Electromotive Force & Potential Difference (CIE IGCSE Physics)

• The electromotive Force (e.m.f.) is the name given to the potential difference of the power source in a circuit
• It is defined as

The electrical work done by a source in moving a unit charge around a complete circuit

• Electromotive force (e.m.f.) is measured in volts (V)

The EMF is the voltage supplied by a power supply: 12 V in the above case

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• The definition of e.m.f. can also be expressed using an equation

• Where
  • E = electromotive force (e.m.f.) (V)
  • W = energy supplied to the charges from the power source (J)
  • Q = charge on each charge carrier (C)
    Note: in circuits the charge carriers are electrons
• This equation should be compared to the definition of potential difference (below) as the two are closely related

• As charge flows around a circuit energy is transferred from the power source to the charge carriers, and then to the components
  • This is what makes components such as bulbs light up
  • The potential difference between two points in a circuit is related to the amount of energy transferred between those points in the circuit
• Potential difference is defined as

The work done by a unit charge passing through a component

• Potential difference is measure in volts (V)

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)

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• The definition of p.d. can also be expressed using an equation

• Where
  • V = potential difference (p.d.) (V)
  • W = energy transferred to the components from the charge carriers (J)
  • Q = charge on each charge carrier (C)
    • In circuits the charge carriers are electrons
• This equation should be compared to the definition of e.m.f. as the two are closely related due to conservation of energy

• Potential difference is measured using a voltmeter, which can be either
  • Digital (with an electronic read out)
  • Analogue (with a needle and scale)
• Voltmeters are connected in parallel with the component being tested
  • The potential difference is the difference in electrical potential between two points, therefore the voltmeter has to be connected to two points in the circuit 

Analogue or Digital?

• 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 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

Voltmeters are connected in parallel to the component being tested