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First exams 2025

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Potential Difference (CIE AS Physics)

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Katie M

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Katie M

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Defining potential difference

  • A cell makes one end of the circuit positive and the other negative. This sets up a potential difference (p.d.) across the circuit
  • The potential difference across a component in a circuit is defined as:

The energy transferred per unit charge flowing from one point to another

  • Potential difference (sometimes called voltage) is measured in volts (V). One volt is the same as one joule per coulomb (J C-1)
    • If a bulb has a potential difference of 3 V, every coulomb of charge passing through the bulb will transfer 3 J of energy to the bulb
  • The potential difference of a power supply connected in series is always shared between the components in the circuit

Potential difference in a series circuitPotential difference in a circuit, downloadable AS & A Level Physics revision notes

The potential difference is the voltage across each component in a circuit

  • Potential difference is measured using a voltmeter
  • A voltmeter is always set up in parallel so that it can measure the difference in electrical potential on each side of the component
    • This is a measure of how much energy has been transferred to the component

Potential difference in a parallel circuitVoltmeter in a circuit, downloadable AS & A Level Physics revision notes

Potential difference can be measured by connecting a voltmeter in parallel between two points in a circuit.

Calculating potential difference

  • The potential difference is defined as the energy transferred per unit charge
  • Another measure of energy transfer is work done
  • Therefore, potential difference can also be defined as the work done per unit charge

V space equals W over Q

    • V = potential difference (V)
    • W = work done (J)
    • Q = charge (C)

Worked example

A lamp is connected to a 240 V mains supply and another to a 12 V car battery. Both lamps have the same current, yet the 240 V lamp glows more brightly. WE - Potential difference question image, downloadable AS & A Level Physics revision notes

Explain in terms of energy transfer why the 240 V lamp is brighter than the 12 V lamp.

ANSWER:

  • Both lamps have the same current, which means charge flows at the same rate in both
  • The 240 V lamp has a potential difference 20 times greater than the 12 V lamp
  • Potential difference is the energy transferred (work done) per unit charge
  • This means the energy transferred by each coulomb of charge to the 240 V lamp is 20 times greater than for the 12 V lamp
  • This makes the 240 V lamp shine much brighter than the 12 V lamp

Examiner Tip

Think of potential difference as being the energy per coulomb of charge transferred between two points in a circuit

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Katie M

Author: Katie M

Expertise: Physics

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.