Magnetism

(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.

(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.

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.

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.

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.

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.

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.

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).

temporary, induced

Reason — Due to magnetic induction, a piece of magnetic material acquires the magnetic properties temporarily in presence of another magnet near it.

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.

both (i) and (ii)

Reason — The correct statements are :

1. They are closed and continuous curves.
2. Outside the magnet, they are directed from the north pole towards the south pole of the magnet.

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.

neutral points

Reason — At each neutral points, the resultant magnetic field is zero.

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).

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.

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.

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.

Reason — The lower ends of both the iron nails will repel each other since they have same polarities (north pole in this case).

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.

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, hence, the position of neutral points depends on the direction of the magnet. Therefore, the statement "The position of neutral points does not depend on the direction of the magnet" is incorrect.

Lodestone is the first known pieces of magnets, an ore of iron oxide (Fe₃O₄) found in large quantities in Magnesia, in Asia Minor.

The pieces of lodestone found in nature were called natural magnets.

The limitations of a natural magnet are —

1. They are found in quite irregular and odd shapes.
2. They are not magnetically strong enough for use.

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 —

1. Natural magnets are of irregular and odd shapes.
2. Natural magnets are not magnetically strong enough for use.

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.

The temporary magnetism acquired by a magnetic material when it is placed near (or in contact with) a magnet, is called induced magnetism.

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.

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.

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.

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.

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.

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.

The magnetic field lines have the following properties —

1. They are closed and continuous curves.
2. Outside the magnet, they are directed from the north pole towards the south pole of the magnet.
3. The tangent at any point on a field line gives the direction of magnetic field at that point.
4. 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.

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.

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.

(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:

The evidences of the existence of earth's magnetic field are —

1. A freely suspended magnetic needle always rests in geographic north-south direction.
2. An iron rod buried inside the earth along north-south direction becomes a magnet.

Magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone are shown below:

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.

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.

(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) 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.

(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.

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.

Uniform Magnetic Field.

The magnetic axis of the earth makes an angle of 17° with the axis of rotation of the earth.

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.

both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because the strength of magnetic field of an electromagnet can be increased by the following two ways :

(i) by increasing the number of turns of winding in the solenoid, and

(ii) by increasing the current through the solenoid.

Reason (R) is true because on increasing the number of turns of winding in the solenoid, the magnetic field strength of an electromagnet also increases and vice versa and hence, reason correctly explains the assertion.

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 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.

An electromagnet can be made in the following two shapes:

1. I-shape or bar magnet.
2. U-shape or horse-shoe magnet.

The principle behind making both the magnets is the same. An electromagnet can be made by winding an insulated copper wire over a piece of soft iron in U-shape or a solenoid.

The factors on which the strength of the magnetic field of an electromagnet depends are —

1. The number of turns of wire wound around the coil — By increasing the number of turns of wire wound around the coil, the magnetic field of an electromagnet can be increased.
2. The amount of current flowing through the wire — By increasing the current through the wire, the magnetic field of an electromagnet can be increased.

The labelled circuit diagram for making an electromagnet from a soft iron bar is shown below:

The differences are as follows —

Below labelled circuit diagram shows an electromagnet made by winding a wire on a U-shaped piece of soft iron:

Construction :

An electric bell is shown in below figure :

An electric bell consists of the following main parts:

(i) A horse-shoe electromagnet (M) with a soft iron core, wound with coils (CC) in opposite directions.

(ii) A soft iron armature (A) fixed to a metallic spring strip (SS).

(iii) A hammer (H) attached to the armature.

(iv) A gong (G) struck by the hammer.

(v) An adjusting screw (S’) to set the position of the armature.

(vi) A switch or bell-push (K).

(vii) A battery connected across the terminals (T₁ and T₂).

When the switch K is not pressed, the strip SS touches the screw S’, and the armature A remains away from the electromagnet poles.

Working :

When the switch K is pressed, current flows through the coils (CC) and magnetises the electromagnet. This attracts the armature A, causing the hammer H to strike the gong G, producing sound.

As the armature moves, contact between the strip SS and the screw S’ breaks, stopping the current. The electromagnet loses magnetism, and the spring strip pulls the armature back to its original position. Contact is re-established, current flows again, and the cycle repeats.

This make and break action continues rapidly, causing the hammer to strike the gong repeatedly while the switch is pressed.

soft iron

Reason — An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet.

(A) south pole

(B) north pole

(C) current is switched on, current is switched off

Reason —

(A) When current is passed through the winding of the solenoid by closing the key K, the end P of the bar becomes the south pole, since current at this face is clockwise.

(B) When current is passed through the winding of the solenoid by closing the key K, the end Q of the bar becomes the north pole, since current at this face is anticlockwise.

(C) The bar shows magnetic properties only when an electric current flows through the solenoid and it loses the magnetic properties as soon as the current is switched off (since soft iron has low retentivity), thus it is a temporary magnet.

increasing the number of turns, increasing the current

Reason — The strength of a magnet can be increased by —

1. Increasing the number of turns of windings in the solenoid.
2. Increasing the current through the solenoid.

(B) and (C)

Reason — The correct statements for an electromagnet are :

1. An electromagnet can produce a strong magnetic field.
2. The polarity of electromagnet can be reversed by reversing the direction of current in the solenoid.

It becomes magnetised

Reason — When the current flows through the soft iron piece in the form of a closed ring, it becomes magnetised. The north and south poles will be present on opposite sides of the ring.

Magnetic compass

Reason — A permanent magnet is used to make a magnetic compass.

the direction of current at the ends of the magnet.

Reason — The magnetic polarity at the two ends of a horse-shoe magnet depends on the direction of current at the ends of the magnet.

option 1

Reason —

Options (1) and (3) are correct : Here field lines go from north to south direction outside the magnet which matches the physical rule of magnetic field direction i.e., magnetic field lines originates from north pole and terminate at south pole outside the magnet.

Option (2) is incorrect : As field lines are going from S to N outside the magnet which is opposite of correct magnetic field line direction.

Option (4) is incorrect : Some field lines are coming from north to south while some are going from south to north forming a closed loop outside the magent but magnetic field lines always move from north to south direction outside the magnet.

An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet.

The strength of a magnet can be increased by —

1. Increasing the number of turns of windings in the solenoid.
2. Increasing the current through the solenoid.

(a) The polarity at X — south pole and at Y — north pole.

(b) The strength of the electromagnet so formed can be increased by increasing the current in the coil.

Advantages of an electromagnet over a permanent magnet are —

1. An electromagnet can produce a strong magnetic field.
2. The strength of the magnetic field of an electromagnet can easily be changed by changing the current (or the number of turns) in it's solenoid.

If alternating current is used instead of direct current then the core of electromagnet will get magnetized, but the polarity at it's ends will change. Since attraction of armature does not depend on the polarity of electromagnet, so the bell will still ring on pressing the switch.

Soft iron is used for making the armature of an electric bell because it acquires magnetic properties only when an electric current flows through the solenoid and loses the magnetic properties as soon as the current is switched off.

No, electromagnet is not used as a permanent magnet. It is a temporary strong magnet.

The material used for preparing an electromagnet is a piece of soft iron.

This device is called an electromagnet.

Electromagnets are used in electrical devices such as electric bell, electric motor, relay, microphone, etc.

An electric bell uses an electromagnet to function.

The soft iron bar acquires the magnetic properties only when an electric current flows through the solenoid and loses the magnetic properties as soon as the current is switched off. That's why soft iron is used as the core of the electromagnet in an electric bell.