Potential Difference (CIE IGCSE Physics)

• 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

The potential difference is the difference in the electrical potential across each component: 5 volts for the bulb (on the left) and 7 volts for the resistor (on the right)

• 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

• 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 

Potential difference can be measured by connecting a voltmeter in parallel between two points in a 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 not at zero, you will need to subtract this from all your measurements
  • They should be checked for zero errors before using

Voltmeters can be either analogue (with a scale and needle) or digital (with electronic read-out)

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

Extended tier only

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

• 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

• This can be rearranged using the formula triangle below:

Energy charge and potential difference formula triangle

Formula triangle for the energy transferred, voltage and charge equation

• Check out this revision note on speed, distance and time if you need a reminder on how to use formula triangles