I-V characteristics required practical

In the I-V characteristics required practical, you change the potential difference across a component, measure the current through it each time, and plot current against potential difference. You do it for three components: a filament lamp, a resistor and a diode. It is required practical 4 in AQA's GCSE Physics and required practical 16 in Combined Science Trilogy. This guide covers the practical itself; our guide to I-V characteristics explains why each graph has its shape.

Aim

AQA's specification states the aim as: use circuit diagrams to construct appropriate circuits to investigate the I-V characteristics of a variety of circuit elements, including a filament lamp, a diode and a resistor at constant temperature.

Equipment

AQA's student sheet lists:

• a digital ammeter and a digital voltmeter;
• a milliammeter, for the diode;
• a variable resistor, a battery or power supply, and connecting leads;
• a filament lamp, a resistor and a diode;
• an extra resistor labelled P, to protect the diode.

The teacher notes suggest a 12 V, 24 W filament lamp, such as a ray box lamp. With a variable power supply you can change the potential difference directly and leave out the variable resistor.

Method

These steps follow AQA's student sheet, which runs the three parts in one lesson, in this order.

Activity 1: filament lamp

1. Set up the circuit in the diagram.
2. Record the ammeter and voltmeter readings in a table.
3. Adjust the variable resistor and record the new readings. Repeat to get several pairs of readings.
4. Swap the connections on the power supply. The readings should now be negative.
5. Record several more pairs of readings with the supply reversed.
6. Plot current against potential difference. With negative values, the origin goes in the middle of the graph paper.

Activity 2: resistor

Swap the supply leads back, replace the lamp with the resistor, and repeat steps 2 to 6. This time you should get a straight line passing through the origin.

Activity 3: diode

Swap the supply leads back. If you can, reduce the supply to less than 5 V. Connect resistor P in series, replace the ammeter with the milliammeter, and replace the resistor with the diode. Then repeat steps 2 to 6. The voltmeter still goes across the diode alone.

Variables

Results table

Use one table for each component, with positive and negative readings:

For the diode, record the current in mA, from the milliammeter.

How to analyse the results

Plot current on the y-axis against potential difference on the x-axis, then work out the resistance at each reading with:

AQA's handbook warns that many students think the gradient of the graph is the resistance. It isn't: the resistance at any point is 1 ÷ the gradient of a straight line from that point to the origin, which is the same as V ÷ I.

Example results for a filament lamp

Modelled readings for a 12 V, 24 W lamp, to show the shape: for illustration, not measured. Your own results will differ.

The resistance rises as the filament gets hotter. At its rated 12 V, the lamp takes 24 ÷ 12 = 2.0 A, so its resistance is 6.0 Ω.

For the resistor, current is directly proportional to potential difference, so the gradient of its line is 1 ÷ R. The Practical Data Lab finds the gradient of your best-fit line, with its uncertainty, from your readings.

Now try one in the course. Read the resistance from the graph, then press Mark to check it there.

Risks

Errors and improvements

• The resistor warms up. The aim says constant temperature, so keep the current small and switch off between readings.
• Too few readings where the graph bends. Take more readings where the lamp curve flattens and where the diode's current starts to rise.
• Diode currents too small to read. Use the milliammeter, as AQA's sheet does.

Common mistakes

• Putting the voltmeter in series. It goes in parallel across the component; the ammeter goes in series.
• Taking the gradient as the resistance. Use R = V ÷ I at the point.
• Plotting only positive values. Reverse the supply to show the full characteristic, especially for the diode.
• Axes the wrong way round. AQA plots current on the y-axis and potential difference on the x-axis.

The Circuit Builder has a ready-made Lamp I-V experiment with this circuit. Drag the variable resistor and select the lamp to see its I-V curve. It has LEDs but no ordinary diodes.

Try it

Circuit Builder

Build any DC circuit with meters, lamps, LEDs and sensors, and see live readings

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Practise physics in a short timed paper

GCSE Physics Higher gives you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so electricity and required practicals turn up regularly but not in every paper.

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