Electro Magnetic Field - EMF - induced into a 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)
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)
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
For an air filled cylindrical wire coil the inductance can be calculated with the empirical formula
L = μo n2 A / (l + 0.45 d) (3)
μo = permeability for vacuum µ0 = 4π 10−7 H/m ≈ 1.2567 10−6 H/m
A = cylinder cross section area (m2)
n = number of wire turns
l = length of coil (m)
d = diameter coil (m)
The equation is valid for l > 0.5 d.