Electromotive Force
Electricity and Magnetism

Episode 119: Preparation for EMF and internal resistance topic

Teaching Guidance for 16-19 IOP TAP

It is essential to become well-versed in the use of an oscilloscope for displaying voltages. If you have access to more than one CRO, you are likely to find that they are not all identical, so it is vital to develop an understanding of how they work, rather than just getting by with one particular model.

Start by looking at the available CROs. (You may also have interfaces which allow you to use a computer as a CRO.) Learn to use the simplest one, and work up to the more complex models.

You should also find out how you can allow students to use CROs. Do you have enough for them to work individually, or in small groups? Or will you have to include one or two CRO-based experiments in a circus?

Once you are confident with a CRO, you will find it useful in many areas, such as the display of sound waves.

Main aims of this topic

EMF and internal resistance


Students will:

  • describe energy changes in an electric circuit
  • define the EMF of a cell
  • demonstrate how the load on a cell reduces its terminal voltage
  • solve problems involving terminal voltage, lost volts, internal resistance and EMF
  • measuring EMF and internal resistance
  • use an oscilloscope – measuring constant and varying voltages
  • describe alternating current in terms of r.m.s. and peak values

Prior knowledge

Students should be able to apply the equations  V = I × R and  E = Q × V , and Kirchhoff’s laws, to simple circuits.

Where this leads

Once students have completed this (and earlier) electrical topics, they should have a clear understanding of electric circuits. They should understand about current, voltage and resistance, and how these relate to energy transfers.

This will give them a much better understanding when they progress to studies of electromagnetism and electronic circuits.

Electromotive Force
appears in the relation ε=V+Ir
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