How EMF is Induced? Types of Induced EMF
Types of Induced EMF
There are two types of Induced emf.
- Dynamically induced emf
- Statically induced emf
In the first case, usually the field is stationary and conductors cut across it (as in d.c. generators). But in the second case, usually the conductors or the coil remains stationary and flux linked with it is changed
by simply increasing or decreasing the current producing this flux (as in transformers).
1. Dynamically Induced EMF
In Fig. (1) a conductor A is shown in cross-section, lying m2 within a uniform magnetic field of flux density B Wb/m2. The arrow attached to A shows its direction of motion. Consider the conditions shown in [Fig. 1 (a)]
when A cuts across at right angles to the flux. Suppose ‘l’ is its length lying within the field and let it move a distance dx in time dt. Then area swept by it is = ldx. Hence, flux cut = l.dx × B webers.
Change in flux = Bldx weber
Time taken = dt second
Hence, according to Faraday’s Laws the e.m.f. induced in it (known as dynamically induced e.m.f) is
rate of change of flux linkages = Bldx/dx = Bl dt/dt = Blv volt where dx/dt = velocity
If the conductor A moves at an angle θ with the direction of flux [Fig. 1 (b)] then the induced e.m.f. is
e = Blυ sin θ volts = l B υ × (i.e. as cross product vector υ and B).
The direction of the induced emf is given by Fleming’s Right-hand rule or Flat-hand rule and most easily by vector cross product given above. It should be noted that generators work on the production of dynamically induced emf in the conductors housed in a revolving armature lying within a strong magnetic field.
2. Statically Induced EMF
It can be further sub-divided into two parts.
a. Mutually induced emf
b. Self-induced emf
a. Mutually-Induced EMF
Consider two coils A and B lying close to each other (Fig. 2). Coil A is joined to a battery, a switch and a
variable resistance R whereas coil B is connected to a sensitive voltmeter V. When current through A is
established by closing the switch, its magnetic field is set up which partly links with or threads through the coil B. As current through A is changed, the flux linked with B is also changed. Hence, mutually induced emf is produced in B whose magnitude is given by Faraday’s Laws and direction by Lenz’s Law.
If, now, battery is connected to B and the voltmeter across A (Fig. 3), then the situation is reversed and now a change of current in B will produce mutually-induced emf in A.
-min.jpg "Mutually-Induced E.M.F")
It is obvious that in the examples considered above, there is no movement of any conductor, the flux variations being brought about by variations in current strength only. Such an emf induced in one coil by the influence of the other coil is called (statically but) mutually induced emf.
b. Self-Induced EMF
This is the e.m.f. induced in a coil due to the change of its own flux linked with it. If current through the coil (Fig. 4) is changed, then the flux linked with its own turns will also change, which will produce in it what is
called self-induced e.m.f. The direction of this induced e.m.f (as given by Lenz’s law) would be such as to oppose any change of flux which is, in fact, the very cause of its production. Hence, it is also known as the opposing or counter e.m.f of self-induction.
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