26.4 - Energy in capacitors
Capacitors store energy. A battery or other device performs work as its electric field
pulls electrons from the positive plate of a capacitor and pushes them to the negative
plate. The force of this field drives electrons away from the attractive force of the
positive plate and toward the repulsive force of other electrons already on the negative
plate. This work increases the electric potential energy of the capacitor.
To the right you see two equations that express the energy stored in a capacitor. The
first equation is stated in terms of the charge on the capacitor; the second is stated in
terms of the potential difference across the capacitor. We can derive these equations by
calculating the work required to move each charge from one plate to the other.
The example problem asks you to find the amount of energy stored in a capacitor with a
potential difference of 25,000 V across its plates. This is representative of the voltages
found in television sets. The stored energy in a TV capacitor can give you a dangerous
shock, even after the set is unplugged. This is why the back panels of televisions are
labeled with warnings against attempting to disassemble them yourself. We recommend
that you follow this advice!
Energy in a capacitor
Battery’s field causes electrons to move
Charges create electric field
Electric field stores energy