Energy Density is defined as the total amount of energy in a system per unit volume. For example, the number of calories per gram of food. Foodstuffs that has low energy density provide less energy per gram of food which means that you can eat more of them since there are fewer calories.
 It is denoted by letter U.
Magnetic and electric fields can also store energy.
In the case of the electric field or capacitor, the energy density formula is given by
E = E is the electric field
The energy density formula in case of magnetic field or inductor is given by,
B =Magnetic field
Regarding electromagnetic waves, both magnetic and electric field are equally involved in contributing to energy density. Therefore, the formula of energy density is the sum of the energy density of the electric and magnetic field.
Example 1:
Find the energy density of a capacitor if its electric field, E = 5 V/m.
Solution:
Given,
E = 5V/m
We know that,
ϵ0 = 8.8541× 10−12F/m
The energy density formula of the capacitor is given by
  = (1 × 8.8541×10−12×52 )/2
U= 1.10×10−10 FV2/m3
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