What is the relation between energy density and electric field?

Explanation

To find the relation between energy density and electric field, we can follow these steps:

### Step 1: Understand the Concept of Energy Density
Energy density is defined as the energy stored per unit volume in an electric field. It is denoted by \( u \) or \( \mu \).

### Step 2: Identify the Formula for Energy Stored in a Capacitor
For a parallel plate capacitor, the energy \( U \) stored can be expressed in terms of the electric field \( E \) and the permittivity of free space \( \epsilon_0 \). The formula for the energy stored in a capacitor is given by:
\[
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