What must be the magnitude of a uniform electric field if it is to have the same energy density as that possessed by a 20 mT magnetic field?

Explanation

To find the magnitude of a uniform electric field (E) that has the same energy density as a 20 mT magnetic field (B), we can follow these steps:

### Step 1: Understand the energy density of electric and magnetic fields
The energy density (u) of an electric field is given by the formula:
\[ u_E = \frac{1}{2} \epsilon_0 E^2 \]
where:
- \( \epsilon_0 \) is the permittivity of free space (\( \approx 8.85 \times 10^{-12} \, \text{F/m} \))
- \( E \) is the magnitude of the electric field.
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