Capacitors are energy storage devices which have the ability to store an electrical charge across its plates. Thus capacitors store energy as a result of their ability to store charge and an ideal capacitor would not loose its stored energy.
The simplest construction of a capacitor is by using two parallel conducting metal plates separated through a distance by an insulating material, called a the dielectric as summarised below.
- A capacitor consists of two metal plates separated by a dielectric.
- The dielectric can be made of many insulating materials such as air, glass, paper, plastic etc.
- A capacitor is capable of storing electrical charge and energy.
- The higher the value of capacitance, the more charge the capacitor can store.
- The larger the area of the plates or the smaller their separation the more charge the capacitor can store.
- A capacitor is said to be “Fully Charged” when the voltage across its plates equals the supply voltage.
- The symbol for electrical charge is Q and its unit is the Coulomb.
- Electrolytic capacitors are polarized. They have a +ve and a -ve terminal.
- Capacitance is measured in Farads, which is a very large unit so micro-Farad ( μF ), nano-Farad ( nF ) and pico-Farad ( pF ) are generally used.
- Capacitors that are daisy chained together in a line are said to be connected in Series.
- Capacitors that have both of their respective terminals connected to each terminal of another capacitor are said to be connected in Parallel.
- Parallel connected capacitors have a common supply voltage across them.
- Series connected capacitors have a common current flowing through them.
- Capacitive reactance, XC is the opposition to current flow in AC circuits.
- In AC capacitive circuits the voltage “lags” the current by 90o.
The basic construction and symbol for a parallel plate capacitor is given as: