THE MAGNETIC FIELD

We know that a magnet exerts a force on another magnet.similarly,we sea that a current carrying wire exerts a force on a compass needle.We describe this type of interaction by introducing the concept of magnetic field. The region of space around a magnet or a current,where the magnetic influence of the source magnet or source current is felt by a test magnet is called magnetic field.
The magnetic field is represented by magnetic lines of force.Typical magnetic fields are illustrated by such line in the figure below.
    
The magnetic field,like the electric field,is a vector field.It is specified at each of its points by a vector B called the magnetic induction.The complete specification of the magnitude as well as the direction of B is given in the following section. However, the direction of B due to a current in a straight wire is easily found by the right-hand rule.This rule states that if the current carrying wire is grasped by the right-hand rule with the thumb in the direction of the current

.the figures encircle the wire in the direction of the field.This figure illustrates this rule for a straight current-carrying wire.In this case, the lines of induction are circles concentric with the wire. Th magnetic induction fields of a current in a closely wound circular coil and in a long narrow closely wound solenoid are also found by the right-hand rule and are shown in the figure.