Magnetic effect of electric current
In 1820, Hans Christian Oersted , a Danish scientist, discovered the connection between electricity and magnetism.
From various observations, Oersted concluded that current or moving charges in a conductor produced a magnetic field around it.
To represent the magnetic field produced by a straight current carrying conductor on paper, we adopt the convention for magnetic field emerging out of the plane of the paper, which is denoted by a dot.
If the magnetic field is going into the plane of the paper, it is denoted by a cross.
The fields due to all the charges should add vectorially. This is called the Principle of Superposition.

Magnetic field on the axis of a circular current loop
Lsn 3: Motion of a charged particle in an external magnetic field
F =B qv
When external force acting on a body is perpendicular to the motion of the body, the work done on the body is said to be zero.
The radius of the circular path of a charged particle moving in an external uniform magnetic field is directly proportional to the momentum of the charged particle.