Compare the expression for magnetic energy density with electrostatic energy density stored in the space between the plates of a parallel plate capacitor.

Explanation

The enectrostatic energy stored
`U=(1)/(2)(q^(2))/(C)=(q^(2))/(2epsilon_(0)A)xxd" …(i)"`
(where C is the capacitance, `C=(epsilon_(0)A)/(d)`, q is the charge upon capacitor)
Now the electric field in the space (between plates of capacitor)
`E=(q)/(epsilon_(0)A)" ...(ii)"`
From equation (i) and (ii),
`U=(1)/(2)((q)/(epsilon_(0)A))^(2)xxepsilon_(0)xxAxxd`
`rArr" "U=(1)/(2)epsilon_(0)E^(2)xx"Volume"`
Hence `(U)/("Volume")=(1)/(2)epsilon_(0)E^(2)" ...(iii)"`
This expression can be compared with magnetic energy per unit volume which is equal to `(B^(2))/(2mu_(0)).` ....(iv)