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 and 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.

Episode 125-1: Super capacitor (Word, 27 KB)

Demonstration: Some capacitors in use
To emphasize the wide range of situations in which capacitors are used, show a few examples.

Episode 125-2 Where to find capacitors in everyday useĀ (Word, 24 KB)

Student experiment and 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.

Episode 125-3 Charging and discharging capacitorsĀ (Word, 42 KB)

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 emphasise 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.

Episode 125-4: Questions on charging capacitors (Word, 49 KB)

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Episode 125: Introducing capacitors (Word, 125 KB)