Projects
Episode 125: Introducing capacitors
It is helpful to start this topic by discussing capacitors, rather than the more abstract notion of ‘capacitance’.
Summary
Demonstration: A ‘super-capacitor’. (10 minutes)
Demonstration: Some capacitors in use. (10 minutes)
Student experiment + discussion (40 minutes): Charging and discharging capacitors.
Student questions: Charge storage. (20 minutes)
Demonstration:
A super capacitor
You should be able to capture the attention of your students with a short demonstration of a ‘super-capacitor’. This will allow the term ‘capacitor’ to be introduced and shows that (electrical) energy storage is one feature of these devices.
Demonstration:
Some capacitors in use
To emphasize the wide range of situations in which capacitors are used, show a few examples.
TAP 125-2 Where to find capacitors in everyday use
Student experiment + discussion (40 minutes): Charging and discharging capacitors The transient nature of the charge/discharge process can be looked at in a qualitative way using a range of capacitors and resistors and monitoring the current with an (analogue) ammeter. This is a good student experiment but you may have to give some initial guidance in how to discharge the capacitor between observations by connecting a lead across its terminals. |  |
TAP 125-3 Charging and discharging capacitors
The experimental work can be followed by a discussion which should bring out the following observations:
- that current flows for a short time (meters deflect briefly)
- that the current is initially large and then decreases
- that there is the same current in the wires connecting the capacitor to both the positive and negative terminals of the supply (meters deflect identically)
- that the value of the capacitor and resistor alter the magnitude of the current and the time for which it flows.
- further discussion should emphasize the underlying physics explanations:
- that electrons are being removed from one plate, while others are being added to the other, during the ‘charging’ process
- that as this charge increases the potential difference across the capacitor increases. (The experiment uses a number of cells and assumes that each has the same emf)
- that this causes a reduction in the flow of charge
- until the pd across the capacitor equals the emf of the supply.
- thus a ‘charged’ capacitor has equal but opposite charges on the two plates so that the total charge is zero.
[There is some advantage in looking at the time variation of the current and/or voltage using either an oscilloscope or datalogger but this can be left until Episode 129.]
Student questions:
Charge storage
Questions will reinforce the discussion or get the students to think through the ideas about charge and its storage for themselves.
TAP 125-4 Questions on charging capacitors
Download Word version of Episode 125 (116 KB)
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