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It has been found that whenever a current-carrying conductor is placed in a magnetic field, it experiences a force which acts in a direction perpendicular both to the direction of the current and the field.

-         Fig. 12(a) shows the field Bet up by the poles.

-         Fig. 12(b) shows the conductor field due to flow of current in the conductor.

-         Fig. 12(c) shows the resultant field produced when the current-carrying-conductor of [Fig. 12(b) (i)] is inserted in the pole-field, which axis of the conductor at right angles of the flux.

On the upper side of the conductor in Fig. 12 (c) the magnetizing forces of the field and of the current in the conductor are additive, while on the lower side these are subtractive. This explains why the resultant field is strengthened above and weakened below the conductor. Thus the conductor has a force on it which tends to move it downwards. 

Fig. 12 (a). Pole-field

Fig. 12(b) Conductor field

Fig. 12 (c)

Fig.12 (d)

Fig. 12. Force on a current-carrying conductor lying in a magnetic field.

Fig. 12 (d) shows that when the current in the conductor is reversed, the direction of the force is also reversed.

The force developed in the conductor is given by the relation:

F = BIl newtons                                                                                          … (3)

(=µ0µr HIl newtons)                                                                                   … [3 (a)]

where F = force developed in the conductor

B = flux density, T (Wb/m2)

I = current in the conductor, A

l = exposed length of the conductor, m

µ0 = absolute permeability

µr = relative permeability

H = magnetising force.

The direction of this force may be easily found by Fleming’s left hand rule which states as follows:

“Hold your left hand with index finger, middle finger and thumb at right angles. If the index finger points in the direction of the flux from north to south and middle finger points in the direction of the imposed voltage and its resulting conventional current flow, then the thumb will point in the direction of the force that is developed.

Fig. 13. Fleming’s left hand rule.