Energy of electric field and magnetic field I am studying about inductors and capacitors. They store their energy in the form of electric field and magnetic field.Energy is required to create fields.But if we take an isolated charge and nothing else,electric field lines will be created .Does that take energy? I guess not .Then why does energy is stored when electric field lines are formed between the plates of the capacitor?
 A: An  isolated charge (like, an electron) is produced by ionizing an atom, using energy to pull a single electron free of the atom and pulling that charge
far from the opposite-charge ion.  So, it DOES take energy to isolate the
charge.   The isolated charge has an E field around it, but the original
uncharged atom had none.   
Similarly, when you apply a battery to two plates of a capacitor, there
is battery power required to put charge onto the plates, creating the
field between those plates.  Disconnecting the battery may HIDE the energy
source, but the field between the plates is evidence that there was
energy supplied (and is still available).
A: Even electrostatic fields still contain potential energy in them. The issue here is, an electron (for example) cannot interact with itself. This means that its own field cannot give it a potential energy. So yes, in the case of a lone charge completely isolated, it will create an $\vec E$ field which contains potential energy, but it will do no work until another charge is brought into the system.
Capacitors store half the work the battery does to charge it, in an $\vec E$ field. The energy stored in it is $U = \frac{1}{2}QV$. You can find out why it is half here. Basically it has to do with $I^2 R $ losses.
