Electrical Engineering ⇒ Topic : Typical Electromagnetic Fields

Typical Electromagnetic Fields

The current carrying conductor may be in the form of a straight wire, a loop of one turn, a coil of several turns. The shape of the magnetic field would eventually depend upon the shape of conductor. By way of illustration, we shall discuss magnetic fields produced by some current carrying conductor arrangements.

(1) Long straight conductor. If a straight long conductor is carrying current, the magnetic lines of force will be concentric circles around the conductor as shown in Fig. (a). In Fig. a (i), the conductor is carrying current into the plane of paper (usually represented by a cross inside the X-section of the conductor). Applying right-hand rule, it is clear that direction of magnetic lines of force will be clockwise. In Fig. a (ii), the conductor is carrying current out of the plane of paper (usually represented by a dot inside the X-section of the conductor). Clearly, the direction of magnetic lines of force will he anticlockwise.

                                                                       figure (a)

(2) Parallel conductors. Consider two parallel conductors A and B placed close together and carrying current into the plane of the paper as shown in Fig. a (i). The magnetic lines of force will be clockwise around each conductor. In the space between A and B, the lines of force due to the conductors are in the opposite direction and hence they cancel out each other. This results in a field that entirely surrounds the conductors as shown in Fig. a (ii).

                                                                                              figure (a)

If there are several parallel conductors placed close together and carrying current into the plane of the paper as shown in Fig. b (i), the magnetic field envelops the conductors. If the direction of current is reversed, the direction of field is also reversed as shown in Fig. b (ii).

                                                                                         figure (b)

(3) Coil of several turns. Consider a coil of several turns wound on a hollow tube or iron bar as shown in Fig. a (i). Such an arrangement is called a *solenoid. Suppose current flows through the coil in the direction shown. In the upper part of each turn (at points 1, 2, 3, 4 and 5), the current is flowing into the plane of the paper and in the lower part of each turn (at points 6, 7, 8 and 9),current is flowing out of the plane of paper. This is shown in the cross-sectional view of the coil in Fig. a (ii). It is clear that a clockwise field entirely surrounds the conductors 1, 2, 3, 4 and 5 while anticlockwise field completely envelops the conductors 6, 7, 8 and 9. As a result, the field becomes similar to that of a bar magnet with flux emerging from one end of the coil and entering the other

                                                                                                                   figure (a)

It is clear that left-hand face of the coil [See Fig. a (ii)] becomes a N-pole and right-hand face S-pole. The magnetic polarity of the coil can also be determined by the right-hand rule for coil. Grasp the whole coil with right-hand so that the fingers are curled in the direction of current. Then thumb stretched parallel to the axis of the coil will point towards the N-pole end of the coil [See Fig. (b)] It may be noted that both right hand rules (for a conductor and for a coil) discussed so far can be applied in reverse.

If we know the direction of magnetic field encircling a conductor or the magnetic polarity of a coil, we can determine the direction of current by applying appropriate right-hand rule.

                                                                              figure (b)