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Bar magnets are rectangular and narrow objects that are made up of steel/iron/ferromagnetic materials which have the ability to generate a magnetic field.

All bar magnets have two poles: the north pole and the south pole which are painted red and blue for identification purposes respectively.

However, in science laboratories, they are painted red completely and the north pole is indicated by a white dot.

Properties of bar magnets

• Bar magnets are made up of ALNICO which is a mix of aluminum, cobalt, and nickel due to which they are usually dark grey or black in color

• They have a north pole and a south pole at the opposite ends.

• They are permanent magnets, they keep their magnetism even when removed from the magnetic field.

• Like poles repel and unlike attract [ north-north repels, south-south repels, north-south attracts]

• The field lines are most concentrated at the poles; therefore, magnetism is strongest at the poles and weakest at the center.

Properties of the magnetic field around a bar magnet

• The magnetic lines are always closed and continuous on both sides of the bar magnet.

• The lines move from north pole to south pole.

They form loops.

• The magnetic lines do not intersect.

• These magnetic field lines spread out more as they move away from the magnetic poles.

• Inside the bar magnet, the magnetic field is from south pole to north pole In the air they move from north pole to south pole.

Bar Magnet Magnetic Field Lines

Magnetic Field Lines of Attraction Between Opposite Poles

Magnetic Field Lines of Repulsion Between Similar Poles

Uses of bar magnet

• By rubbing the magnet on objects like paper clips, safety pins we can magnetize them.

• Bar magnet can function as a compass: When it is attached to a string and suspended, the bar magnet aligns itself towards the north pole of the earth.

• They are used in electric bells.

• They are used in a wide variety of medical procedures

• They are used in magnetic laboratory magnetic experiments as stirring rods

• They are also used in a wide variety of devices like telephones and radios.

Pole strength: It is measured by moving a bar magnet along an ‘infinite’ wire and then by measuring the amount of current produced.

The formula used for calculating pole strength is,

P = strength of the magnetic pole

W = work done in moving the magnet around the wire

I = electric current in the wire

Law of magnetism: Opposite poles attract each other and like poles repel each other.

If a magnet is cut into two, they will become separate individual magnets with new north poles and south poles respectively.

To understand more about the magnetic field around a bar magnet, let us look into a very simple experiment which uses iron filings.

Bar magnets and iron filing experiment

1. Take a sheet of paper and fix it on a plane horizontal surface with the help of an adhesive so that it doesn't move around.

2. Now, at the center of the paper place the bar magnet.

3. Then sprinkle the iron filings around the magnet in a uniform manner.

4. Gently tap the paper, so that the fillings move closer to the bar magnet.

5. Now, observe the pattern in which the iron filings have arranged themselves.

This represents the magnetic lines of force.

6. Now, use a compass in order to find the direction of magnetic lines.

7. If needed, you can even redo the experiment with more than one magnet placed in various different positions.

This will help in finding the changes in the magnetic field caused due to the interaction of one magnet with another magnet.

Iron Filings showing Magnetic Field Lines around a Bar Magnet

Observation

From the above picture, one can understand that the iron filings align themselves according to the lines of magnetic force.

The lines indicate entry and exit of the magnetic field.

The magnetic field leaves from one of the magnets and enters again from another pole of the magnet across the length of the magnet.

You can also notice that despite the fact, there are many poles all along the length of the magnet;

most of the iron filings are concentrated towards the ends of the magnet.

In reality, the magnetic lines of force exist in three dimensions and are not two dimensional

How are bar magnets different from electromagnets?

Though electromagnets and bar magnets have the same magnetic fields, and in a few cases, they might even look similar to one another, there are a few differences between them.

They are:

• Electromagnets are mostly made up of coils of wires, which may not or may surround a ferromagnetic core.

But the majority of the bar magnets are blocks of ferromagnetic materials.

Electromagnets behave like bar magnets only when an electric current is passed through them.

• Electromagnets do not have their own magnetic field and are dependent on an external electric source to produce a magnetic field.

But bar magnets can generate their own magnetic fields.

• Bar magnets can generate a constant magnetic pull and are also permanent magnets.

But electromagnets, have magnetic properties which can be turned off or on depending on the electric supply.

This property of electromagnets makes them ideal for creating simple motors.

• The power of bar magnets is constant and can be determined via the material it is made up of.

But the magnetic force of the electromagnet can change, and it is determined by measuring the amount of electricity flowing through it.

How does earth behave like a bar magnet?

The earth has its own giant magnetic field, which extends hundreds of miles into space and this is what protects the humans, plants, and animals from the harmful radioactive solar winds.

The earth's magnetic field is generally compared to the magnetic field of a bar magnet.

The earth has two poles, namely the north pole and the south pole.

One can find the lines of electromagnetic forces running along the length of earth's poles in a long loopy arc from one pole to another.

It is thought that the magnetic field occurs due to thermal convection of the molten metal present deep inside the earth's surface.

They keep circulating deep and create powerful electric currents.

Magnetic Field of the Earth

AlNiCO and Neodymium Bar Magnet

Neodymium bar magnets and Alnico bar magnets are important topics to talk about.

The primary component of Alnico magnets is cobalt, nickel, iron, and aluminum.

Both of these magnets have a very strong magnetic field and they have the ability to retain their magnetic properties even under very high heat.

Neodymium magnets are made up of a mixture of neodymium, iron, and boron.

Both of these magnets are very powerful magnets but are also very brittle.

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