Investigating Ohm's Law
Vary the voltage across a resistor, record current readings, plot a V-I graph and find the resistance. Compare it with a filament lamp. Then test yourself and study the notes.
- Physics
- Lower to upper secondary
- About 30 minutes
- Includes exam and notes
Reviewed by Courseversity Academic Reviewer, Scientific Content Reviewer. Review date: 2026-10-28.
1. Learn: Ohm's law
What you will learn
- Draw and use a circuit to investigate how current depends on voltage.
- Connect an ammeter in series and a voltmeter in parallel.
- Record readings in a table and plot a voltage-current graph.
- Calculate resistance using R = V / I and from the gradient of a graph.
- Explain why a filament lamp is not an ohmic conductor.
Current is the flow of electric charge. Voltage (potential difference) is the energy transferred per unit charge. Resistance is how much a component opposes current.
Ohm's law says that for a conductor at constant temperature, the current is directly proportional to the voltage across it. You can write this as V = I x R.
SAFETY WARNINGS
- ELECTRICITY: Use only low-voltage supplies from batteries or a school power pack. Never touch mains electricity or bare wires connected to the mains.
- HOT COMPONENTS: Resistors and lamps can become hot when current flows. Switch off between readings and do not touch them until they cool.
- SHORT CIRCUITS: Never connect the terminals of a power supply directly together; it can cause burns and damage.
- WATER: Keep water away from electrical equipment.
- THIS PAGE: This is a virtual simulation. It does not replace supervised practical work.
Equipment
- Power supply. Provides an adjustable low voltage.
- Resistor. The component under test. Its resistance should stay constant if its temperature stays constant.
- Ammeter. Measures current in amperes. Connected in series.
- Voltmeter. Measures potential difference in volts. Connected in parallel across the component.
- Switch. Allows the circuit to be switched off between readings to avoid heating.
- Connecting leads. Join the components in the circuit.
- Filament lamp. A second component whose resistance changes as it heats.
2. Prepare
Predict before you start.
- If the voltage doubles, what should happen to the current in a resistor at constant temperature?
- What shape should the V-I graph be?
- Will a lamp behave in the same way? Why or why not?
3. Experiment
The simulation needs JavaScript. You can still read the notes below and take the examination if your browser allows scripts.
4. Observe
Complete the experiment step first. Your observation tasks will appear here.
5. Analyse
Calculations and conclusions will appear here once you have your readings.
6. Examine
Test what you have learned with a short practical examination. Questions are marked on the server, and every answer is explained afterwards. Marks help you learn; they are not an official grade.
7. Review answers
8. Experiment Learning Notes
These notes are always open to read, before and after the examination. After you submit an exam, sections that match questions you missed are highlighted.
A. Experiment overview
This experiment investigates how the current through a component depends on the voltage across it, and uses the results to find resistance.
It matters because all electrical devices depend on these relationships. The skills here (circuit building, measurement, graph plotting) are core physics skills.
B. Essential scientific principles
Ohm's law: the current through an ohmic conductor is directly proportional to the potential difference across it, provided the temperature stays constant.
In a metal wire, free electrons carry the current. When the temperature rises, the metal ions vibrate more, so electrons collide with them more often. This is why the resistance of a filament lamp increases as it gets hot.
C. Important apparatus and their functions
| Apparatus | Function and how it is connected |
|---|---|
| Ammeter | Measures current. Connected in series, so the same current flows through it. |
| Voltmeter | Measures potential difference. Connected in parallel across the component. |
| Power supply or cell | Provides the voltage. |
| Resistor | The component being tested. |
| Switch | Opens or closes the circuit. |
D. Essential experimental procedures
- Build the circuit with the power supply, switch, ammeter and resistor in series.
- Connect the voltmeter in parallel across the resistor only.
- Check the circuit before closing the switch.
- Set a low voltage and close the switch.
- Record the voltage and current from the meters.
- Switch off between readings so the component does not heat.
- Increase the voltage in steps and repeat for at least five different voltages.
- Plot the graph and draw a line of best fit.
E. Important observations and expected results
- As voltage increases, current increases in the same proportion for the resistor.
- The graph is a straight line through the origin.
- For the lamp, current increases more slowly at higher voltages because the filament heats up and its resistance rises.
- The lamp graph is a curve, not a straight line.
F. Important formulas, calculations and graphs
Worked example. A resistor has 6.0 V across it and 0.12 A through it. R = 6.0 / 0.12 = 50 ohms.
Gradient of a V-I graph: with V on the vertical axis and I on the horizontal axis, the gradient = change in V / change in I = resistance. Use a large triangle on the line of best fit.
