Answer
$E = 1.5\times 10^8~V/m$
Work Step by Step
We can write an expression for the energy density of a magnetic field:
$u_B = \frac{B^2}{2~\mu_0}$
We can write an expression for the energy density of an electric field:
$u_E = \frac{\epsilon_0~E^2}{2}$
We can equate the two expressions to find $E$:
$u_E = u_B$
$\frac{\epsilon_0~E^2}{2} = \frac{B^2}{2~\mu_0}$
$E^2 = \frac{B^2}{\epsilon_0~\mu_0}$
$E = \frac{B}{\sqrt{\epsilon_0~\mu_0}}$
$E = \frac{0.50~T}{\sqrt{(8.854\times 10^{-12}~F/m)(4\pi\times 10^{-7}~H/m)}}$
$E = 1.5\times 10^8~V/m$
