Electrical Engineering ⇒ Topic : Magnitude of Mutually Induced E.M.F.

Magnitude of Mutually Induced E.M.F.

Consider two coils A and B placed adjacent to each other as shown in Fig. (a). If a current I₁ flows in the coil A, a flux is set up and a part Φ₁₂ (mutual flux) of this flux links the coil B. If current in coil A is varied, the mutual flux also varies and hence an e.m.f. is induced in the coil B. The e.m.f. induced in coil B is termed as mutually induced e.m.f. Note that coil B is not electrically connected to coil A ; the two coils being magnetically linked.

                                                                          FIGURE (A)

The larger the rate of change of current in coil A, the greater is the e.m.f. induced in coil B. In other words, mutually induced e.m.f. in coil B is directly proportional to the rate of change of current in coil A i. e.,

Mutually induced e.m.f. in coil B ∝ Rate of change of current in coil A

where M is a constant called mutual inductance between the two coils. The unit of mutual inductance is henry (H). If in exp. (i), em= 1 volt, dl₁ /dt = 1 A/sec, then, M = 1 H.

Hence mutual inductance between two coils is 1 henry if current changing at the rate of

1 A/sec in one coil induces an e.m.f. of 1 I' in the other coiL

Mutual inductance comes into picture when two coils are placed close together in such a way that flux produced by one links the other. We say then that the two coils are coupled. Each coil has its own inductance but in addition, there is further inductance due to the induced voltage produced by coupling between the coils. We call this further inductance as mutual inductance. We say the two coils are coupled together by mutual inductance. The terms magnetic or inductive coupling are sometimes used.

Note. The magnitude of mutually induced e.m.f. in coil B (secondary) is em= M dI₁/dt where dI₁ is the change of current in coil A (primary). However, the magnitude and direction of mutually induced e.m.f. in coil B should be written as

The minus sign is because the mutually induced e.m.f. sends current in coil B in such a direction so as to produce magnetic flux which opposes the change in flux produced by change in current in coil A. In fact, minus sign represents Lenz's law mathematically.