Capacitance (Edexcel IAL Physics)

Spec point — 99

Capacitance

• 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

Worked Example A parallel plate capacitor has a capacitance of 1 nF and is connected to a voltage supply of 0.3 kV.

Calculate the charge on the plates.

Answer:

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

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

Exam Hint 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 letter ‘C’ is used both as the symbol for capacitance as well as the unit of charge (coulombs). Take care not to confuse the two!