Conceptual Physics

26.0 - Introduction

Capacitors store charge, and in doing so, they store energy. They are used to store energy in devices ranging from camera flashes to defibrillators, the systems used to “shock” the human heart back into its proper rhythm. Capacitors are employed for other purposes as well. Because it takes a fixed amount of time to charge or discharge a given capacitor, capacitors are used in circuits where timing is essential, such as the tuner of a radio. At the right, you see the design of a basic capacitor. It consists of two conducting plates separated by air. There is a wire connected to each plate, and these wires are attached to a source of potential difference that has caused the plates to become charged. The amounts of charge on the plates are equal in magnitude but opposite in sign. You can launch the simulation and try an experiment that demonstrates one of the capacitor’s fundamental properties. In the simulation, you can change the potential difference across the plates with the ǻV controller next to the capacitor. When you change the potential difference, note what happens to the amount of charge stored by the capacitor. The approximate charge is displayed visually on the capacitor plates; for a more precise value you can rely on the charge readout gauge displayed below. Is the charge proportional to the potential difference?

The "capacitor tree" (left) at the Fermilab National Accelerator Laboratory stored huge amounts of energy for high-energy accelerator experiments.

26.1 - Capacitors

Capacitor: A device with

two conducting plates that

can hold equal but opposite

amounts of charge.

Capacitance: The ratio of

the charge on one of the

capacitor’s plates to the

potential difference between the plates.

The simplest capacitors consist of two parallel metal plates separated by a narrow gap. We use this configuration, a parallel-plate capacitor, for the definitions above. A battery (or other source of potential difference) causes the plates to become electrically charged: The plates contain equal but opposite amounts of charge. An insulator, which can be as simple as an air gap, separates the plates. The capacitance of a capacitor tells how much charge it can store for a given potential difference between the plates; specifically, it equals the value of the positive charge on one plate in coulombs, divided by the potential difference in volts, a relationship shown in Equation 1. The amount of charge on one plate is represented by the letter q. The potential difference is also measured as a positive value, so charge, potential difference and capacitance are all positive values. The geometry of a capacitor and the nature of its insulator are the two factors that determine its capacitance. Capacitors with larger plates, or with plates separated by a narrower distance, have greater capacitance. The other factor that can affect capacitance is the nature of the insulator between the

Various capacitors

Capacitors

Composed of two conductors Separated by insulator Equal, opposite charges on conductors