In a capacitor, energy is stored in which form?

• A. Magnetic energy
• B. Electrical energy
• C. Kinetic energy
• D. Chemical energy

Answer: (B)

In a capacitor, energy is stored as electrical energy in the electric field between the plates. When the plates are charged, separated charges create a field, and the energy you’ve put in shows up as that field’s stored energy. Mathematically, the energy can be written as W = 1/2 C V^2 or W = 1/2 QV, and the energy density is proportional to the square of the electric field, u = 1/2 ε E^2. Magnetic energy would require a magnetic field from a changing current (like in an inductor), which isn’t how a capacitor stores energy. Kinetic energy would involve moving mass or charges, but in a steady-state capacitor the charges are held stationary on the plates. Chemical energy involves bonds in molecules, not the electric field between capacitor plates. So the stored energy form is electrical energy.