It is the technique of the properties and relationship between magnetism and electric current. When electric current passes through the coil which is made of magnetic material is turned to be magnetized and becomes a magnet. This phenomenon is called electromagnetism.
The magnetic field around a straight current-carrying conductor is composed of circular lines of force or field and their plane perpendicular in the direction of the current-carrying conductor and their centers in the position at the center of the current-carrying conductor.
Magnetic Field around the Solenoid:
When we wind many turns of an insulated wire around a cylinder, the resulting coil is called a solenoid. Each turns line in a plane perpendicular to the axis of the cylinder. To obtain a magnetic field pattern due to a current flowing through a solenoid, we fix it on cardboard with its axis lying on the board. Iron filings arrange themselves along the lines of force when a large current is sent through the solenoid. The field pattern is shown below figure:
One end of the solenoid behaves like an ‘N’ pole and the other ‘S’ pole polarities are reserved when we change the direction of the flow of current. We may consider a solenoid as equivalent to a large number of circular turns of wire placed side by side. Each turn develops opposite polarities at its two faces but the neighboring turns neutralize each other polarities except at the two extreme turns where the polarities are exhibited. For long solenoid, the inside field is uniform and the intensity is given by,
H = 4πni
Where ‘n’ = number of turns per unit length of the solenoid
i = current in ampere.