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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Electomagnetic Induction Page 3

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Self and Mutual Induction


Self Induction:


An e.m.f is induced in a coil, due to the change of current in the same coil. This phenomenon is called self induction.

Definition of 1 henry: The self inductance of a coil is 1 henry, if an e.m.f. of 1 volt is induced in a coil when the current passing through the same coil changes at the rate of 1 ampere / second.

L =|








1 henry

1 volt


1 ampere




Mutual Induction:


An e.m.f. is induced in a (secondary) coil due to the changes of current in a neighbouring (primary) coil. This is called mutual induction.



Induction Coil


Principle: It works on the principle of electromagnetic (mutual) induction.



Primary coil - few turns
Secondary coil - A large number of turns
Soft iron core
Make and Break arrangement - hammer (H) and screw (s) through platinum points P1 and P2 .


(i) The repeating cycle of make and break:


(1) key closed


(2) current in primary


(3) soft iron core magnetised


(4) hammer attracted


(5) contact between hammer and screw lost


(6) current through primary stops


(7) core demagnetised


(8) hammer returns to original position



contact between hammer and screw re-established


(ii) In secondary coil:


Current changes in primary coil cause flux changes in secondary and therefore induce e.m.f.
Rate of rise of current in primary coil during make is much slower than the rate of fall of current during break.
Therefore, e.m.f. induced in secondary coil during make is much lesser (negligible) than e.m.f. induced during break.
The output at secondary terminal is, therefore, high, intermittent, unidirectional (
d.c.) e.m.f..




Action of Condenser:

The condenser reduces the sparking between contact points of
the hammer and screw.

Condenser discharges during break and sends a current in opposite direction through the primary coil. This increases the rate of change of flux through secondary coil during break, and hence increases the induced e.m.f. to almost twice its value.


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