Magnetic Field can be defined as a region near a magnet where magnetic effects can be experienced.

When a charged particle is placed near a current-carrying wire, there is no force on the given charge particle. But, when the charge is projected with some velocity near the same current-carrying wire, it experiences some force. This force exerted by the moving charge is defined as Magnetic Force.

This force depends on the following:

1. The magnetic force (F) directly depends on the value of the given charge.
2. It directly depends on the velocity with which the charge is moving.
3. It also depends on the value or magnitude of the magnetic field.

Origin of Magnetic Fields

Research on magnetic fields started in 1269.

• In 1269, there was a French scholar named Petrus Peregrinus de Maricourt. He placed a spherical magnet on the surface and then sprinkled iron filings. He found that iron fillings were placed in a manner where he noticed that the resulting iron fillings (hence, field lines) crossed at the two points of the magnet. He then named these two points “poles”. He stated that: Magnets always have two poles-North and South poles.  

• In 1600, William Gilbert stated that Earth is also a Magnet.

• In the 18th century, magnetic poles were found to  attract and repel each other.

• In 1820, the Danish physicist Oersted (Hans Christian Oersted) noticed the deflection in a magnetic compass needle near a current-carrying wire. This was his most remarkable discovery. 

Oersted Experiment (Relation b/w Electricity and Magnetism)

Oersted took a magnetic compass, an electric circuit with current flowing through the wire. We can also refer to it as electrons moving in the given wire. He then placed a magnetic compass near the current wire.  He investigated the phenomenon and noticed that: 

1. There is deflection in the magnetic needle due to the current flowing through the wire.
2. He also noticed that reversing the direction of current flowing through the wire also changes the direction of orientation of the magnetic needle.
3. After that, he put some iron fillings near the wire and found that the iron filings were arranged in concentric circles.

He concluded that the current or moving charges (electrons) produce magnetic fields in the surrounding region.

Results:

1. A Magnetic field arises when a charge is in motion. This is because moving charge means the flow of current. This flow of current will be in the direction of charge if the charge is positive and will be in the opposite direction if the given charge is negative, like an electron.
2. The magnetic field can be created using a bar magnet itself.

We now know that our Earth is also a magnet. So, let us learn more about Earth’s magnetic field and see the origin of the Earth’s magnetic field.

Origin of Earth’s Magnetic Field

The reasons physicists gave earlier for the origin of the Earth’s magnetic field were: 

Theory 1: In the era of 600 BC, there was an island named Magnesia in Greece. There were some shepherds on that island. They realised that their wooden shoes (with iron nails) were getting stuck on the surface of the Earth. 

Then, it was imagined that there was a long bar magnet deep inside Earth.

But later on, this theory failed as there were no perfect proofs for this. What was the reason behind the failure of this theory?

Contradiction:

The temperature inside Earth is very high. At a very high temperature, no material can show the magnetic property. This is because the temperature T is much higher than Curie’s temperature. Hence, the magnetic properties of the magnet become weak—also, magnetic properties due to a magnetic material can’t exist. 

So, the theory failed as it could not explain the magnetic properties with a very high temperature.

Theory 2: The metallic molten fluids present inside the Earth (like iron Fe, nickel Ni) rotate with the Earth. At high temperatures, atoms of molten iron get more ionisation energy. So, because of this ionisation energy, the electron will get free from the atom and hence, get ionised, and ions are now rotating.

We can say that there is an electric current due to the motion of ions. Hence, because charges are moving, they will generate magnetic fields. 

Some proofs: 

1. Moon has no metallic fluid, and hence, there is no magnetic field.
2. The rotation of Venus is very slow, so the current will be less and hence, a weak magnetic field.
3. The rotation of Jupiter is very slow, so the current will be more and hence, a strong magnetic field.

Note: The origin of Earth’s magnetic field is still an area of research.

Conclusion

The magnetic field is formed around a bar magnet or a current-carrying wire. It does not exert any force on the charge at rest. If the charge moves with some velocity, it will experience a magnetic force. 

We have learned that magnetic fields can be created by a moving charge or a bar magnet. But the general reason for the presence of Earth’s magnetic field is the presence of metallic iron.