MAGNETIC FIELD DUE TO A CURRENT IN A SOLENOID (wire is wounded in the form of a cylindrical coil)
The magnetic field strength is increased if the number of turns in the coil is increased or the strength of current in the coil is increased. So in order to create a stronger magnetic field force (and consequently, more field flux) with the same amount of electric current, we can wrap the wire into a coil shape ( it looks like a helical spring), where the circling magnetic fields around the wire will join to create a larger field with a definite magnetic (north and south) polarity:
Since the magnetic lines of force through the coil point in the same direction, hence one face of the coil acts as a large area of north polarity because it is sending out magnetic lines of force and the other face acts as a large area of south polarity as magnetic lines of force are entering it. Thus, the coil has a magnetic field similar to a magnetised iron disc of same radius as that of the coil.
As shown in the above LHS figure; the field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field in a given region is uniform.
The magnitude of magnetic field B produced by a current-carrying solenoid at its centre is:
Formula which we have given above is applicable when there is only one turn of a circular wire. If we have circular coil having N turns of wire, then the magnetic field will become N times. Thus, the magnetic field at the centre of a circular coil of N turns having radius r and carrying current I is given by:
Magnetic field produced by a circular coil carrying current is directly proportional to both, number of turn (N) and current (I), but inversely proportional to its radius (r). Thus, the strength of magnetic field produced by a current carrying circular coil can be increased by (i) increasing the number of turns of wire in the coil, (ii) increasing the current following through the coil and (iii) decreasing the radius of the coil.
ELECTROMAGNET AND POLARITY:
The polarity of the faces of the coil depends on the direction of current and is determined by the clock rule. Looking at the face of the coil, if the current around the face is in an anticlockwise direction then the face has north polarity, while if the current at that face is in the clockwise direction, the face has south polarity. This can be tested by using a compass needle.
The magnetic field, thus produced, is very much similar to that of a bar magnet and one end of the coil acts like a magnetic north pole while the other acts like a south pole.
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