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Magnetic Field and Field Lines

We have already introduced the concept of an electric field surrounding a charge. In the same way we speak of the magnetic field surrounding a magnet. We know that a magnet attracts small pieces of iron even when they are lying at a certain distance away from it. Thus, the magnetic force, like the electric force or the gravitational force, acts at a distance. The idea of force acting at a ‘distance' can be easily understood by introducing the concept of a field. We imagine a magnet as giving rise to a magnetic field, which exists, in the whole space surrounding it.

The phenomenon of magnetism is "mediated" by the magnetic field -- i.e., an electric current or magnetic dipole creates a magnetic field, and that field, in turn, imparts magnetic forces on other particles that are in the fields.

All materials are influenced by magnetic field to greater or lesser degree.


Magnetic Field

Space around a magnet in which magnetic force is exerted is called a magnetic field.

Bar Magnet

The space around a magnet in which the force of attraction and repulsion can be detected is called the magnetic field.

The force that one magnet exerts on another can be described as the interaction between one magnet and the magnetic field of the other. As in the case of an electric field, placing a `test object’ at that point and finding the force that the field exerts on it can measure the strength of the magnetic field at any point. The test object is chosen to be unit North Pole. The strength of magnetic field at a point of space surrounding a magnet is defined as the force experienced by a hypothetical unit North Pole placed at that point and the direction of the force gives the direction of the field. Magnetic field is a vector quantity that has both direction and magnitude. The direction of the magnetic field is taken by convention that the field lines emerge from North Pole and merge at South Pole. The strength of the magnetic field can be observed from the degree of closeness of the field lines.


Natural Magnets

Natural magnets called lodestones were found near Magnesia in Greece. They contain magnetite, a natural magnetic material Fe3O4.


Artificial Magnets


Magnetic Field Lines or Magnetic Lines of Force


A convenient method to describe the magnetic field around a magnet is to draw magnetic field lines around it.

Magnetic lines of force are the imaginary lines introduced by Michel Faraday to visualize the magnetic field.

Magnetic field lines were introduced by Michael Faraday (1791-1867) who named them ‘Lines of force’. The lines of force of a field are a very useful concept in physics. Lines of force are a graphical representation of a field. The line of force is the path along which an isolated unit North Pole would move in a magnetic field.

Magnetic lines

  • Place a magnet on a cardboard and gently sprinkle some iron filings uniformly over it.
  • The iron filings are found to arrange themselves in a pattern as shown in the following figure.

  • Why do iron filings arrange themselves in a definite pattern?
  • This is due to the reason that each piece of iron filing becomes a small magnet and experiences a force in a certain direction in the magnetic field due to the magnet.
Properties of Magnetic Field Lines
  1. A magnetic field line is directed from north – pole to south – pole outside the magnet
  2. A magnetic field line is a closed and continuous curve. (we have not shown magnetic field lines inside the magnet where these are directed from south pole to north pole)
  3. The magnetic field lines are crowded near the pole where the magnetic field is strong and are far apart near the middle of the magnet and far from the magnet where the magnetic field is weak
  4. The magnetic field lines never intersect each other because if they do so, there would be two directions of magnetic field at that point which is absurd.
  5. In case the field lines are parallel and equidistant, these represent a uniform magnetic field. The Earth’s magnetic field is uniform in a limited space.

What are magnetic field lines? How is the direction of a magnetic field at a point determined?


Magnetic field lines are the curved paths along which the iron filings arrange themselves in a magnetic field due to force exerted on them by the magnetic field. These lines give us a pictorial representation of the magnetic field. The tangent at any point on the magnetic field line gives the direction of the magnetic field at that point.


Field lines converge

where the magnetic force is strong, and spread out where it is weak. The magnetic force is strongest near the poles where they come together. The behavior of field lines in the Earth's magnetic field is very similar. No two-field lines are found to cross each other. If it crosses, it will mean that at the point of intersection, the compass needle will point towards two directions, which is not possible.


You are given the magnetic field pattern of a magnet. How will you find out from it where the magnetic field is strongest?

The place where the magnetic field lines crowded more is the strongest part.


1) The north pole of one magnet attracts the south pole of other magnet.

2) The north pole of one magnet repels the north pole of other magnet.

You can feel the magnets pushing apart.

  • This is called repulsion
  • Note - south poles also repel each other.

  • You are given a bar magnet and a rectangular bar. How will you check whether the given bar is a magnet or magnetic substance or a nonmagnetic substance?
  • When will the repulsion take place?
  • Note; we conclude that, (magnetic) repulsion identifies the existence of two magnetic fields.

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