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Emf & Internal Resistance

Physics: AQA A Level, topic 5.1.11

Electromotive Force

Electromotive force or EMF is the total voltage produced by the cell. Not all of this is seen by the circuit.

Internal resistance

Internal resistance

  • A cell produces a set amount of voltage, this voltage is what makes the current flow around the circuit.
  • Some of this voltage has already been used by the internal resistance of the cell itself.
  • EMF is measured in Volts (V).
Calculations

Calculations

  • The EMF of a cell can be calculated by combining the internal resistance of the cell and the resistance of the circuit.
  • The equation for the EMF is:
  • EMF = (Internal resistance + circuit resistance) x current
  • ϵ=(r+R)I{\epsilon}=(r+R)I
    • Where ϵ{\epsilon} is the EMF, r is the internal resistance and R is the circuit resistance.

Internal Resistance

The internal resistance of a cell causes a loss of voltage before the circuit begins.

Sources of resistance

Sources of resistance

  • Most cells have some internal resistance, they reduce the voltage output of the cell before it reaches the circuit.
  • Internal resistance arises because the cell is made up of chemicals which will resist the flow of charge.
Inside a cell

Inside a cell

  • The inside of a cell is very complex and different chemicals have different resistances.
  • A cell can be thought of as a swirl of chemicals which generate power but also provide resistance.

Investigating Internal Resistance

In order to know what the internal resistance of a cell is we must conduct an experiment.

Finding the emf

Finding the emf

  • Firstly connect the cell to two wires, one to the positive and one to the negative terminal.
  • Use a voltmeter to measure the voltage between these two wires, this is known as the open-circuit voltage and is the same as the emf.
Finding the current and voltage

Finding the current and voltage

  • Next, connect a resistor of known resistance between the two wires.
  • Use a voltmeter to measure the voltage across the resistor.
  • Use an ammeter to measure the current flowing in the circuit.
Calculation

Calculation

  • The current, voltage, emf and internal resistance are linked in this equation:
  • Voltage = emf - (current x internal resistance)
  • V=ϵ−IrV={\epsilon}-Ir
  • We can rearrange this to give the internal resistance:
  • r=ϵ−VIr=\frac{{\epsilon}-V}{I}

Test yourself on Emf & Internal Resistance

  • What is the correct equation for the EMF of a circuit?

  • Why is the EMF of a circuit larger than the voltage?

  • Which of these statements about cells are true?

  • What is the open circuit voltage?

  • What is the equation for voltage in terms of EMF?

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