Potential Difference (Cambridge (CIE) IGCSE Physics): Revision Note

Potential difference

• Potential difference is defined as:

The work done by a unit charge passing through a component

• Potential difference is measured in units of volts (V)

• The potential difference between two points in a circuit is related to the amount of energy transferred between those points

• As electrons flow through a cell, they gain energy

  • For example, in a 12 V cell, every coulomb of charge passing through gains 12 J of energy

• As electrons flow through a circuit, they lose energy

  • For example, after leaving the 12 V cell, each coulomb of charge will transfer 12 J of energy to the wires and components in the circuit

Measuring potential difference

• Potential difference can be measured using a voltmeter

• Voltmeters must be set up in parallel with the component being measured

  • This is because potential difference is the difference in electrical potential between two points

  • Therefore, a voltmeter has to be connected to two points in the circuit 

• Voltmeters can be:

  • digital (with an electronic read-out)

  • analogue (with a needle and scale)

Analogue voltmeters

• Analogue voltmeters are subject to parallax error

  • Always read the meter from a position directly perpendicular to the scale

• Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary

  • Always double-check exactly where the marker is before an experiment

  • If it is not at zero, this value must be subtracted from all measurements

  • They should be checked for zero errors before using

Digital voltmeters

• Digital voltmeters can measure very small potential differences, in mV or µV

• Digital displays show the measured values as digits and are more accurate than analogue displays

• They’re easy to use because they give a specific value and are capable of displaying more precise values

  • However digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down

• Digital voltmeters should be checked for zero error

  • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results

Calculating potential difference

Extended tier only

• Potential difference, energy transferred and charge are related by the equation:

$V=\frac{W}{Q}$

• Where:

  • $V$ = potential difference, measured in volts (V)

  • $W$ = energy transferred to the components, measured in joules (J)

  • $Q$ = charge moved, measured in coulombs (C)

• One volt is equivalent to the transfer of 1 joule of electrical energy by 1 coulomb of charge, or 1 V = 1 J/C