## The Magnetic Field Due to a Long Straight Wire

The magnetic field around a long straight wire carrying a current is shown below.

The sense of the magnetic field – clockwise or anticlockwise – is given by the right hand grip rule. If the right hand grips the wire with the thumb pointing in the direction of the current, the fingers will curl in the direction of the magnetic field.

The magnetic field forms concentric circles around the wire, decreasing in strength with increasing distance from the wire. Experimentally it is found that the field is

1. Proportional to the current I in the wire.

2. Inversely proportional to the radial distance r from the wire.

3. Dependent on the medium surrounding the wire. Every material has a quantity called the permeability,The permeability of a vacuum is The permeability of air is very nearly equal to this and is often taken to be equal to it.

We can write