# I/V characteristic of a filament lamp

##### Class practical

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

#### Apparatus and materials

Filament lamp 12 V, 24 W

Power supply, 0 to 12 V, DC to supply up to 4 A

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

Rheostat, e.g. 8 ohm rated at 5 A

#### 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 lamp, from 1.0 V to 10.0 V in intervals of 1 volt. 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 lamp.

c Analysis. Plot a graph of current/A (y-axis) against potential difference/V (x-axis).

The resistance of the lamp at a particular potential difference = potential difference/current.

Use the graph to calculate the resistance of the lamp at a number of different potential differences.

Describe how the resistance changes with potential difference.

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

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

Describe how the conductance changes with potential difference.

#### Teaching notes

1 The aim of this experiment is to develop confidence in setting up simple circuits and in taking careful measurements. The analysis is fairly straightforward but students may well need reminding to convert mA into A where necessary.

2 It is often stated that the resistance of a component is the gradient of a V against I graph. This is not usually the case. Resistance is the ratio of V/I. It is therefore best to encourage students to take V/I ratios at specific points.

3 In the case of a filament lamp it is, in fact, the resistance that increases (rather than the number of charge carriers falling) due to increased lattice vibrations.

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

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