Physics
Study Material

# Bar Magnet

A bar magnet is a rectangular piece of object made up of substance having ferromagnetic property like cast iron, mild steel.

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Posted by Mahak Jain, 10/11/2021

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A magnet is an object that produces a magnetic field around itself. They can be of any shape and size. Depending upon the shape we have bar magnet, coin magnet, cylindrical magnet, etc.

Magnet can be of two types,

• Natural magnet
• Artificial magnet

## What is bar magnet?

A bar magnet is a rectangular piece of object made up of substance having ferromagnetic property like Iron, steel. The magnet has two poles,

• A North pole
• A South pole

The magnet when suspended freely it align itself so that the north pole points towards the magnetic north pole of the earth.

There are two types of bar magnet,

1. Rectangular bar magnet
2. Cylindrical bar magnet

Both these forms are known as a bar magnet due to their resemblance to a shape of a block, whose one side is longer than the adjacent side, commonly referred to as a ‘Bar’.

### Rectangular bar magnet

The bar magnetic with sharp and clear edges is called as rectangular magnet. Rectangular bar magnet have greater magnetic field and strength as compared to other magnet. The bar magnet have many applications in engineering and manufacturing industries due to greater magnetic field.

### Cylindrical bar magnet

The bar magnet with round edges is called a cylindrical magnet. Cylindrical bar magnets have various applications in experimental, educational, and research uses.

## Properties of Bar Magnet

The properties of bar magnet are,

• A bar magnet has two poles one is north and another is south pole.
• The magnetic force of a bar magnet gets stronger towards the poles, force gets weaker in the middle of the magnet.
• The same of pole of magnet repel each other and opposite pole of magnet experiences attraction.
• The bar magnet attracts other ferromagnetic substance.

## Where are poles of bar magnet located ?

The two poles of magnet are north and south pole. These two poles are located at the end of bar magnet just before the ends.

## Pole strength of magnet

Pole strength of magnet is strength of magnet at which it attracts magnetic material.

Thus, Pole strength = magnetic dipole moment (M) / effective length of magnet (2l)

Unit of pole strength = Ampere × meter (Am)

## Magnetic field of bar magnet

The imaginary lines that can be drawn along the magnetic field that is acting around any magnetic substance.

The properties of magnetic field lines are,

• The magnetic field lines moves from the north Pole to the south pole.
• The magnetic field lines of a magnet (or a solenoid) form continuous closed loops. Magnetic filed line is unlike the electric dipole where these field lines begin from a positive charge and end on the negative charge or escape to infinity
• The arrows show the direction of the magnetic lines of force.
• The tangent to the field line at a given point represents the direction of the net magnetic field B at that point.
• The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B.
• The magnetic field lines are dense around poles therefore the pole has strongest magnetic field.
• The magnetic field lines do not intersect, for if they did, the direction of the magnetic field would not be unique at the point of intersection.

## Direction of magnetic field lines

The direction of magnetic field lines is from North pole to South pole outside the magnet. while field lines are directed from south pole to north pole inside the magnet.

## Bar magnet as an equivalent solenoid

A solenoid is a type of electromagnet which produces magnetic field when current flowing through it. It is a coil of wire with a length greater than it’s diameter.

Magnetic filed due to solenoid is,

## Difference between bar magnet and solenoid

• The bar magnet has a permanent magnet whereas a solenoid has when an electric current is passes through.
• When a bar magnet is cut into two halves, both the pieces act as a magnet with the same magnetic properties whereas when a solenoid is cut into two halves, they will have weaker fields.
• The poles of the bar magnet are fixed unlike solenoid where the poles can be changed
• The strength of the magnetic field of a bar magnet is fixed whereas the magnetic field strength of a solenoid depends on the electric current passes through it.

## Why does a compass needle get deflected when brought near a bar magnet ?

A compass is made by using a magnetized piece of iron therefore it acts as a small magnet. When the compass brought near the bar magnet then  the field lines of compass interact with that of the bar magnet. Therefore the compass needle gets deflected near the bar magnet.