Episode 109: Electrical characteristics

In this episode, students measure the current and voltage characteristics for several components, and identify ohmic and non-ohmic behaviour.

Summary

  • Student experiment: Further characteristics (40 minutes)
  • Student experiment – alternative version: Further characteristics (40 minutes)
  • Discussion: The results (10 minutes)
  • Student questions: On characteristics (30 minutes)
Apparatus to determine V-I characteristic of filament lamp

Student experiment: Further characteristics
Students determine the V-I characteristics for a carbon resistor, semiconductor diode and filament lamp.

This activity is best carried out individually (if space and apparatus allows this) so that each student has to construct and test his/her own circuit. One of the dangers of always working in pairs is that some students who lack confidence in circuit building will always avoid having to do it. This activity gives good practice in building testing and using a circuit designed to measure current and voltage.

They will need a reasonable amount of time to set up, check the circuit and begin to take readings. Make sure they all have correct circuits and that their meters are set to appropriate ranges (many take a long time to come to terms with multimeters). I would suggest about 40 min on the practical work itself. This should allow them all to collect data for all three components.

Some will work much faster than this so it may be worth having some additional activities available (e.g. the characteristic for thermistor).

Data collection can be handled in two ways: Simply record the data into a prepared table, or record directly into Excel or a similar spreadsheet package.

Episode 109-1: Electrical characteristics (Word, 59 KB)

Student experiment – alternative version
If you have access to datalogging equipment, this is a good opportunity to get students to set up and record results automatically.

If a pc projector is available it is worth collecting one set of data to use at the end of the practical session (you could generate this yourself or else harvest a reliable set from one of the students/groups).

Episode 109-2: Electrical characteristics – datalogging alternative  (Word, 31 KB)

Discussion: The results
In either case it is useful to bring the class together at the end of the practical session (say, 15 minutes before the end of the lesson) to discuss results. If you have collected some sample data you can show them how to process this in real time using Excel and a PC projector. Use this to instruct them about trend lines (don’t join the dots and don’t let Excel take over!). For some or all it may be worthwhile to recommend plotting by hand.

This may be the first time they have plotted graphs in Physics that include points in more than one quadrant, so this can be illustrated and discussed. Use terms such as ‘ohmic’ and ‘non-ohmic’ and encourage them to do the same. Reinforce the idea that an ohmic conductor is distinguished by a straight-line graph that passes through the origin. Remind them that resistance is the ratio of V to I not the gradient of the graph (particularly important when discussing the filament lamp).

Student questions: On characteristics
Students will get confused between V against I and I against V graphs. Both will be encountered so they should be prepared.

Episode 109-3: Lamp and resistor in series (Word, 41 KB)

Episode 109-4: Using non-ohmic behaviour (Word, 109 KB)

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Episode 109: Electrical characteristics (Word, 214 KB)