Defining capacitance
- Capacitors are electrical devices used to store energy
- In electronic circuits, they are commonly used as a backup store of energy in case of power failure
- The circuit symbol for a parallel plate capacitor is two parallel lines
Capacitor circuit symbol
The circuit symbol for a capacitor consists of two parallel lines perpendicular to the wires on either side
- Capacitors possess capacitance, which is defined as:
The charge stored per unit potential
- The greater the capacitance, the greater the charge stored on the capacitor
- Capacitors come in different forms, such as:
- isolated spherical conductors
- parallel plates
Isolated spherical conductors
- An isolated spherical conductor can store charge on its surface, which means it can act as a capacitor
- When the conducting sphere is connected to a high-voltage supply:
- electrons move on to, or off of, the surface of the sphere
- the remaining charges are of the same type, so they repel
- the surface is conducting, allowing them to move and become evenly distributed
- As the potential difference of the supply increases, the charge on the conductor also increases
- The capacitance of the sphere is equal to the ratio of the charge to the potential
Capacitance of an isolated spherical conductor
Conducting spheres act like capacitors due to their ability to store charge on their surfaces
Parallel plate capacitors
- A parallel plate capacitor is made up of two conducting metal plates connected to a voltage supply
- The negative terminal of the voltage supply pushes electrons onto one plate, making it negatively charged
- The electrons are repelled from the opposite plate, making it positively charged
- There is a dielectric between the plates which ensures charge does not flow freely between the plates
Capacitance of a parallel plate capacitor
A parallel plate capacitor is made up of two conductive plates with opposite charges building up on each plate
Examiner Tip
The ‘charge stored’ by a capacitor refers to the magnitude of the charge stored on each plate in a parallel plate capacitor or on the surface of a spherical conductor. The capacitor itself does not store charge.