Question 1

Question 1(a) Draw the pattern of magnetic field lines near a bar magnet placed with it's 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?

Accepted 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.

Question 2

Question 2Figure given below shows a bar magnet placed on the table top with it's 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.

Accepted 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.

Question 3

Question 3You 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]

Accepted 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.

Question 4

Question 4A 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.

Accepted 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.

Question 5

Question 6Explain the mechanism of attraction of iron nails by a magnet when brought near them.

Accepted 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.

Question 6

Question 7A 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?

Accepted 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.

Question 7

Question 8Explain the method of plotting magnetic field lines by using a small compass needle.

Accepted 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).

Question 8

Question 10The figure given below shows two iron nails attached to the end of a magnet. Choose the correct figure:

Accepted Answer

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

Question 9

Question 11A freely suspended bar magnet is taken to the north pole of the earth. It comes to rest :in any directionparallel to the earth’s surfacenearly vertical with the south pole in a downward directionnearly vertical	with the north pole in a downward direction.

Accepted Answer

nearly vertical with the south pole in a downward directionReason — 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.

Question 10

Question 7'Induced magnetism is temporary'. Comment on this statement.

Accepted 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.

Question 11

Question 10State four properties of magnetic field lines.

Accepted 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.

Question 12

Question 11Explain 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.

Accepted 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.

Question 13

Question 12Can two magnetic field lines intersect each other? Give reason to your answer.

Accepted 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.

Question 14

Question 13In figure below, draw at least two magnetic field lines between the two magnets.

Accepted 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:

Question 15

Question 14State two evidences of the existence of earth's magnetic field.

Accepted 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.

Question 16

Question 15Sketch four magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone.

Accepted Answer

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

Question 17

Question 16What 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.

Accepted 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.

Question 18

Question 17What is a neutral point? How is the position of a neutral point located with the use of a compass needle?

Accepted 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.

Question 19

Question 18State the positions of neutral points when a magnet is placed with it's axis in the magnetic meridian and with it's north pole (i) pointing towards the geographic north, (ii) pointing towards the geographic south.

Accepted 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.

Question 20

Question 5Describe the construction and working of an electric bell with the help of a neat and labelled diagram.

Accepted Answer

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.

Magnetism

Exercise 10A Long Answer Type

Exercise 10A Multiple Choice Type

Exercise 10A Short Answer Type

Exercise 10B Long Answer Type