Magnetic Field

The force exerted by a magnet on the various magnetic substance is called magnetic force. The magnetic force is a non contact force that is it is effective from a distance. Obviously, there must be some influence of the magnet on the space around it. This influence reduces as we go further away from the magnet. The area where the magnetic force is applicable is called magnetic field. This magnetic field causes attraction or repulsion between them. A magnetic compass always points in the N—S direction when it is placed away from any magnet. If we bring a magnet near the compass, the compass needle will show deflection and will get aligned in a new direction depending upon the placement of the bar magnet. This shows that a magnet changes the magnetic properties of space around it.

This can be verified by placing a compass needle near a magnet. The needle gets deflected due to the effect of the magnet.

If you move the compass needle above, below and to the side of the magnet, the needle gets deflected all around the magnet. Take the needle further and further away, the influence of the magnet becomes weaker and weaker, until at some distance away it is not felt at all.

Thus, there is a region around the magnet where its magnetic influence can be felt. This region is called the magnetic field of the magnet.

Magnetic Field Lines:

Magnetic field lines show the direction of the magnetic field. To find the magnetic field line place the compass near one end of the bar magnet and stepwise move the compass in a direction in which north pole of compass points and each time mark the position of compass without disturbing the bar magnet.

Finally, you will get a line starting from North Pole and ending at South Pole of bar magnet as shown in the figure. You can draw many such lines around the magnet by changing your starting point from North Pole, using compass. These lines are called Magnetic Field Lines. They are a way to visualize the magnetic field. When drawn the distance between them is an indication of the strength of the field. The closer they are the stronger is the field. Also, the direction of the tangent to the field line is the direction of the magnetic field at that point. A free north pole would move along the magnetic field line.

Properties of Magnetic Field Lines: The various properties are

1. Magnetic field is pictorially represented by magnetic field lines.

2. These lines indicate the direction in which a North Pole would move if it can be isolated

3. Tangent to the curves at a point on magnetic field lines gives the direction in which a test north pole will experience a force if it is placed at that point.

4. Inside the bar magnet, field lines are directed from south to north.

5. Magnetic field lines start North Pole and go towards the South Pole and inside the magnet they move from south to north. Hence it is a closed curve.

6. Strength of magnetic field at a point is shown by the density of the field lines. The closer the field lines, the greater will be the strength of the magnetic field, and vice—versa.

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Sample Questions

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Question : 1

An electron beam is moving horizontally towards east. If this beam passes through a uniform magnetic field directed downwards, find the direction in which the beam will be deflected.
A. North
B. South
C. East
D. West
Right Option : A
View Explanation

Question : 2

A magnetic field line is used to find the direction of ______________
A. Magnetic Compass		B. Magnetic Field
C. A bar Magnet		D. East West Direction
Right Option : B
View Explanation

Question : 3

Two objects repel each other.This replusion could be due to
A. frictional force only	B. electrostatic force only	C. magnetic force only	D. either a magnetic or an electrostatic force
Right Option : D
View Explanation