How a Capacitor Stores Energy: Plates, Farads, Dielectrics

A capacitor is a two-terminal electronic part that stores energy by keeping opposite electric charges on two conductors separated by an insulator. It does not make energy; it holds some for a short time and gives it back. The key relationship is that the energy stored depends on the capacitance and on the square of the voltage, so voltage matters more than size.

The structure

Wikipedia describes a capacitor as a passive component with two terminals, built from two closely spaced conductors insulated from each other. In the simplest form these are two flat plates. The insulating layer between them is called the dielectric. When a voltage is applied, charge collects on the plates, positive on one and negative on the other, and an electric field forms across the gap. The energy is held in that field.

What a farad measures

Capacitance is the ratio of charge to voltage, written C = Q/V. The LibreTexts electronics text defines one farad as one coulomb of charge per volt. A capacitor with a larger capacitance holds more charge at the same voltage. Wikipedia gives the same definition. The LibreTexts page notes practical values range from picofarads to farads, which is a spread of many orders of magnitude.

What sets the capacitance

For two parallel plates, capacitance is C = εA/d. Here A is plate area, d is the gap and ε is the permittivity of the dielectric. Three consequences follow:

• Bigger plates give more capacitance.
• A smaller gap gives more capacitance.
• A dielectric with higher permittivity gives more capacitance.

LibreTexts points out that the relative permittivity is the ratio to the permittivity of vacuum, about 8.85 x 10^-12 farads per metre. It also says the highest permittivity is not always the best choice, because temperature stability, leakage, equivalent series resistance and breakdown strength also matter.

How much energy is stored

Both Wikipedia and LibreTexts give the stored energy as W = ½CV². Doubling the capacitance doubles the energy, but doubling the voltage quadruples it. As an example calculation, a 100 microfarad capacitor charged to 10 volts holds ½ x 0.0001 x 100, which is 0.005 joules. That is a small amount, which is why ordinary capacitors suit short bursts and smoothing rather than long-term storage.

Voltage rating and breakdown

Capacitors are rated by maximum voltage, not power. LibreTexts explains that breakdown strength limits the rating, so halving the plate spacing doubles the capacitance but halves the voltage rating. Wikipedia notes that exceeding the breakdown voltage can cause a short circuit that may damage the part and, because the short draws current from the rest of the circuit, can be explosive.

How it behaves in a circuit

LibreTexts states that voltage across a capacitor cannot change instantly. Once fully charged by a steady DC source, it acts like an open circuit and no DC current flows. Wikipedia summarises the practical result: capacitors block direct current while passing alternating current. That makes them useful for smoothing power-supply output, tuning radio circuits and acting as memory cells in DRAM.

Families of capacitors

The Wikipedia article names several construction types:

• Ceramic: early versions used porcelain.
• Electrolytic: the first were invented by Charles Pollak, using an oxide layer on aluminium.
• Film: polymer films replaced paper as the industry developed during World War II.
• Supercapacitors: electric double-layer capacitors, first described in a 1957 patent by H. Becker.

Where the idea came from

Wikipedia dates the earliest storage of charge to October 1745, when Ewald Georg von Kleist found that charge could be held in a water-filled glass jar. The following year Pieter van Musschenbroek developed a similar device, which became known as the Leyden jar.

In summary, a capacitor is two conductors, a gap and an electric field. Its capacitance comes from geometry and material, and its energy grows with the square of the voltage.

Sources

• Capacitor, Wikipedia
• 8.2: Capacitance and Capacitors, LibreTexts Engineering/08%3A_Capacitors/8.2%3A_Capacitance_and_Capacitors)