CIE A Level Physics (9702) exams from 2022

Revision Notes

19.1.1 Capacitance

Defining Capacitance

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

Exam 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.

Calculating Capacitance

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

Calculating Capacitance equation 1

 

  • 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)
  • 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

Capacitance of a Spherical Conductor

  • The capacitance of a charged sphere is defined by the charge per unit potential at the surface of the sphere
  • The potential V is defined by the potential of an isolated point charge (since the charge on the surface of a spherical conductor can be considered as a point charge at its centre):

Calculating Capacitance equation 2

  • Substituting this into the capacitance equation means the capacitance C of a sphere is given by the expression:

C = 4πε0r

Worked example: Charge on parallel plates

Calculating_Capacitance_Worked_example_-_Charge_on_Parallel_Plates_Question, downloadable AS & A Level Physics revision notes

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

Calculating Capacitance equation 1

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 Tip

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!

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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