What is Lenz's Law?
Faraday’s law says that the induced emf is given by
V = - dψ/dt
The direction of the induced emf is given by Lenz’s law,
which says that the induced voltage is in the direction such that, if the
voltage caused a current to flow in the wire, the magnetic field produced by
this current would oppose the change in ψ. The negative sign indicates the
opposing nature of the emf.
A current flowing in a simple coil produces a magnetic
field. Any change in the current will change the magnetic field, which will in
turn induce a back-emf in the coil. The self-inductance or just inductance L
(H) of the coil relates the induced voltage to the rate of change of current
V = L dI/dT
Two coils placed close together will interact. The magnetic
field of one coil will link with the wire of the second. Changing the current
in the primary coil will induce a voltage in the secondary coil, given by the
mutual inductance M (H)
V2 = M dI1/dT
Placing the coils very close together, on the same former,
gives close coupling of the coils. The magnetic flux linking the primary coil
nearly all links the secondary coil. The voltages induced in the primary and
secondary coils are each proportional to their number of turns, so that
V1/V2 = N1/N2
and by conservation of energy, approximately
I1/I2 = N2/N1
A two-winding transformer consists of two coils wound on the
same ferromagnetic core. An autotransformer has only one coil with tapping
points. The voltage across each section is proportional to the number of turns
in the section.
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