AQA A Level Physics

Revision Notes

5.4.2 Required Practical: Investigating EMF & Internal Resistance

Required Practical: Investigating EMF & Internal Resistance

Aims of the Experiment

The overall aim of the experiment is to investigate the relationship between e.m.f and internal resistance by measuring the variation of current and voltage using a variable resistor

  • Independent variable = voltage, V (V) & current, I (A)
  • Dependent variable = resistance, R (Ω)
  • Control variables:
    • E.m.f of the cell
    • Internal resistance of the cell

Equipment List

Required practical 6 - Equipment List Table, downloadable AS & A Level Physics revision notes

  • Resolution of measuring equipment:
    • Voltmeter = 1 mV
    • Ammeter = 0.1 mA

Method

Required practical 6 - Apparatus, downloadable AS & A Level Physics revision notes

  1. The cell and the resistor, labelled r, should be connected in series and considered to be a single cell
  2. With the switch open, record the reading V on the voltmeter
  3. Set the variable resistor to its maximum value, close the switch and record V and the reading I on the ammeter – make sure to open the switch between readings
  4. Vary the resistance of the variable resistor up to a minimum of 8-10 readings and record values for V and I for each resistance. Ensure to take readings for the whole range of the variable resistor
  • An example of a suitable table might look like this:

Required practical 6 - Data Table, downloadable AS & A Level Physics revision notes

Analysing the Results

  • The relationship between e.m.f. and internal resistance is given by

E = I (R + r)

  • Where:
    • E = electromotive force (V)
    • I = current (A)
    • R = resistance of the load in the circuit (Ω)
    • r = internal resistance of the cell (Ω)

 

  • This can be simplified into the form:

E = IR + Ir = V + Ir

  • Rearranging this equation for V:

V = –rI + E

  • Comparing this to the equation of a straight line: y = mx + c
    • y = V (V)
    • x = I (A)
    • Gradient = –r (Ω)
    • Y-intercept = E (V)

 

  1. Plot a graph of V against I and draw a line of best fit
  2. Measure the gradient of the graph and compare it with the manufacturer’s value of the resistor
  3. The y-intercept will be the e.m.f and the gradient will be the negative internal resistance:

Required practical 6 - Graph, downloadable AS & A Level Physics revision notes

Evaluating the Experiment

Systematic Errors:
  • Only close the switch for as long as it takes to take each pair of readings
    • This will prevent the internal resistance of the battery or cell from changing during the experiment
Random Errors:
  • Only use fairly new cells otherwise the e.m.f. and internal resistance of run-down batteries can vary during the experiment
  • Wait for the reading on the voltmeter and ammeter to stabilise (stop fluctuating) before recording the values
  • Take multiple repeat readings (at least 3) for each voltage and current and calculate a mean to reduce random errors

Safety Considerations

  • This is a very safe experiment, however, electrical components can get hot when used for a long period
  • Switch off the power supply right away if you smell burning
  • Make sure there are no liquids close to the equipment, as this could damage the electrical equipment

Worked Example

In an experiment, a student uses a variable resistor as an external load. The current flowing through the circuit is measured with a suitable milliammeter and the potential difference across the variable resistor is measured with a voltmeter for a range of resistance values.

The data collected was as follows:

Required practical 6 - Sample Data, downloadable AS & A Level Physics revision notes

Plot a graph of these results and determine the e.m.f. and the internal resistance directly from the graph.

Step 1: Plot the data on a graph of V against I and draw a line of best fit

Required practical 6 - Worked Example Graph (1), downloadable AS & A Level Physics revision notes

Step 2: Draw the largest triangle possible in order to calculate the gradient

Required practical 6 - Worked Example Graph (2), downloadable AS & A Level Physics revision notes

      Step 3: Determine the e.m.f. and the internal resistance from the graph

V = –rI + E

  • From this equation:
    • Gradient = –r (Ω)
    • Y-intercept = E (V)
  • Therefore:
    • Internal resistance, r = 22.7 Ω
    • E.m.f. E = 1.60 V

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top