Capacitance (OCR A Level Physics)

Revision Note

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Katie M


Katie M




  • 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:

Capacitor circuit symbol, downloadable AS & A Level Physics revision notes

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:

Capacitance Equation_2

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

Capacitor, downloadable AS & A Level Physics revision notes

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.

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

Capacitance Equation_2

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 × 107 C = 300 nC

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 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!

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Katie M

Author: Katie M

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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