Like electric field emerging from electric monopole i.e +p, -e or +e, magnetic field also require a magnetic monopole about which there are many theories but we do not have single magnetic monopole in our hands. However magnetic field can be defined in terms of magnetic force acting on a single charge.
Moving charge i.e current always produces magnetic field or moving charge is always surrounded by magnetic field. Its the intrinsic property of charge. For example a wire carrying current produces magnetic field around itself whose direction is given by right hand rule. Point your thumb of right hand in a direction of current and curl all of your other fingers around the wire. The magnetic field in this case will either be clockwise or counter clockwise.
Alright, now come towards the description of magnetic field. When a charge moves in a magnetic field, the magnetic force on it is always proportional to the vsinθ, B and q. Where θ is the angle between velocity of the charge and the external magnetic field B and q is the charge.
The force is
F=qvBsinθ
or F=q(vXB)
This equation shows that if v and B are perpendicular to each other, the force is maximum and if they are parallel to each other, the force is minimum.
If q is positive, the force is directed in the direction vXB and if q is negative, the force on it due to the external magnetic field is directed opposite to the direction of vXB.
By right hand we can find the direction of the force. Curl your fingers from v to B, the thumb gives the direction of the force. In case of negative charge the direction will be opposite to the direction of the thumb.
When v and B are perpendicular, θ=90 degrees
and F= qvB
then
B=F/qv
the SI unit of magnetic field is Newton-sec/columb-m which is given the name of Tesla.
from last equation magnetic field can be defined as
The magnetic field is one tesla if charge of one columb moving with velocity 1 m/s perpendicular to B, experiences the force of one newton in that magnetic field.
The other unit is gauss designated by G or Gs.
1 Tesla = 10000 Gs
