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  • When an electric current flown through a wire, a magnetic field is built up around the wire itself. This can be seen using a cardboard, iron filing, and a current-carrying wire. When the wire passed though the cardboard and the current flowing, iron flowing, iron filings are sprinkled onto the card board. They can be seen arranging themselves in a magnetic field (Fig. 5.).

The magnetic lines of forces are referred to as flux. Just as in a natural magnet, the field is strongest near the wire and diminishes as the distance from the wire increases.

Fig. 5. Magnetic field around a wire that is carrying electric current.

Flux around a wire does have direction. Flux direction is determined by the direction of electron flow within the wire. As shown in Fig. 6, the North pole of the compass needle indicates the direction of flux or magnetic field around the wire . The dot in the centre of the wire on the left indicated the point of the current direction arrow coming toward the observer; the × at the right represents the tail of the current arrow pointing away from the observer. If the direction of electron flow within the wire is reversed, the compass needles will reverse themselves, indicating a change in flux direction.

Right hand rule (or right hand screw rule)

The direction of the magnetic field can be found by using right hand rule or the right hand screw rule. The right hand rule states as follows:

“Grasp the wire in the right hand, with the thumb pointing in the direction of the current. The fingers will curl around the wire in the direction of the magnetic field”. 

Fig. 6. Compasses indicate the direction of flux around a wire.

Fig. 7. Right hand rule (or right hand screw rule).

Fig.7, illustrates the rule

The right hand screw rule can be explained as follows:

As a wood screw is turned clockwise it moves (or progresses) into the wood. The horizontal direction of the screw is analogous to the direction of current in a conductor. The circular motion of the screw shows the direction of the magnetic flux around the conductor.

Magnetic effect of electric currents

It has been observed that whenever an electric current flows, a magnetic field is invariably associated with it. The magnitude of the field depends on the magnitude of the current, and the nature of field depends upon the shape of the conductor.

The relationship between magnetism and the electric current for various shapes of current carrying conductor is given below:

  1. Magnetic field surrounding a conductor:

Fig. 8 (a) Current coming towards the observer (or coming out of the page)

The following points are worth noting:

  • The strength of the magnetic field is proportional to the current.
  • If the current is doubled, the magnetic field is doubled.
  • If the current is reduced to zero, the magnetic field is also reduced to zero.
  • When the direction of current is reversed, the magnetic field also reverses its direction.

2. Magnetic field due to currents in two parallel conductors:

The following points are worth noting:

  • When the currents flow in opposite direction in the parallel conductors, the conductors mutually repel, so that in such a case, the current carrying conductors tend to move away from each other [Fig. 9 (a)].
  • When the currents flow in the same direction in parallel conductors, the conductors mutually attract, so that in such a case, the current carrying conductors tend to move towards each other [Fig.9 (b)].

Fig. 9(a). Currents in the opposite direction

Fig. 9(b). Currents in the same direction

Fig. 9 . Force between parallel current current carrying conductors.

3.  Magnetic field due to a circular loop:

When a single conductor is looped to form a single turn of coil, the magnetic fields are set up as small loops around the coil shown as AA in Fig. 10 (a). The magnetic lines enter the inside of the coil on one face and leave it on the other. The direction of the field in the centre of coil will be perpendicular to the plane of the loop as shown in Fig. 10.

Fig. 10. Magnetic field due to circular loop.

4. Magnetic field due to solenoid:

A solenoid is a coil of wire wound on a long straight core. When a current passes through a solenoid a magnetic field is produced. The lines are approximately parallel to the axis of the solenoid until they reach the ends where they
begin to diverge (magnetic field pattern long similar to bar magnet).

The solenoid produces strong magnetic fields for such applications as relay coils, transformer coils and circuit breakers.

The right hand rule for determining the direction of flux from a solenoid states as follows:

“Hold the solenoid in right hand such that the fingers point in the direction of current flow in the coils. The thumb will point towards N pole of the field”.