In magnetism, bar magnets consist of two poles, North pole and south pole. The bar magnet can be either in the shape of a square or oblong depending upon the requirement. A bar magnet is usually made of Ferromagnetic substances which can be iron or steel.

Bar magnets are generally of two types:

• Natural Magnets- these types of magnets exist naturally and have usually very weak magnetism. It includes Lodestones.
• Artificial magnets- These are the man-made magnets and can be of very strong magnetic fields. Their strength can be increased or decreased depending upon the usage or requirement. The magnets that are shaped in the form of bars are called as Bar magnets.

Solenoid

A solenoid is an electromagnet formed by passing the electric current through a coil. The solenoid is used to generate magnetic fields in a controlled way by controlling the amount of electric current passing through it. The length of coil used in solenoid has to be more than that of its diameter.

When electric current is passed through the solenoid, a magnetic field is produced with both negative as well as positive poles like that of a permanent magnet. The poles of a solenoid either repel or attract depending upon the type of poles. If both negative or positive poles are brought closer it will repel and if opposite poles are brought closer, it will attract. The magnetic field in a solenoid is produced by moving the soft iron core with ferromagnetic properties in and out of the solenoid which cuts the electric field lines inside the coil, leading to the production of Magnetic field.

Uses of Solenoid

• It is used for setting up the ignition in automobiles.
• It is also used as relay i.e., to bring the metals together.
• Solenoids being versatile,thus are extremely useful in the activation of doorbells, activating paintball guns in the automatic factory machines.

Similarities between bar magnet and Solenoid

• In both cases of solenoid and bar magnet, directive property is shared which means that when they are suspended to a thread , they will direct towards the North-south direction.
• The alignment of the magnets can be carried through an external magnetic field.
• The axial magnetic fields produced by solenoid and bar magnet are always the same and identical.
• The magnetic moment produced is also identical.

Behavior of a Solenoid

The magnetic poles of a current carrying solenoid are determined by using a bar magnet :

• A solenoid is made by winding the insulated copper wire around the soft iron core. When current is passed through the solenoid, a magnetic field is produced in the solenoid the same as that of a bar magnet.
• The north pole of the solenoid is repelled by the negative terminal of the battery.
• As per the principle, like poles repel and unlike poles attract, the end of the solenoid connect to the negative terminal of the battery acts as north pole and the otherpole acts as south pole, which means that one acts as north pole and other end acts as south pole of the solenoid.

Derivation

To calculate the magnetic field produced by a solenoid, a bar magnet can be used to demonstrate the magnetic field produced by a solenoid. Consider a solenoid of radius a and length 2l and let the number of turns per unit length be n. Let the current passing through the solenoid be I. To determine the magnetic field through the whole section, consider a section dx at a distance of x from O so that OP=r

Magnetic field produced due to n turns at the axis of the solenoid

Integrating X from -I to +I, we get

On solving the above integral,

Therefore,

Where, m is the magnetic moment of the solenoid which is equal to that of the bar magnet.

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