I/V characteristic of a carbon resistor

Class practical

An example of the behaviour of a simple component, giving students opportunities to construct a circuit, gather data and perform some analysis.

Apparatus and materials

Power supply, 0 to 12 V, DC

Carbon film resistor - e.g. about 100 ohms, 1 W

Multimeters, 2, or 1 ammeter and 1 voltmeter of suitable ranges

Rheostat, e.g. 200 ohms, 2 W

Health & Safety and Technical notes

Some components may become hot enough to burn fingers.

Procedure

a Set up the circuit as shown below.

b Use the variable power supply and the variable resistor to vary the potential difference across the resistor, from 1.0 V to 4.0 V, in intervals of 0.5 V. Record pairs of potential difference and current values in the table.

You can record results for currents in the opposite direction by reversing the connections on the resistor.

Analysis
Plot a graph of current/A (y-axis) against potential difference/V (x-axis). Remember to include the readings for ‘negative’ voltages.

The resistance of the resistor is equal to the ratio of potential difference to current.

Use the graph to calculate the resistance of the resistor at a number of different currents.

Describe how the resistance changes with current. Is the resistance of the resistor the same for current in both directions?

The conductance of the resistor at a particular potential difference = current/potential difference.

Use the graph to calculate the conductance of the resistor at a number of different potential differences.

Teaching notes

1 The aim of this experiment is to develop confidence in setting up simple circuits and in taking careful measurements. If you decide that all students should attempt all the experiments in this collection, it may be sensible to start with this very simple one. The analysis is straightforward but students may well need reminding to convert mA into A where necessary. The graph should be a straight line through the origin. Many students will realize that, if the gradient of the line is constant and if it passes through the origin, all V/I values will be the same. However, there is much confusion about such ideas! See point 2 below.

2 It is often stated that the resistance of a component is the gradient of a V against I graph. Only for ohmic conductors (as in this experiment) does this happen to be true. Resistance is the ratio of V/I, so it is generally best to encourage students to take V/I ratios at specific points.

3 In this case a higher potential difference raises more electrons into the conduction band so the use of the term conductance is probably helpful.

4 Using a potential divider, as shown below, will enable students to get a full range of readings.

5 How Science Works extension: This experiment provides an excellent opportunity to focus on the range and number of results, as well as the analysis of them. Typically it yields an accurate set. The rheostat enables students to select their own range of results. You may want to encourage them to initially take maximum and minimum readings with the equipment and then select their range and justify it.

If they don’t think of it themselves, suggest that students take pairs of current and voltage readings as they increase the voltage from 0 V to the maximum. They then repeat these readings while reducing the voltage from the maximum to 0 V. This may help them to identify whether the resistance of the resistor remains constant when it is heated. (Turning the equipment off immediately after readings are taken and allowing the resistor to cool provides an alternative to this procedure but will considerably lengthen the time needed for the experiment. It is also possible to put the carbon resistor into a beaker of water to maintain the resistor at constant temperature.) Students could also change the direction of the current and repeat the other procedures.

You can use the fact that resistors are sold with a specified tolerance (and thus a variation in value) as the basis for a discussion about what a ‘true’ value really means in this case. Compare calculated resistance values with the manufacturer’s stated value or value range. Students can also be encouraged to identify the sources and nature of errors and uncertainties in the experimental method.

This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.

This experiment was safety-tested in January 2007