Question

Derive an expression for the force between long straight conductors carrying current and hence define one ampere

Answer 1

When current is passed through a straight conductor in a magnetic field it produces a force that can be found by applying Fleming's left hand rule i.e.

Stretch the thumb, forefinger and central finger of your left hand such that they are mutually perpendicular. The first finger points in the direction of magnetic field and the second in the direction of current then the thumb will point in the direction of motion i.e. the force acting on the conductor.

For one ampere

It is constituted by the flow of one coulomb of charge per second i.e. 1A= 1C/1 sec