[Physics Class Notes] on Magnetic Field Pdf for Exam

The magnetic field is a region or space where there is an influence of a magnet. There are different types of magnets. Bar magnets, rod magnets, horseshoe magnet, ring magnet etc. All types of magnets have two poles; North Pole which is indicated by ‘N’ and South Pole which is indicated by ‘S’. Irrespective of shape every magnet has a field around it. For a better understanding of the magnetic field, place an iron nail in a particular distance from the magnet. Suddenly it gets attracted towards the magnet. If the iron nail is far away from the magnet, it will not have any influence. The area where the power of the magnet exists is known as magnetic field or B field.

The magnetic field can be illustrated in two different ways; vector field and magnetic field lines.

Vector Field

Vector field is the mathematical description of the magnetic field. It is considered that the magnetic field has both magnitude and direction. The vector field can be drawn as a set of vectors drawn on a grid. The direction of each vector points in the direction of the compass. The length of the vector depends upon the strength of magnetic force. 

Magnetic Field Lines

Magnetic field lines are imaginary lines around the magnet. The magnitude of a field is indicated by its line’s density. Near the South and North Pole of a magnet, the magnetic field is stronger and will get weaker when it moves away from the poles. This concept can be clarified by doing a simple experiment. Fix a sheet of white paper on a table and place a bar magnet at the center. Sprinkle some iron filings around the magnet. Gently tap the table. It can be seen that the iron filings align themselves in a specific pattern which represents the field of that magnet. If these patterns are observed clearly, it can be seen that iron filings are accumulated near the poles, whereas concentration is less in the region away from the poles.

Magnetic Field Lines have various Properties:

1. The magnetic field lines never intersect each other.

2. It takes the least resistant path between the opposite magnetic poles. Path of magnetic lines of force of a bar magnet is closed loops from one pole to the other.

3. The length of magnetic field lines will be the same.

4. As the field lines move from higher permeability region to the lower permeability region, their density will decrease.

5. Within a material magnetic field, lines flow from south pole to the north pole and in the air, their direction of flow will be from north pole to south pole.

6. Density of the magnetic field depends upon the distance from the pole. As the distance from the pole increases, their density decreases.

7. The magnetic field is a vector quantity because it has both magnitude and direction.

How to draw Magnetic Field Lines?

Magnetic field lines can be drawn by using a compass, bar magnet and a chart paper. First, fix the paper on a drawing board. Place the bar magnet at the center and mark the position with a pencil. Keep the compass near any one pole of the magnet. Make sure that there is no other magnetic material nearby. It can be seen that the compass arrow is pointing in some directions. Mark a dot in that direction. Move the compass from that position and place it on the dot in such a way that the base of the arrow is at the dot. Mark a new dot in the direction where the arrow of the compass is pointing now.

Until the compass reaches the opposite pole of the magnet, repeat this procedure. Join the dots. Again come back to the previous position and repeat the same steps by starting from a new spot. After drawing a number of lines, it can be seen that the lines are forming a closed loop and this seems to start from one pole of the magnet and ends in another pole. This is the method to draw magnetic field lines. If these lines are compared with the alignment of iron filings, the similarity of the patterns can be noticed. Depending upon the type of magnets, magnetic field lines also will vary.

How is the Magnetic Field produced?

The magnetic field is not only produced by the magnet but also can be produced by a moving charge or electric currents. We all know that matter is made up of tiny particles called atoms. The nucleus of an atom consists of protons and neutrons, with electrons revolving around it. Spinning and orbiting of protons and neutrons or nucleus of an atom produce the magnetic field. The direction of the magnetic field is decided by the direction of orbit and spin. The magnetic field is mathematically represented by the symbol ‘B’. Its unit is Tesla (T). 

Earth’s Magnetic Field

Evidence of earth’s magnetic field was first given by Sir William Gilbert in the year 1600. Based on some experiments he found that earth shows some magnetic properties and it has a magnetic field. If a magnet is suspended freely from a thread and is allowed to rotate in a horizontal plane, it will align automatically in the north-south direction and come into rest. Alignment of the magnet will be in such a way that the north pole of the magnet is attracted towards the geographical south and south pole of the magnet is attracted towards the geographical north. 

The second evidence is that there are some neutral points in the magnetic field lines. Magnetic field due to the magnet, which is used to draw field lines, is canceled by the earth’s magnetic field. Without earth’s magnetic field these neutral points cannot be seen. Third evidence is that a soft iron becomes a magnet if it is buried under the earth in the north-south direction. 

 

The Hypothesis for the source of Earth’s Magnetic Field

1. The Earth’s core is in the form of hot molten liquid and it contains ions. These ions are circulating in the form of current loops inside the liquid and as a result, the magnetic field is produced.

2. The Earth is rotating about its axis and matter on the earth is made of charged particles. These charged particles also rotate about earth’s axis in the form of current loops and are responsible for the production of the magnetic field.

3. The Earth’s outer layer consists of ionized gasses. When the earth is rotating, movement of ions produces electric current and a magnetic field is produced due to this.

Characteristics of the Earth’s Magnetic Field

1. Earth’s magnetic field is uniform.

2. The magnetic field strength at the surface of the earth is approximately 10-4 Tesla

3. The magnetic field of the earth is extended up to a height of 5 times the radius of the earth.

 Applications of Magnets in Real Life

1. Magnets are used in electric bells.

2. They are used in the construction of generators and electrical motors.

3. Magnets are used to find the geographical directions.

4. Magnets play an important role in the separation of magnetic and non-magnetic materials from the scrap.

5. In the medical field also, magnets are widely used in treating pain of different body parts.

Operating the Magnetic Field

Each magnet has a field surrounding it, regardless of its shape. Put an iron nail at a certain distance from the magnet in order to obtain a better understanding of the magnetic field. At that moment, the nail becomes drawn to the magnet. The iron nail will not be able to exert any influence on the magnet if it is far away from it. The magnetic field that exists around magnets is called the B field.

The magnetic field is created by moving magnets or electric charges. Magnetism operates inside the magnetic field surrounding magnetic materials or moving charges. A magnetic field line represents a magnetic field. A visual tool that provides information about the direction and strength of the magnetic field.

The magnetic field lines can be drawn using a compass needle. It is recommended that the compass needle be placed near the magnet on a piece of paper. Check the direction of the compass needle and mark it. Move the compass needle to various positions and mark the directions. By joining the points, you can see the magnetic field lines.