’s magnetic field are equal in magnitude and opposite in direction. Therefore, the resultant magnetic field is zero.
(c) Neutral points are useful because they help in determining the strength and direction of magnetic fields. By locating these points, the magnetic field of a magnet can be compared with Earth's magnetic field.
(d) These points are not fixed. Their positions change with the strength and orientation of the magnet and also with the direction and strength of Earth's magnetic field.
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?
(b) How is an electromagnet different from a permanent magnet?
(c) How can the strength of an electromagnet be increased?
(d) Why are electromagnets preferred in cranes?
(e) Which material is preferred for making the core of an electromagnet?
Answer
(a) An electromagnet produces magnetism only when electric current flows through its coil. When the current is switched off, the magnetic field disappears, so the electromagnet loses its magnetism.
(b) An electromagnet is a temporary magnet that works only when current flows through it, and its strength can be controlled. A permanent magnet, on the other hand, has permanent magnetism and does not require electric current. Its strength cannot be controlled.
(d) Electromagnets are preferred in cranes because their magnetism can be switched on and off. This allows heavy iron objects to be lifted easily and released when required.
(e) Soft iron is preferred for the core of an electromagnet because it gets magnetized easily and loses its magnetism quickly when the current is switched off.