A current-carrying circular wire(or loop) behaves like a thin disc magnet whose one face is a north pole and the other is a south pole.The polarity (north or south)of two faces of a current-carrying circular coil(or loop)can be determined using the clock face rule.
According to the clock face rule,look at one face of a circular wire (or coil) through which a current is passing:
(i) if the current around the face of the circular wire (or coil) flows in the
clockwise direction,then that face of the circular wire (or coil) will be the south
pole(S-pole)
(ii) if the current around the face of circular wire (or coil) flows in the Anticlockwise
direction,then that face of circular wire (or coil) will be a north pole(N-pole)
Suppose the direction of current in the front face of a circular wise is clockwise. In that case, the direction of current in the back face of this circular wire will be anticlockwise(and vice versa). This means that the front face of this current-carrying circular wire will be a south pole, but its back face will be a north pole.
Finding poles using Biot-Savart law:
Although Biot-Savart law gives the magnetic field due to current in a straight conductor or a loop, we would obviously like to know where the magnetic field's north pole or south pole is.
Consider a bar magnet. Its lines of force start from the south pole and go to the north pole inside the magnet and continuously from the north pole to the south pole outside the magnet. Let us now consider a current loop. From Biot-Savart law, we see that the magnetic lines of force are closed loops around the conductor.
At the centre of the coil, they are practically straight lines parallel to each other. Thus, the lines of force seem to enter from the observer side of the coil and move out from the other side. This is similar to when a bar magnet is placed with its south pole facing the observer and north pole away from the observer.
The side where the observer sees the current circulating clockwise behaves as the south pole. The other side, where the current is circulating anticlockwise (facing the coil), behaves as the north pole.
Comparison of magnetic field lines of
force:
Comparison of magnetic field lines of force for a bar magnet and a
circular current carrying conductor.