Unit conversion: 250 mA = 250 / 1000 = 0.25 A.
G. Scientific interpretation and conclusions
If the graph is a straight line through the origin, current is proportional to voltage and the component obeys Ohm's law at constant temperature. The gradient gives the resistance.
For the filament lamp, the curve shows that resistance is not constant: the resistance increases with temperature. So the lamp is non-ohmic.
H. Sources of error and experimental improvements
| Problem | Effect | Improvement |
|---|---|---|
| Component heating up | Resistance increases; the graph curves | Switch off between readings; use low currents |
| Meter connected incorrectly | Wrong or no readings | Ammeter in series, voltmeter in parallel |
| Zero error on a meter | All readings shifted (systematic) | Check zero before starting and correct readings |
| Reading the scale at an angle | Parallax error | Read the meter straight on |
| Too few readings | Weak evidence for a pattern | Take at least five readings and repeat |
I. Laboratory safety and precautions
SAFETY WARNINGS
- ELECTRICITY: Use only low-voltage supplies from batteries or a school power pack. Never touch mains electricity or bare wires connected to the mains.
- HOT COMPONENTS: Resistors and lamps can become hot when current flows. Switch off between readings and do not touch them until they cool.
- SHORT CIRCUITS: Never connect the terminals of a power supply directly together; it can cause burns and damage.
- WATER: Keep water away from electrical equipment.
- THIS PAGE: This is a virtual simulation. It does not replace supervised practical work.
- Use only low-voltage supplies.
- Do not touch hot components.
- Switch off before changing the circuit.
- Never short-circuit a supply.
- Keep liquids away from electrical equipment.
J. Examination-relevant practical knowledge
These are important practical skills linked to this experiment. This is not a claim about how often any question has appeared in past papers; check your own syllabus and past papers.
- Drawing the circuit with correct symbols and meter positions.
- Explaining how to vary the voltage and why readings are repeated.
- Plotting a graph with labelled axes and a line of best fit.
- Calculating resistance from V and I or from a gradient.
- Explaining why the resistance of a lamp changes.
- Identifying sources of error and improvements.
K. Common mistakes and misconceptions
| Common mistake | Why it is incorrect | Correct scientific explanation |
|---|---|---|
| Connect the voltmeter in series. | A voltmeter has a very high resistance, which would block the current. | Connect the voltmeter in parallel across the component. |
| Resistance is the same for every component. | Different components have different resistances. | Resistance depends on the component and, for some, on temperature. |
| Join the points dot to dot. | Readings have small errors. | Draw a smooth line or straight line of best fit. |
| A curved V-I graph means the readings are wrong. | Some components are not ohmic. | A lamp graph curves because its resistance rises as it heats. |
| Use milliamperes directly in V = IR. | The formula needs amperes. | Convert mA to A by dividing by 1000. |
L. Practical examination tips
- Draw the circuit with a ruler and standard symbols.
- Use a table with headings and units for V and I.
- Plot on a large graph with sensible scales and labelled axes.
- Draw a line of best fit and use a large triangle for the gradient.
- Show your working for resistance with units.
- Link conclusions to the shape of the graph.
M. Real-world and professional applications
- Electronics: engineers choose resistors to control current in circuits.
- Lighting: filament lamp behaviour shows why resistance depends on temperature.
- Safety devices: understanding current and resistance helps design fuses and protection circuits.
- Sensors: thermistors and light-dependent resistors use changing resistance to measure conditions.
N. Essential points to remember
These key points summarise the experiment.
- Ohm's law: for an ohmic conductor at constant temperature, current is proportional to voltage.
- V = I x R, where V is in volts, I in amperes and R in ohms.
- A voltmeter is connected in parallel; an ammeter is connected in series.
- The V-I graph of a resistor at constant temperature is a straight line through the origin.
- The gradient of a V-I graph is the resistance.
- Resistance is measured in ohms.
- A filament lamp is not ohmic: its resistance increases as it gets hotter.
- The V-I graph of a filament lamp is a curve that becomes less steep.
- Switch off between readings so the component does not heat up.
- Use a table with headings and units.
- Plot voltage on the vertical axis and current on the horizontal axis, or the reverse, but state which.
- Draw a line of best fit through the points, not join dot to dot.
- Convert milliamperes to amperes by dividing by 1000.
- Repeat readings to improve reliability.
About these notes. Written from standard school physics practical methods.
9. Practise again
Mastery comes from repeating the practical and meeting new questions. Run the experiment again (the starting values change), then use Additional practice questions after marking.
Repeat the experiment Go to the examination
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