Figure 1: Charge accumulation. Positive charges accumulate on one plate attracting negative charges on the other.

A capacitor is an energy storage device. It consists of two metal plates (with a large surface area) separated by a thin insulating material. The larger the surface area of the plates, the more charge can be stored on them. Because the two plates are isolated from each other, electrical current does not actually flow across them. Instead, electrical charge accumulates on the plates, storing energy, which can also be discharged. Positive charge accumulates on one plate, attracting negative charges to accumulate on the other plate, as shown in Figure 1. Current will flow to the capacitor's plates until the accumulated positive and negative charges produce a voltage differential equal to that supplied by the circuit. If the circuit voltage then drops on the charged capacitor, the capacitor will give up its charge.

The voltage V across a charged capacitor is:

where C is the capacitance (Farads) and Q is the accumulated charge of the capacitor (Coulombs). One Coulomb is equal to an Amp-Second.

Figure 2: An electrolytic capacitor.

There are a few different capacitor types. Two of the most common capacitor types are ceramic as shown in Figure 2 and electrolytic capacitor as shown in Figure 3. The circuit diagram symbols for these two capacitor can be seen in Figure 4. Ceramic capacitors are not polarised so the orientation in the circuit doesn't matter. Electrolytic capacitors on the other hand are polarised. The positive lead must be connected on the positive side of the voltage potential.

Figure 3: A few ceramic capacitor.

Figure 4: Capacitor circuit diagram symbols.

References What is a Capacitor?

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