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Chapter 9 – Magnetic Field and Field Lines

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

Have you ever been fascinated by how bits of iron can stick to really old magnets? The closer you take a magnet to any iron object, the pull you feel gets stronger. This ‘pull’ you feel is the magnetic field. You will learn about magnets, magnetic fields and magnetic field lines in the paragraphs below.

What is a magnetic field?

A magnetic field is an invisible space around a magnet. It is used to describe the distribution of the magnetic force of a magnet. All magnets have a definite region around it in which the forces of attraction and repulsion can be detected. This space is known as the magnetic field.

What are magnetic field lines?

Magnetic field lines are the imaginary lines drawn around a magnet to show its magnetic field distribution. The magnitude of a magnetic field can be determined by the density of these imaginary magnetic field lines. This magnetic field is comparatively much stronger near the poles of the magnet. And, it gets weaker as we move away from the poles.

We will clarify the concept of a magnetic field by performing a simple experiment. Firstly, fix a sheet of white paper on a table and then mark its center. Next, fix a bar magnet at the center mark. Then carefully sprinkle some iron filings around the bar magnet. Gently tap on the table. You will observe that the iron filings are making a symmetrical pattern that is dense near the north and south poles of the magnet. You will also notice that these lines are more or less continuous and the fact that it gets less dense near the sides of the magnet. These lines that are formed represent the magnetic field lines of the bar magnet and the extent of its magnetic field.

A few properties of magnetic field lines

  • Magnetic field lines are imaginary lines around a magnet that represents the magnetic field of the magnet.
  • A magnetic field line is always a closed and continuous curve.
  • A magnetic field line is always directed from the north pole to the south pole outside the magnet and from the south pole to the magnet’s north pole. 
  • The magnetic field lines are densest near the magnet’s poles, where the magnetic field is stronger. The magnetic field lines are far apart near the magnet’s middle, where the magnetic field is weak.
  • The magnetic field lines do not intersect each other because if they do so, these will point to two different directions of the magnetic field at one point, which is not scientifically possible.

A magnetic field around a magnet can be produced by a magnet, a moving charge, or by electric currents. The magnetic field is mathematically represented by the symbol ‘B’. Its unit is Tesla.

We know that all matter is made up of tiny microscopic particles called atoms. The nucleus of an atom consists of positively charged particles called protons and neutral subatomic particles called neutrons. The negatively charged particles, also called electrons, revolve around this nucleus in circular orbits. This uniform directional movement creates a magnetic field. The direction of orbit and spin decides the direction of the magnetic field. This property is more evident in matter than can be magnetised, such as iron, and less in matter such as plastic or other non-metals.

Suppose a piece of magnet is suspended freely from a thread and is allowed to rotate in a horizontal plane. In that case, it will automatically align itself in the geographical north-south direction and then gradually come to rest. The real magnetic north and south poles are, however, different from the geographical north and south poles. The Earth’s uniform magnetic field determines these geographical poles. This magnetic field generated by the Earth’s magnetic properties is uniform in nature and extends upto five times the Earth’s radius itself. 

1. What are magnetic fields and field lines?

A magnetic field is a region around a magnet where the magnet’s magnetic properties can be felt and realized. Magnetic field lines are the imaginary lines drawn to explain the magnetic field’s distribution around a magnet. 

2. What is the difference between the magnetic field and magnetic field lines?

The magnetic field is the space around a magnet where the magnet’s magnetic properties in question can be felt. It is a real phenomenon. However, magnetic field lines are imaginary and represent the magnetic field around a magnet. 

3. How are magnetic field lines drawn?

Magnetic field lines are always drawn from the north to the south poles outside the magnet and from the south pole to the magnet’s north pole.

4. What is the formula for a magnetic field?

The dimensional formula for a magnetic field is given by [M^1 T^-2 I^-1]. Where, M= Mass, I= Current, L= Length, T= Time. The SI unit is Tesla(B). 

5. Which is the stronger- magnetic field or electric field?

An Electric field is a stronger force than a magnetic field in clinical conditions if there are no interfering forces.

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High School Physics

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