PHYSICS [2 Mark Question Answer]

The strength of an electromagnet can be increased by following ways:

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

The device formed is an electromagnet.
Use:
For separating the magnetic substances such as iron from other debris.

The polarity at X is North and at Y is South.

By increasing the number of turns of winding in the solenoid, the strength of the electromagnet can be increased.

An electromagnet is made by winding an insulated copper wire around a soft iron core either in the shape of a solenoid or U-shape. The strength of magnetic field of an electromagnet depends on:

1. The number of turns of wire wound around the coil, and
2. The amount of current flowing through the wire.

If an a.c. source is used in place of a battery, the core of the electromagnet will get magnetized, but the polarity at its ends will change. Since attraction of armature does not depend on the polarity of the electromagnet, the bell will still ring on pressing the switch.

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

The process in which a piece of magnetic material acquires the magnetic properties temporarily in presence of another magnet near it is called the magnetic induction.
When a piece of iron is placed near or in contact with a magnet, the piece of iron becomes a magnet i.e., it acquires the property of attracting iron filings when they are brought near its ends. Thus, a piece of iron behaves as a magnet as long as it is kept near (or in contact with) a magnet.

An artificial magnet is a magnetized piece of iron (or other magnetic material). Artificial magnets are required because natural magnets have odd and irregular shape and they are not magnetically very strong. Artificial magnets can be given desired shape and made very strong.

The pieces of lodestone found in nature are called the natural magnets. Limitations of a natural magnet are as listed below:
(i) They are irregular and odd shaped.
(ii) They are not magnetically very strong.

(i) Neutral points will be in east-west direction. (ii) Neutral points will be north-south direction.

Neutral points are the points where the magnetic field of the magnet is equal in magnitude to the earth's horizontal magnetic field, but it is in opposite direction. Thus the net magnetic field at the neutral points is zero.
Since the net magnetic field is zero at neutral points, the compass needle remains unaffected (i.e. it comes to rest pointing in any direction) at these points and hence, they can be detected.

It can be concluded that magnetic field at that point is zero. This is because the earth's magnetic field at that point is neutralized by the magnetic field of some other magnetized material.

Magnitude of magnetic field at neutral points is zero. It is so because at these points, the magnetic field of the magnet is equal in magnitude to the earth's horizontal magnetic field, but it is in opposite direction. Hence, they cancel each other.

Solution is as follows:

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

In (b) The North Pole of one magnet is facing the South Pole of the other. So, the field lines will be

In (a) The North Pole of two magnets is facing each other. So, the field lines will be

The iron filings take up a definite pattern (curved lines). This happens because each piece of iron filing becomes a magnet to the magnetic induction of the magnet. It thus experiences a force in the direction of magnetic field of the bar magnet at that point and aligns itself along curved lines.

Properties of magnetic field lines:

1. They are closed and continuous curves.
2. They are directed from the North Pole towards the South Pole outside the magnet.
3. The tangent at any point on a field line gives the direction of magnetic field at that point.
4. Two magnetic lines never intersect each other.

A magnetic field line is a continuous curve in a magnetic field such that tangent at any point of it gives the direction of the magnetic field at that point.

When a piece of magnetic material is brought near a magnet, it first becomes a magnet by induction and then it is attraction. Thus, we say that induction precedes attraction.

When a piece of soft iron is placed a little distance away from the needle, the needle induces magnetism to the piece of soft iron. Thus, soft iron piece starts behaving like a magnet and it attracts the magnetic needle towards it.

Several soft iron pins can cling one below the other from the pole of a magnet because the magnet induces magnetism in an iron nail which gets attracted by the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it. This process continues until force of attraction on first nail is sufficient to balance the total weight of all nails in chain.

When two pins are hung by their heads from the same pole of a magnet, they acquire same polarity. Because like poles repel each other, their pointed ends move apart.

When iron nails are brought near one end of a magnet, the nearer end of piece acquires an opposite polarity by magnetic induction. Since unlike poles attract each other, therefore, iron nails are attracted towards the end of the magnet. Thus, the iron nail first becomes a magnet by induction and then it is attracted.