When the magnetic field is at its full strength (no longer expanding), there is no relative motion and no
induced emf, as shown on the meter in the figure below.
When the switch is opened, the magnetic field collapses, which induces an emf of the opposite polarity in
An emf is induced across the secondary whenever the magnetic field linking the secondary is increased or
decreased. Two circuits arranged so that a change in magnitude of current in one causes an emf to be
induced in the other show the effects of MUTUAL INDUCTANCE.
Q. How is energy transferred from a primary coil to a secondary coil?
The symbol for mutual inductance is M or Lm. The unit of measurement of mutual inductance is the henry,
the same unit of measurement used for inductance. The mutual inductance of two circuits is 1 henry when
1 volt of emf is induced in the secondary as the current in the primary is changing at the rate of 1 ampere
per second. The formula expressing this is:
where M is the mutual inductance of the two circuits in henrys,
Es is the emf induced in the secondary, and
Δlp/Δt is the rate of change of current in the primary in amperes per second.
Q. What do the symbols (M) and (Lm) represent?