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MAGNETISING FORCE OF A LONG STRAIGHT CONDUCTOR

Fig. 14 shows a straight solid conductor in which the steady current I is flowing upwards in the direction indicated. Suppose that the field strength at point A distant r meters from the centre of the conductor is H. Then, it means that if a unit N-pole is placed at A, it will experience a force of H newtons. The direction of this force would be tangential to the circular line of force passing thought A, If this unit N-pole is moved once round the conductor against this force, then work done is

= force × distance

= H × 2πr joules.

According to work law (It states that the net work in joules done on or by a unit N-pole in moving around any single completed path in a magnetic field is numerically equal to the ampere-turns linked with its path), this must be equal to the ampere-turns included within the path

Fig. 14. Magnetic field produced by current in a straight solid conductor.

Ampere-turns = NI

= 1 × I = I

Example 2. A long straight conductor carries steady current of 20 A. Find:

i. The intensity of magnetic field produced at a point 300 mm from the axis of the conductor.

ii. Flux density of field at that point.

Solution. Current carried by the conductor, I = 20 A

Distance, r = 300 mm = 0.3 m

(i)                           Magnetic field intensity, H :