But a capacitor is a much simpler device that can't produce new electrons; it only stores them. Like a battery, a capacitor has two terminals – positive and negative. Inside the capacitor, the terminals connect to two metal plates separated by a dielectric (an insulating substance such as ceramic or glass that is highly resistant to electric current) that keeps the plates from touching each other and allows them to hold opposite charges, maintaining an electric field.
The tutorial below demonstrates a capacitor functioning in a direct current circuit that powers an electric motor used to lift a small weight. The tutorial illustrates one reason capacitors are useful. As you will see, if you removed the capacitor from this circuit, the battery alone would do a poor job of powering the motor – even though the voltages across both are equal.
Begin by moving the Switch Position slider to the right until you reach the Battery position. This will close the Knife Switch and create a circuit that includes the Capacitor and the Battery. Notice that electrons (shown here as yellow particles) flow from the negative terminal of the battery to the capacitor, where they are stored. (Electrons flow opposite the direction of conventional current.) For every electron stored on that capacitor plate, the opposite capacitor plate must lose one (flowing to the battery’s positive terminal) to maintain the capacitor in equilibrium. In the tutorial, red particles appear in the left (positive) side of the capacitor plate to denote the positive charge left behind by each parting electron. In this way the capacitor continues to charge until it attains the voltage of the battery charging it.
Next, move the slider farther right to the Motor position to create a new circuit. The electrons on the capacitor's negative terminal are drawn to the positive terminal; they rush along the now open path that leads there, straight through the motor. This electrical current powers the pulley to lift a weight until the charge dissipates. Notice that as the electrons reach the opposite plate, the positive charges disappear.
Repeat the process using the slider to raise the weight a bit more. Hit the blue Reset button to start the process over.
The battery alone could not have caused the weight to move much, because batteries discharge slowly and gradually. Capacitors, on the other hand, discharge quickly, providing greater current over a shorter period of time – enough current, in our example, to lift the weight. This feature of capacitors makes them useful for powering flash bulbs in cameras, for example.
The first capacitor was the Leyden jar, invented in 1745. Originally, the units of capacitance were "jars," but now are measured in farads in honor of the great English experimental physicist Michael Faraday.