# Ohm's law

##### Class practical

This experiment looks at the relationship between current and potential difference (p.d.) for a length of resistance wire.

#### Apparatus and materials

Ammeter (1 amp), DC

Voltmeter (5 volt), DC

Eureka wire (34 SWG), 10-cm length

Power supply, low voltage, DC

Leads, 4 mm, 6

Crocodile clips, 2

Rheostat (10 ohms, at least 1A)

#### Health & Safety and Technical notes

Low voltage power supplies are not continuously variable. Many have nominal voltages of 1, 2, 4, 6 etc. In this case the current could be 1, 2, 4, 6 A since 10 cm of SWG 34 Eureka is about 1 Ω.

#### Procedure

**a **Set up the circuit shown. The length of Eureka wire acts as the resistance in the circuit.

**b** By adjusting the power supply, you can vary the p.d. across the Eureka wire. The ammeter will show corresponding values of the current through the wire. Keep the current small so that the temperature of the wire does not increase. (Adjust the rheostat at the beginning and then keep it constant.) Record a series of values of p.d. and current.

**c** Calculate the ratio p.d./current for each pair of values. Comment on the result.

**d **Draw a graph to represent the same data. From the graph, deduce a value for the ratio p.d./current.

#### Teaching notes

**1** Students should collect pairs of results for the potential difference across the wire, and current through the wire.

**2** A graph is then plotted of current against potential difference. It is a matter of taste whether current is plotted on the y-axis or the x-axis. It normally depends on what the experimenter is trying to find out.

The independent variable is normally the potential difference and so it could be plotted along the x-axis. The resultant current, the dependent variable, would be plotted on the y-axis. This would show how the current varies with potential difference. However, the ratio p.d./current is important to us, and so the axes would have to be reversed if the ratio were needed from the slope of the graph.

**3** Students should also calculate the ratio p.d./current for all pairs of results. (This is best done in a table.) It will be found that the ratio is constant, and this is defined to be the resistance of whatever we have connected the voltmeter across. The unit of resistance, one volt per amp, is known as the ohm.

**4 **The straight line graph through the origin indicates that the current is proportional to the potential difference driving it. It is this proportionality which is Ohm's law.

**5 **V/I = R is a definition of resistance and is NOT Ohm's law. Only if the resistance is constant as the potential difference increases is the material said to be Ohmic.

**6** Ohmic materials play only a small part in our lives. Electronic materials such as those based on semiconductors play an increasing role and are non-ohmic: they do not obey Ohm's Law. Ohm's Law assumed a position of great importance in the nineteenth century when telegraph lines were designed and electrical engineering was developing.

**7** 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 resistor into a beaker of water to maintain the resistor at a 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 was safety-checked in January 2007*