## What is Magnetic Field Intensity?

*The force of attraction or repulsion experienced by an unit north pole strength placed in the magnetic field of a magnet is called the magnetic field intensity.*

__Magnetic Field__

*Magnetic field is defined as the space around a magnet in which its influence of magnetic force of attraction or repulsion is experienced by small magnets.*

Magnetic field intensity is also known as ** strength of a magnetic field**. It is denoted by ( B ) .

If, ( q_m ) is the *pole strength* of a magnet then by *Coulomb’s law of magnetic force*, the force experienced by the unit North pole will be –

F = \frac {\mu_0}{4 \pi} \left ( \frac {q_{m} \times 1}{r^2} \right )

= \left ( \frac {\mu_0}{4 \pi r^2} \right ) q_{m}

Since, at a point at distance ( r ) in the field, the term \left ( \frac {\mu_0}{4 \pi r^2} \right ) becomes a constant. Hence, it can be replaced by another term ( B ) which is called *intensity or strength of magnetic field**.*

Therefore, \quad F = B \times q_{m}

Or, \quad B = \left ( \frac {F}{q_{m}} \right )

= \frac {\text {Magnetic force on unit North pole}}{\text {Pole strength}}

*Thus, strength or intensity of magnetic field at a point is defined as the ratio of force acting on an imaginary unit north pole placed at that point to the pole strength of the magnet.*

__Field Lines__

*Field lines are imaginary curved lines representing the magnetic field of a magnet. *

The tangent at any point of field lines gives the direction of magnetic field at that point.

Field of a magnet may be of two types –

- Uniform magnetic field.
- Non-uniform magnetic field.

__Uniform Magnetic Field__

*If magnetic field has same strength and same direction at all points in the its region, then it is called a uniform magnetic field.*

Uniform magnetic field lines are represented by equidistant set of parallel field lines. Figures (A), (B) and (C) represent uniform magnetic field lines in different directions.

- Figure (A) represents uniform magnetic field lines which are parallel to the plane of paper.
- Figure (B) represents uniform magnetic field lines which are perpendicular to the plane of figure. These are in inward direction i.e., penetrating into the plane of figure.
- Figure (C) represents uniform magnetic field lines which are perpendicular to the plane of figure. These are in outward direction i.e., emerging out from under the plane of figure.

__Non-uniform Magnetic Field__

*If magnetic field has different strength at different points in the field region, it is called a non-uniform magnetic field as shown in figure (D). *

In a non-uniform magnetic field, the magnitude and direction of magnetic field vary at different points in the field region. It is represented by converging, diverging or unequally spaced field lines.

__Properties of Field Lines__

Field lines of a magnet have the following properties :

- Field lines are continuous and closed curves travelling from North pole to South pole at outside the magnet body and from South pole to North pole inside the magnet body.
- The tangent at any point on the field line gives the direction of intensity of field ( B ) at that point.
- Two field lines do not intersect or cross to each other.
- Widely spaced field lines represent weak field and closely spaced field lines represent strong field.
- Magnetic field lines are imaginary but they represent full aspect of a magnetic field which is a real phenomenon.

__Difference between Electric & Magnetic Field Lines__

Electric Field Lines | Magnetic Field Lines |

These are generally discontinuous. | These are continuous and closed curves. |

These imaginary lines not exist within the charged conductor. | These imaginary lines exist within the body of magnet. |

Electric field lines do not exist within the charged conductor, the electric field inside the charged conductor is zero. | Magnetic field lines exist inside the material and magnetic field is never zero inside a magnet body. |

__Magnetic Flux__

*Magnetic flux is defined as the number of field lines passing through a given surface. It is denoted by ( \phi ) .*

SI unit of magnetic flux is ** weber** ( Wb ) .

__Magnetic Flux density__

*The number of field lines passing normally through unit area of a surface is called as magnetic flux density. It is denoted by ( B ) .*

If ( \phi ) is the number of flux passing normally through a surface of area ( A ) , then magnetic flux density is given by –

B = \left ( \frac {\phi}{A} \right )

Magnetic flux density is a *vector quantity*.

In SI system its unit is ** tesla** ( T ) \ \text {or ( Wb-m}^2 )

In CGS system its unit is ** gauss** ( G )

1 G = 10^{-4} T

See numerical problems based on this article.