What is a magnetic field?

Both electrical currents and magnetic materials produce a magnetic effect on them- a magnetic field. The magnetic field is a vector- it has both magnitude and direction.

In our previous articles on magnetism, we have learnt about the magnetic effects of electric current flowing through a straight wire. We also learned how to find the direction of the magnetic field. We also know that the strength of the magnetic field varies with the force of the electric current. In this article, we shall learn more about magnetic fields, with a particular focus on magnetic fields around a circular loops, coils and solenoids.

Let us do a small activity to find out about the magnetic field produced by a loop of wire. Take a straight wire and bend it to produce a circular loop. We have previously learned that the strength of the magnetic field is inversely proportional to the distance from the centre of the wire. Therefore, in the loop model, we will note that the magnetic field lines from each point on the loop converge at the centre of the loop to form a straight line. Using the right-hand thumb rule, we can infer that each point on the loop contributes to the magnetic field at the centre of the loop, and the direction of the field is the same.

The above method is a little-complicated way to find the direction of the magnetic field in a circular loop. There is a simpler method- the Maxwell?s corkscrew rule.

Imagine driving a corkscrew. If we drive in the direction of the current, then the direction of the corkscrew is the direction of the magnetic field.

What would happen to the magnetic field if we use many coils of the wire to make a circular loop? The direction of the magnetic field produced by each point on each coil would be the same. Therefore, according to vector physics, they would just add up. Therefore, more the number of coils, greater is the strength of the magnetic field.

magnetic field

Magnetic field through a solenoid

A solenoid is many turns of insulated copper wire wound in the form of a cylinder. The following diagram shows the direction of the magnetic fields through a current carrying solenoid. You can note that one end of the solenoid behaves like the magnetic north pole, and the other behaves like the south pole. In fact, the magnetic field produced by a solenoid is similar in all aspects with that of a bar magnet. This property is of great relevance in physics and everyday life. Using solenoids, we can now produce unyielding magnets that can be activated by switching on the current. . Such magnets are called as electromagnets.

Can you think of some uses for electromagnets?

Electromagnets are used in electric motors and generators. They are used in transportation- like subway cars. They are used to lift heavy loads- like an electromagnetic crane. They are also used in space crafts. The list of uses of electromagnets is endless.

 

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