The expression for the magnetic energy stored in a solenoid in terms of magnetic field \(B\), area \(A\) and length \(l\) of the solenoid is:
1. \( \frac{1}{\mu_0}B^2Al\)
2. \( \frac{1}{2\mu_0}B^2Al\)
3. \( \frac{2}{\mu_0}B^2Al\)
4. \( \frac{3}{2\mu_0}B^2Al\)

The ratio of magnetic energy per unit volume and electrostatic energy stored per unit volume in a parallel plate capacitor is:

$1. \frac{1}{{\mathrm{\epsilon }}_0{\mathrm{\mu }}_0}{\left(\frac{\mathrm{B}}{\mathrm{E}}\right)}^2$
$2. \frac{1}{{\mathrm{\epsilon }}_0{\mathrm{\mu }}_0}{\left(\frac{\mathrm{E}}{\mathrm{B}}\right)}^2$
$3. \frac{1}{2{\mathrm{\epsilon }}_0{\mathrm{\mu }}_0}{\left(\frac{\mathrm{B}}{\mathrm{E}}\right)}^2$
$4. \frac{1}{2{\mathrm{\epsilon }}_0{\mathrm{\mu }}_0}{\left(\frac{\mathrm{E}}{\mathrm{B}}\right)}^2$