Inductance
The Electro Magnetic Field  EMF  induced into a electric circuit.
Inductance is a property of an electrical conductor by which a change in current flowing through it will induces an electromagnetic field  EMF (e.m.f.)  and an electromotive force in the conductor itself and in nearby conductors by mutual inductance.
The unit of inductance is henry  H.
A circuit has an inductance of one henry when an e.m.f. of one volt is induced by a current changing at the rate of one ampere per second.
The induced electromagnetic field  EMF  in a coil can be expressed as
EMF = n dΦ / dt (1)
where
EMF = electromagnetic field  EMF (volts)
n = turns
dΦ = change in flux (webers, Wb)
dt = time (s)
Alternatively, the induced electromagnetic field  EMF  in a coil of inductance L can be expressed as
EMF = L dI / dt (2)
where
L = inductance (henrys, H)
dI = change in current (amperes)
dt = time (s)
Example  Inductance
The EMF induced in a coil with 500 turns with a change of flux of 30 mWb in 30 ms can be calculated as
EMF = 500 (30 10^{3} Wb) / (30 10^{3} s)
= 500 Volts
Coil Inductance
For an air filled cylindrical wire coil the inductance can be calculated with the empirical formula
L = μ_{o} n^{2} A / (l + 0.45 d) (3)
where
μ_{o} = permeability for vacuum µ_{0} = 4π 10^{−7} H/m ≈ 1.2567 10^{−6} H/m
A = cylinder cross section area (m^{2})
n = number of wire turns
l = length of coil (m)
d = diameter coil (m)
The equation is valid for l > 0.5 d.
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