Inductance can be illustrated by the behavior of a coil of wire which resists any change of electric current that passes through it. The unit of inductance is the Henry (). Thus, a coil has an inductance of if an induced voltage of flows through it with a rate of change of current of .
Calculation Of Self Inductance
In order to calculate the self inductance, consider a circuit of length and cross-sectional area , which is passed by a coil of turns (see Figure 2).
Also, assume that the area of the core is small, and that the magnetic field strength is constant across the area. We can then write that:
where is the current which passes through the coil (for a more detailed discussion on the magnetic field strength see Field Strength ). We also know that the magnetic field strength is related to the magnetic flux density by the equation:
Mutual inductance represents the generation of an electromotive force () in a coil as a result of a change in current in a coupled coil as the one diagramed in Figure 3.
The induced in coil 2 due to changes in coil 1 can be expressed as:
In this definition, represents the ratio between the generated in coil 2 and the change in current in coil 1 responsible for generating this .
In the case of no magnetic saturation, the mutual inductance can be written as:
As the two coils are wound on the same core, we have that and . However, for more complicated cases, such relations are harder to see.
The Induced Voltage Of Two Coils In Series Using Inductance
Consider two coils arranged in series, as diagramed in Figure 5.
There are four induced electromotive forces due to self and mutual inductance. As such, the induced voltage at any instant can be written as:
where is the inductance of the first coil, the inductance of the second coil, and the mutual inductance.
Equation (25) applies only when the turns are additive. If the turns are opposite, equation (25) becomes:
The energy stored in a magnetic field can also be expressed using the (self-) inductance (for a more detailed discussion on the energy stored in a magnetic field see Stored Energy ). For example, equation (6) can also be written as: