Episode 119: Preparation for EMF and internal resistance topic
The work on emf and internal resistance draws on ideas about voltage, current and charge that
were discussed in previous sections. The idea of emf
(electromotive force) has already been introduced but
may well need reinforcement, along with the definition of
the volt, and there is a lot to be gained by beginning this
work with a general discussion of energy transfers within
electric circuits. This should link to real-life examples
involving energy transfer from a chemical battery to
electrical work.
The work on ac and the oscilloscope may not be required
by the specification you are following, but is widely useful
(resourcefulphysics.org)
and may well be worth teaching as an extension. It is best
to introduce the oscilloscope with dc supplies before
teaching about ac, when it can be used to illustrate the link between peak and r.m.s. values.
Episode 120: Energy transfer in an electric circuit
Episode 121: EMF and Internal Resistance
Episode 122: Using an oscilloscope
Episode 123: Alternating Current
Advance warning
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
Students will:
1. Describe energy changes in an electric circuit.
2. Define the emf of a cell.
3. Demonstrate how the load on a cell reduces its terminal voltage.
4. Solve problems involving terminal voltage, ‘lost volts’, internal resistance and emf
5. Measuring emf and internal resistance.
6. Use an oscilloscope - measuring constant and varying voltages.
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7. 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 W = 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.
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