Investigating I-V Characteristics

Section 1 of 7Aim and variables

Aim

• Aim To construct circuits that investigate the I-V characteristics of a fixed resistor at constant temperature, a filament lamp and a diode.

Variables

• Variables Independent variable: the voltage across the component.
• Dependent variablethe current through the component.

Control variables

• Control variables The same power supply and the same wires and equipment throughout,
  • and the temperature of the resistor kept constant by using low voltages.

Section 2 of 7Equipment

Equipment	Purpose
Ammeter	Measures the current, connected in series
Voltmeter	Measures the voltage, connected in parallel across the component
Variable resistor	Changes the current and so the voltage across the component
Fixed resistor (100 to 500 Ω)	First component tested
Filament lamp and diode	Other components tested in turn
Low-voltage power supply	Drives current round the circuit
Wires	Complete the circuit

Section 3 of 7Method

1

Build the circuit with the fixed resistor, the ammeter in series and the voltmeter in parallel across the resistor.

2

Adjust the variable resistor to set a voltage, and read the current from the ammeter.

3

Take three current readings at each voltage and calculate an average.

4

Repeat for eight to ten voltages, increasing by about 0.5 V each time.

5

Switch off between readings so that the component and wires do not heat up.

6

Reverse the connections to the power supply and repeat to obtain readings for negative voltage and current.

7

Replace the resistor with the filament lamp and then the diode, repeating the whole procedure.

Section 4 of 7Processing and analysing results

1Tabulate

• Tabulate Record voltage, the three currents and the mean current in a clear table with units in the headings.

2Plot

• Plot Draw a graph of mean current against voltage for each component.

3Describe the shape

• Describe the shape A straight line through the origin means a linear conductor;
  • a curve means a non-linear conductor.

4Compare

• Compare Expect a straight line for the resistor and curves for the lamp and diode, and compare with the known I-V graphs.

Section 5 of 7Safety and evaluation

Reliable results

Safety

• Safety A thin wire carrying a high current gets very hot, so never touch it while the circuit is on.
• Components, especially the filament lamp, become hot, so handle them carefully.
• Switch off straight away if you smell burning and keep liquids away from the equipment.

Accuracy

• Accuracy Repeating the readings and averaging reduces the effect of random errors.
• Using a low voltage stops the heating effect changing the resistance of the resistor.

Identifying an unknown component

• Identifying an unknown component Zero current for negative voltages and a rapid rise for positive voltages indicates a diode or LED.

Section 6 of 7Key terms

Term	Meaning
Independent variable	The variable that is deliberately changed, here the voltage.
Dependent variable	The variable that is measured, here the current.
Control variable	A variable kept the same so that the test is fair.
Mean	The average of repeated readings, found by adding them and dividing by the number of readings.
Non-linear conductor	A component with a curved I-V graph whose resistance changes.

Section 7 of 7Exam technique

1

Placing the ammeter in parallel or the voltmeter in series, which gives wrong readings.

2

Leaving the circuit on between readings so that the components heat and change resistance.

3

Plotting voltage on the vertical axis when the question asks for an I-V graph, with current on the vertical axis.

4

Taking only one reading at each voltage, which makes results less reliable.

5

Forgetting to reverse the supply to obtain negative values, which are needed to show diode behaviour.