What is a capacitor?

1. What is a capacitor? A capacitor is a device consisting of two metal plates that stores electric charge. Between these two plates is an insulator (a material that does not conduct electricity) to prevent charges from moving between the two plates. One of these plates is a negative plate, it holds negative charges, the other, is a positive plate, it holds positive charges. One common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of electrical circuits in many common electrical devices.

3. What are its uses and/or functions? • Store energy • Change their behavior with frequency • Come about naturally in circuits and can change a circuit's behavior

4. Energy is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed. (This prevents loss of information in volatile memory.)

5. When we connect the ends of a battery to the ends of a capacitor, the battery will pull negative electric charges from one plate and move them to the other plate. This movement of charges is knows as a current, and it will exist for a short time until the capacitor is fully charged. The result is one plate with an excess of negative charges and the other plate with an excess of positive charges, or, a positive and negative plate. A short time later, negative charges begin moving from one plate to the other.

6. Why do the charges stay on the conductors ? • If the top plate contains positive charge, and the bottom plate contains negative charge, then there is a tendency for the charge to be bound on the capacitor plates since the positive charge attracts the negative charge (and thereby keeps the negative charge from flowing out of the capacitor) and in turn, the negative charge tends to hold the positive charge in place. Once charge gets on the plates of a capacitor, it will tend to stay there, never moving unless there is a conductive path that it can take to flow from one plate to the other. 

7. Voltage and Current relationship There is a relationship between the charge on a capacitor and the voltage across the capacitor. The relationship is simple. For most dielectric/insulating materials, charge and voltage are linearly related. Q = C V where: V is the voltage across the plates. C = capacitance in farads (F)V = voltage in volts (V) Q=charge stored in the capacitor When V is measured in volts, and Q is measured in couloumbs, then C has the units of farads. Farads are really coulombs/volt. C = Q / V A capacitor has a voltage that is proportional to the charge that is stored in the capacitor.

8. Applications Capacitors are used in many devices, such as the flash in cameras. In a flash camera, a capacitor charges up and stores the electric charge (that's the whining sound you hear in some cameras) and releases the charge to a light bulb which creates a flash when the picture is taken.

9. Warning ! Capacitors are used in many other devices, like TV's. The capacitors found in TV's are very powerful, they can hold a lot of charges on their plates. They are very dangerous to touch, then, because if you touched them you would get shocked (the charges would move through YOUR body). Because of the large amount of charges, this can be as dangerous or more dangerous than sticking your finger in a wall outlet. Because of this, TV's have WARNINGS on them not to open them up