Magnetic Field due to Circular Coil Carrying Current
A piece of wire bent in the form of a ring (or coil) is passed through a horizontal cardboard C at two points P and Q at the opposite ends of a diameter of the ring and then some iron fillings are scattered on the cardboard. The ends of the coil are connected to a battery through a rheostat and a key. When a strong electric current is passed through the coil by closing the key and the cardboard is gently tapped we find that the iron filing arrange themselves in a definite pattern representing the magnetic lines of force due to the current carrying coil.
Answer: As the above Middle figure shows; current is flowing anticlockwise through a circular loop. The direction of magnetic field around the conductor can be known by using the right hand thumb rule. As the figure shows, the magnetic field would be towards the plane of the paper when it is inside the loop. On the other hand, the magnetic field would be away from the paper when it is outside the loop. Direction of magnetic field is found by applying the right hand thumb rule to each section of the coil and we find that the concentric lines of force pass through the coil in the same direction. Furthermore that:
(i) The magnetic lines of force are nearly circular near the wire.
(ii) Within the space enclosed by the wire, the lines of force are in the same direction.
(iii) Near the center of the coil, the lines of force are nearly parallel and the magnetic field may be assumed to be practically uniform for a small space around the centre.
(iv) At the centre, the lines of force are along its axis and at right angle to the plane of the coil.
The magnitude of magnetic field B produced by a current-carrying circular wire at its centre is:
(i) Directly proportional to the current I passing through the circular wire and
(ii) Inversely proportional to the radius r of the circular wire.
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