Unit 4: Principles of Electrical and Electronic
Engineering
LO1: Understand fundamental electrical principles
– Maximum Power Transfer
Student activity sheet
Activity 1
The maximum power transfer theorem states that the maximum amount of power will be dissipated by a
load resistance when the value of that load resistance is equal to the equivalent resistance of the
network. But - is this true?
Look at the circuit below which shows a battery with internal resistance RI connected to a load resistor
RL.
RI is the equivalent resistance of this network. The internal voltage of the battery (VI) is also shown – this
is not the voltage on the battery terminals! A current ‘I’ flows in the circuit.
Current I
Battery
Internal Resistance RI
Load Resistor RL
Internal Voltage VI
December 2014
1. If the internal resistance of the battery is 200 and the internal battery voltage is 9v, what value
of load resistance will result in maximum power being transferred?
2. Plot a graph of power dissipated in the load resistor over the range 50 to 400 (in steps of
10). The graph will show load (horizontal axis) vs. load power (vertical axis).
Hint: you will need to tabulate your results before plotting a graph, and work out current and power for
each value of load resistance. Part of the table is shown below with the first two rows completed.
Load Resistor Value RL
Circuit Current I
Load Power P
50
0.036A
0.0648W
60
0.034615A
0.0719W
70
80
An alternative way of creating your table and plotting a graph is to use a spreadsheet.
What do you notice about the graph – and does it prove that the maximum power transfer theorem is
correct?
December 2014
Activity 2
It is not possible, in reality, to measure directly the internal resistance of a battery but we can still
determine its internal resistance.
Look at the following video tutorial which explains how to measure the internal resistance of a battery.
http://youtu.be/HZUDFcOSe48
Devise an experiment to determine the internal resistance of a battery.
If you have access to a battery, load resistor and multimeter (that will measure volts and amps) then
carry out an experiment to determine the internal resistance of the battery.
Write up your findings and show (as in the video) the required calculations. Your report should include a
list of equipment used, method, results (including all calculations) and conclusions.
What value of load resistance connected to your battery will result in maximum power being transferred?
December 2014