Magnetic Field 

Magnetic field is an important topic of IIT JEE Physics. It lays the foundation for other topics also. It is important to master this topic so as to understand other topics easily. The knowledge of magnetic fields facilitates the understanding of topics like electric currents.

A magnetic field is a scientific description of the magnetic influence of electric currents and magnetic materials. Both the magnitude and the direction are considered while defining a magnetic field. It uses two distinct but closely associated fields denoted by B and H. B denotes the magnetic flux density and H refers to the magnetic field strength. Some magnetic fields cause magnetic compass needles and various permanent magnets to line up in the direction of the field. As a result of magnetic field, the electrically charged particles are forced to move in a circular or helical path. In fact, electric motors also operate as a result of the force exerted on electric currents in wires in a magnetic field.

Magnetic Field

Thus it is clear that magnetic fields exist only when the electric current flows. And magnetic fields then exist along with the electric fields. The strength of the magnetic field depends on the strength of the current.   

Electric fields around the wire to an appliance only cease to exist when the appliance is unplugged or switched off at the wall. They will still exist around the cable behind the wall.

As shown in the figure given below, the force of magnetism is illustrated through lines. Here, the force points from the positive pole towards the negative pole. The force of magnetism forces small pieces of iron to line up in the direction the magnetic force points. 

 
Magnetic Field

On Earth, the north (positive) pole of the Earth's magnet is in fact at its South geographic pole. If you use a compass needle, it indicates North. But if the compass needle is placed near a bar magnet it points away from the north (positive) pole of the bar magnet.

Hence, the force of magnetism coming from the magnet is called the magnetic field. This is shown by lines. The magnetic field is strongest at the point where the lines of force come together (turn red as shown in the figure above) and they are weakest where the lines of force are apart ( shown in blue).

We may also define a magnetic field as the space around a magnet in which a torque acts on a magnetic needle, or the space around a magnet in which a net force acts on a magnetic test pole.

There are four types of magnetic field:

(1) Uniform Magnetic Field:

The magnetic field in which the intensity of magnetism is same at all points or is uniform everywhere is called as the uniform magnetic field. In such a magnetic field, the magnetic lines of force are parallel and equidistant.

Example: The best possible example of uniform magnetic field is the magnetic lines of force of earth’s magnetic field.

You may view this video to get an idea on the motion of electric charges in uniform magnetic field

(2) Non-uniform magnetic field:

The magnetic field, in which the intensity of magnetic field at different points is different, is known as non-uniform magnetic field. It is represented by non-parallel lines of force.
 
Non-Uniform Magnetic Field             Non-Uniform Magnetic Field


 (3) Varying Magnetic Field:

The magnetic field which keeps on changing with respect to time is called as a variable magnetic field.

Example: B = B0 sin ω t         or         B = B0 cos ω t 

(4) Non-varying Magnetic Field:

The magnetic field which does not change with time is called as a constant magnetic field. The direction of magnetic field is that in which a force acts on a unit test pole. It can be produced by moving charges, current carrying loops, and variations in electric currents.

Non-varying Magnetic Field

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