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Magnetic Field Due to a Current Through a Circular Loop

Not long after Oersted's discovery of the magnetic effect of a current in a straight wire, Ampere found that the magnetic effect could be increased if the wire is bent into the form a circular loop or coil. Each small segment of the wire is surrounded by circular lines of force described above .The right-hand rule tells us that all these lines go through the coil in one sense, entering at one face and leaving from the other, thus enhancing the magnetic effect. 


The magnetic effect is further increased by using a flat circular coil consisting of a number of identical loops as shown in Figure. The current passes through each loop in the same sense as shown in the diagram for the case in which the current flows in the clockwise sense. The diagram also shows the lines of force of the magnetic field of the current carrying coil. A comparison with the lines of force of a bar magnet shown in Figure indicates that the coil behaves as a bar magnet. The face of the coil from which the lines of force appear to emerge is the North pole of the coil and the face into which they enter must be South pole as shown.

The Clock Rule

The polarity of any face of the coil can be determined using the following so-called clock rule. Looking at the face of the coil, if the current round that face is in the anticlockwise sense, that face has North polarity. On the other hand, looking at the face of the coil, if the current is in the clockwise sense, then that face has South polarity.
  • Direction of magnetic field
  • When curled fingers represent the direction of the current in circular wire then thumb represents the direction of the straight magnetic lines of force.

Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right hand rule to find out the direction of the magnetic field inside and outside the loop.

Ans: The direction of the magnetic field is perpendicularly downward on the plane of the table inside the loop and outwardly outside the loop.



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