6. Magnetic Lines and Their Characteristics
The space surrounding a magnet or magnetic configuration in which its effects are perceptible is called the magnetic field of the given magnet or magnetic configuration.
In order to visualize a magnetic field graphically, Michael Faraday introduced the concept of lines.
According to him a line is an imaginary curve the tangent to which at a point gives the direction of the field at that point.
Regarding magnetic field it is worth noting that :
(i) Outside a magnet, field are from north to to south pole while inside from south to north, i.e., magnetic lines are closed curves i.e., they appear to converge or diverge at poles.
(ii) The number of magnetic lines of field originating or terminating on a pole is proportional to its strength. μ0 lines are assumed to be associated with a unit pole. so if a body encloses a pole of strength m, total lines linked with the body (called magnetic flux) will be μ0(m).
(iii) Magnetic lines of field can never intersect each other because if they intersect at a point, intensity at that point will have two directions which is absurd.
(iv) Magnetic lines of field have a tendency to contract longitudinally like a stretched elastic string (producing attraction between opposite poles) and repel each other laterally (resulting in repulsion between similar poles)
(v) Number of lines of field per unit area, normal to the area at a point, represents the magnitude of field at that point. so crowded lines represent a strong field while distant lines represent weak field. Further, if the lines of force are equidistant and straight the field is uniform otherwise not
(vi) In a region of space where there is no magnetic field, there will be no lines of field. This is why, at a neutral point (where resultant field is zero) there cannot be any line of field.
(vii) Magnetic lines of field originate from or enter in the surface of a magnetic material at any angle.
(viii) Magnetic lines of field exist inside every magnetised material
(ix) As mono-poles do not exist, the total magnetic flux linked with a closed surface is always zero, i.e.,
This law is called Gauss's law for magnetism.
Magnetic field line due to some important structure
1. Straight current carrying wire
2. Circular coil