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: