E-StoreEnergy stored in Magnetic Field
Energy stored in Magnetic Field:
So far we have discussed the inductance in static forms. In earlier chapter we discussed the fact that work is required to be expended to assemble a group of charges and this work is stated as electric energy. In the same manner energy needs to be expended in sending currents through coils and it is stored as magnetic energy. Let us consider a scenario where we consider a coil in which the current is increased from 0 to a value I. As mentioned earlier, the self inductance of a coil in general can be written as
..................................(4.70a)
or 
..................................(4.70b)
If we consider a time varying scenario,
..................................(4.71)
We will later see that 
is an induced voltage.
Therefore in order to maintain the increase of current, the electric source must do an work against this induced voltage.
is the voltage drop that appears across the coil and thus voltage opposes the change of current.
For linear magnetic circuit 
...................................(4.74)
Now, 
...................................(4.75)
where A is the area of cross section of the coil. If l is the length of the coil
...................................(4.76)
Al is the volume of the coil. Therefore the magnetic energy density i.e., magnetic energy/unit volume is given by
...................................(4.77)
In vector form is the energy density in the magnetic field. 
J/mt3 ...................................(4.78)
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