What is Bar Magnet?

In physics, magnetism is a phenomenon exhibited by a magnetic substance that exerts either attractive or repulsive force. A bar magnet is either a rectangular or round and hollow fit comprised of ferromagnetic materials and has a fanciful attractive field surrounded by it.

Properties of Bar Magnet

The following points are the fundamental properties of bar magnet:

• Every bar magnet comprises of two poles the north and south pole. The point when a magnet is suspended unreservedly, the actual magnet adjusts toward the magnetic directions of the earth. (i.e.) north pole (N) of the magnet changes towards the northern tip of the earth same as the south pole (S) of the magnet changes towards the southern end of the earth.
• Like poles (N-N) (S-S) repel each other and unlike poles (N-S) (S-N) attracts one another.
• If we cut a magnet into two halves, the broken pieces of the magnet will have N-S poles, irrespective of how many times we break it.
• The force of the magnet has maximum strength at the pole end.
• A mariner compass is used to identify the direction of the ship. If the magnet is tied with a rope and freely suspended, it won’t stop until it aligned in their respective direction (N-S).

Types of Bar magnet

Typically, there are two kinds of magnets: artificial and natural magnets. Natural magnets are found on the earth naturally and possess a weak magnetic field whereas artificial magnets are man-made and have a very strong magnetic field. We can customize as per our desire. When we shape it in the form of a bar, it is named a bar magnet.

Bar magnets can be classified into two types.

• Permanent magnets.
• Temporary magnets.

Permanent magnets

Permanent magnets are those which hold the attractive field for quite a while even without a magnetic field. It has high remanence induction and coercivity. They are extremely powerful and very brittle and furthermore, they are extremely laborious to demagnetize. Hard magnets are otherwise known as permanent or strong magnets. Examples of hard magnets are neodymium bar magnets, alnico, ferrites.

Of these, neodymium magnets are made up of rare earth materials like neodymium, boron, and iron. Alnico is synthesized by metals such as aluminum, cobalt, and nickel. It is found in applications in different fields like automobiles, computer peripherals, clocks, and fractional HP motors.

Temporary magnets

Temporary magnets are those which remain magnetized only for a minimum interval of time only if the magnetic field is present. It has high retentivity and low coercivity. Their attractive properties are nullified when the magnetic field is eliminated. They are prepared from soft metals. An electromagnet is the best model for temporary magnets. Electromagnets are those magnets created from the iron core of the solenoid. When we pass a current through the wire, it creates a magnetic field from the center of the coil. These are widely used in electrical and electromechanical instruments such as buzzers, transformers, bells, etc.

Magnetic flux density of a bar magnet

The magnetic flux density of a bar magnet is defined as the number of lines of force passing through a unit area of a material. The SI unit is Tesla (T).

The formula is,

$B=\frac{\varphi }{A}$

Where,

ϕ= magnetic flux.

A=area.

Pole strength

It is defined as the ratio of the magnetic moment to its effective length. The unit is an ampere-meter.

$m=\frac{M}{2l}$

Where,

m= pole strength.

M= magnetic moment.

l= effective length.

Classification of magnetic materials

Generally, all the substances are building by atoms and each atom contains electrons, protons, and neutrons.  The electrons create a magnetic field when they start to move in space. The transfer of charge creates a magnetic field and the field strength is known as a magnetic moment. Even though all materials are a buildup of atoms, some materials are non-magnetic too. The orientation of the spin and the electron orbit determine the magnetic field direction.

When the field is given externally, there is a change in the field related to the particle. The electron always occurs in pairs and the spin direction is opposite. Some materials with unpaired electrons react to the magnetic flux when we give from outside are,  

• Diamagnetic materials.
• Paramagnetic materials.
• Ferromagnetic materials.

Diamagnetic materials

The materials which are diamagnetic are repulsed by the magnetic field. Because they are non-magnetic materials. The atom in the field does not have any unpaired electrons; therefore it does not attract any outside magnetic flux. It is easily separated from other materials since it has a low repulsive force and travel from a strong field to a weak field. Examples of diamagnetism are quartz, water, and calcite.

Paramagnetic materials

Paramagnetic materials have unpaired electrons and it is easily attracted towards the external magnetic field. It has a strong magnetic moment. By applying an external magnetic flux, the atomic dipoles arrange themselves towards the electric field and it has a weak attraction to the magnets. The materials which have this property are called paramagnetic materials and this phenomenon is known as paramagnetism. Examples of paramagnetic materials are aluminum, gold, and copper.

Ferromagnetic materials

The materials which have a strong attraction towards the magnetic field while applying from outside and are named ferromagnetic materials. The materials have a high number of oddly paired electrons in the atoms comparing to paramagnetic materials. When they are attracted to an external magnetic field, they behave like a little bar magnet and they are eliminated from the materials by using low-intensity magnetic separators. The materials which exhibit this property are called ferromagnetic materials and this prodigy is known as ferromagnetism.

Generally, permanent bar magnets are made of ferromagnetic metals such as iron, nickel, and cobalt, etc.

Bar magnet and a solenoid

A solenoid is a loop of circular wire, which has N number of turns. When the electric field is applied, it starts to behave like a bar magnet. One end of the solenoid behaves like a north pole and another end is the south pole. Here, the lines of force start from one end and end at another end. The north and south ends are identified with the bar magnet. If the magnet gets attracted towards the solenoid it is south and if it is repelled, it is north.

Uses of a bar magnet

• In physics, bar magnet has various kind of applications .It is used as a stirrer in lab for experiments related to magnetism.
• The fundamental function of this magnet is to pick up small screws ,metal shavings, nails etc.
• Electrical instruments like mobile phones, telephones, radio and television uses bar magnet.
• In medical field, it is utilized for medical procedures.
• In waste management, by using bar magnet we can collect the magnetic waste from pile of mixed wastes.
• Bar magnet is used as a compass to find the directions.

Formulas

The formula of magnetic flux density is,

$B=\frac{\varphi }{A }$             

The formula of pole strength is,

$m=\frac{M}{2l}$

Context and applications

The bar magnet topic is one of the fundamental topics in all physics courses for all students and for undergraduates and postgraduates, especially for bachelors and masters in science (physics), and bachelors of technology (electrical engineering).

Practice Problems

Question 1: What is the SI unit of pole strength?

$$\begin{gathered} a.A{m}^2 \\ b.Am \\ c. A{m}^{-1} \\ d.A{m}^{-2} \end{gathered}$$

Answer: The correct option is b.

Explanation: The SI unit of pole strength is Am (ampere meter).

Question 2: Which of these is the property of bar magnets?

1. Like poles repel each other and unlike poles attract each other.
2. Magnetic force is weak at the poles
3. The magnetic poles can be separated
4. None of these

Answer: The correct option is a.

Explanation: In magnets, there are two poles, the north, and south poles. When we bring two north poles together it repels each other whereas if we bring a north pole and south pole together it attracts each other.

Question 3: What is a bar magnet?

1. Artificial magnet
2. Does not have any shape
3. A rectangular size with permanent magnetic properties
4. None of these

Answer: The correct option is c.

Explanation: A bar magnet is defined as a rectangular size of an object that has permanent magnetic properties. It has two poles namely the north and south poles.

Question 4: Which of the following is not attracted by magnets?

1. Iron
2. Copper
3. Nickel
4. Cobalt

Answer: The correct option is b.

Explanation: A magnet has both attractive and repulsive properties. It attracts pieces of iron, cobalt, and nickel, etc. But it does not attract copper.

Question 5: The direction of the magnetic lines in a bar magnet is from________.

1. Nort pole to south pole
2. South pole to north pole
3. South pole to east pole
4. East pole to west pole.

Answer: The correct option is a.

Explanation: The magnetic lines of force always originates from the north pole and ends in the south pole.