Capacitance (OCR 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
• Capacitors do this by storing electric charge, which creates a build up of electric potential energy
• They are made in the form of two conductive metal plates connected to a voltage supply (parallel plate capacitor)
  • There is commonly a dielectric in between the plates, to ensure charge does not flow across them

• The capacitor circuit symbol is:

The capacitor circuit symbol is two parallel lines

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

The charge stored per unit potential difference (between the plates)

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

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

• Where:
  • C = capacitance (F)
  • Q = charge stored (C)
  • V = potential difference across the capacitor plates (V)

A capacitor used in small circuits

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

• 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

• This capacitance equation shows that an object’s capacitance is the ratio of the charge stored by the capacitor to the potential difference between the plates

Step 1: Write down the known quantities

• • Capacitance, C = 1 nF = 1 × 10⁻⁹ F
  • Potential difference, V = 0.3 kV = 0.3 × 10³ V

Step 2: Write out the equation for capacitance

Step 3: Rearrange for charge Q

Q = CV

Step 4: Substitute in values and calculate the final answer

Q = (1 × 10^-9) × (0.3 × 10³) = 3 × 10⁻⁷ C = 300 nC

• Capacitors are useful because they store electric potential energy
• They have a wide variety of applications, such as:
  • in cameras, i.e. a bright flash of light as the capacitor discharges 
  • in smoothing currents
  • in electronic timing circuits
  • to use as a back up power supply during unexpected power cuts
  • to power devices with memory to store information when they are switched off e.g. calculators