The magnetic field is defined as a region or space in which there is an influence of a magnet. There are various types of magnets such as bar magnets, rod magnets, horseshoe magnet, ring magnet etc. Magnets have two poles. North pole is indicated by ‘N’ and South pole is indicated by ‘S’. In order to understand it better, let us place an iron nail at a particular distance from the magnet. We will see that the nails will get attracted towards the magnet. If the iron nails are at a far-off distance, then it will not get attracted. The area where the magnet pulls the iron nails is known as the magnetic field or B field.
The magnetic field can be explained in two different ways: vector field and magnetic field lines.
Vector field can be described as the mathematical description of the magnetic field. The magnitude field has both magnitude and direction. The vector field can be shown by drawing a set of vectors drawn on a grid. The direction of each vector points towards the direction of the compass. The length of the vector is dependent on the strength of magnetic force.
Magnetic field lines
Magnetic field lines can be defined as imaginary lines around the magnet. The magnitude of a field is shown with its line’s density. The magnetic field is stronger near to south and north pole of a magnet and becomes weaker when it goes away from the poles. This concept can be explained by doing a simple experiment. A sheet of white paper is placed on a table and a bar magnet is placed at the center. Now some iron filings are sprinkled around the magnet. Now the table is tapped gently. It will be seen that the iron filings will arrange themselves in a specific pattern which shows the magnetic field. The iron filings will collect near the poles, whereas the accumulation will be less in the region away from the poles.
Properties of magnetic field
- The magnetic field lines do not intersect with each other.
- It is less resistant to the path between the opposite magnetic poles. The path of magnetic lines of force of a bar magnet are closed loops from one pole to another.
- In a material magnetic field, the direction of the flow of lines is from south pole to the north pole and in the air, the flow is from north pole to south pole.
- The density of the magnetic field is dependent on the distance from the pole. When the distance from the pole increases, then the density decreases.
- The magnetic field has both magnitude and direction. That is why, it is a vector quantity.
Earth’s magnetic field
In the year 1600, Sir Wiliam Gilbert propagated the earth’s magnetic field. He made some experiments and found that the earth has some magnetic properties and a magnetic field. When a magnet is allowed to suspend freely from a thread and rotate in a horizontal plane, then it will arrange automatically in the direction of north-south and will eventually come to rest. The magnets will be aligned in such a way that north pole of the magnet is towards the geographical south and south pole of the magnet will be towards the geographical north. The second proof is that the magnetic field has some neutral points. The magnetic field used to draw the field lines is cancelled by the earth’s magnetic field. The neutral points cannot be seen without earth’s magnetic field. There is third evidence that a soft iron will turn into magnet if it kept under the earth’s surface in the north-south direction.
The hypothesis for the sources of earth’s magnetic field
- The core of the earth is in the from of hot molten liquid and it has ions. The ions keep circulating in the form of current loops inside the liquid thereby causing the magnetic field.
- The earth rotates on its axis. The earth’s matter is made of charged particles. The charged particles also rotate on the axis of the earth in the form of current loops thereby causing the magnetic field.
- The outer layer of the earth is made up of ionized gases. When the earth rotates, the movement of ions causes electric current thereby producing a magnetic field.
Characteristics of the earth’s magnetic field
- The magnetic field of the earth is uniform.
- The strength of the magnetic field on the surface of the earth is
approximately 10-4 Tesla.
- The magnetic field of the earth is up to a height of 5 times the radius of the earth.