Capacitance (CIE A Level Physics)

• Capacitors are electrical devices used to store energy in electronic circuits, commonly for a backup release of energy if the power fails

• 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

• They can be in the form of:
  • An isolated spherical conductor
  • Parallel plates

• Capacitors are marked with a value of their capacitance. This is defined as:

The charge stored per unit potential difference

• The greater the capacitance, the greater the charge stored in the capacitor

Parallel Plate Capacitors

• A parallel plate capacitor is made up of two conductive 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 commonly a dielectric  in between the plates, this is to ensure charge does not freely flow between the plates

The Workings of a Parallel Plate Capacitor

A parallel plate capacitor is made up of two conductive plates with opposite charges building up on each plate

• The capacitance of a capacitor is defined by the equation:

• Where:
  • C = capacitance (F)
  • Q = charge (C)
  • V = potential difference (V)

• It is measured in the unit Farad (F)
  • In practice, 1 F is a very large unit
  • Capacitance will often be quoted in the order of micro Farads (μF), nanofarads (nF) or picofarads (pF)

A Capacitor with a Capacitance of 47 μF

A capacitor used in small circuits looks like this

• If the capacitor is made of parallel plates, Q is the charge on the plates and V is the potential difference across the capacitor
• The charge Q is not the charge of the capacitor itself, it is the charge stored on the plates or spherical conductor
• This capacitance equation shows that an object’s capacitance is the ratio of the charge on an object to its potential