Ohm’s law and the I–V characteristics practical
By Engr. Mehmood Kakar · Updated
Quick answer
Connect the component in series with an ammeter and a variable resistor, with a voltmeter across the component. Change the p.d. in steps, record the current each time, then reverse the connections for negative values and plot current against p.d. A fixed resistor gives a straight line through the origin, a filament lamp gives an S-shaped curve, and a diode only conducts one way.
Ohm’s law
For a resistor at constant temperature, the current is directly proportional to the potential difference across it. Doubling the p.d. doubles the current, and the resistance stays the same. The equation is V = I R, with V in volts, I in amps and R in ohms.
Components that follow this rule are called ohmic. A filament lamp and a diode are not, and this practical shows the difference.
The circuit
- A power supply (or battery) and a variable resistor in series, so you can change the p.d.
- The component being tested (resistor, filament lamp or diode).
- An ammeter in series with the component, to measure the current.
- A voltmeter in parallel across the component, to measure the p.d.
A diode is usually tested with a protective resistor in series, so the current can’t get large enough to damage it.
Method
- Set the variable resistor so the p.d. is as small as possible and switch on.
- Record the p.d. and the current.
- Adjust the variable resistor to increase the p.d. in steps, recording both readings each time.
- Swap the connections to the supply so the current flows the other way, and repeat to get negative values.
- Repeat the whole set of readings and calculate a mean current for each p.d.
- Plot current (y-axis) against potential difference (x-axis).
For the resistor, switch off between readings so it doesn’t heat up. Ohm’s law only holds at constant temperature.
Worked results table
Results for a resistor (example data):
| p.d. (V) | Current (A) | R = V ÷ I (Ω) |
|---|---|---|
| 1.5 | 0.10 | 15 |
| 3.0 | 0.20 | 15 |
| 4.5 | 0.30 | 15 |
The resistance is the same at every reading, so the resistor obeys Ohm’s law.
The three I–V graphs
- Fixed resistor: a straight line through the origin. Current is directly proportional to p.d., so resistance is constant.
- Filament lamp: an S-shaped curve that flattens at higher p.d. (positive and negative). The filament heats up, so its resistance increases.
- Diode: almost zero current for negative p.d. For positive p.d. the current stays near zero until about 0.6–0.7 V, then rises steeply.
Finding resistance from a graph
At GCSE, read a pair of values from the graph and use R = V ÷ I at that point.
A filament lamp has a current of 0.25 A at 2.0 V and 0.40 A at 6.0 V. Show that its resistance increases. (3 marks)
- At 2.0 V: R = 2.0 ÷ 0.25 = 8.0 Ω
- At 6.0 V: R = 6.0 ÷ 0.40 = 15 Ω
- The resistance rises from 8.0 Ω to 15 Ω as the p.d. increases
The filament gets hotter at higher current, so electrons collide with the vibrating metal ions more often.
Errors and improvements
- The resistor heating up increases its resistance and curves the graph. Keep the current small and switch off between readings.
- Zero errors: check the meters read zero before you start, and subtract any offset.
- Random errors in the readings: repeat each reading and take a mean, and ignore anomalous results when you calculate it.
- Too few points near a curve: take extra readings where the graph bends, such as around 0.6 V for a diode.
Practice
1. A resistor has a current of 0.50 A when the p.d. across it is 12 V. Calculate its resistance.
R = 12 ÷ 0.50 → R = 24 Ω
2. The same resistor has 9.0 V across it. What current should you expect if it obeys Ohm’s law?
I = V ÷ R = 9.0 ÷ 24 → I = 0.375 A (0.38 A to 2 s.f.)
3. Why should the ammeter be in series and the voltmeter in parallel?
The ammeter must have the same current flowing through it as the component. The voltmeter measures the difference in energy per coulomb between the two ends of the component, so it connects across it.
Frequently asked questions
What is Ohm’s law?
For a conductor at constant temperature, the current through it is directly proportional to the potential difference across it. Its resistance stays the same, so V = IR gives a straight-line I–V graph through the origin.
Why does a filament lamp’s resistance increase?
As the current increases, the filament gets hotter. The metal ions vibrate more, so electrons collide with them more often, which increases the resistance. The I–V graph curves and gets flatter.
Why does a diode only let current flow one way?
A diode has a very high resistance in the reverse direction, so almost no current flows that way. In the forward direction, current flows once the p.d. is above a small value.
How do you get negative values in the I–V practical?
Swap the connections to the power supply (or turn the component round) so the current flows the other way. Record those readings as negative p.d. and negative current.
Written by Engr. Mehmood Kakar
Physics educator and engineer (MSc Physics, B.Ed, M.Ed) with 8 years of hands-on teaching, from college students to teacher training. About the author · LinkedIn (opens in a new tab)
Checked against the AQA GCSE Physics (8463) specification on 14 November 2026.