Electricity lesson
Measuring Resistance and Ohmic Conductors: Learn how to measure resistance using a circuit, draw circuit diagrams, and understand the behaviour of ohmic…
Measuring Resistance and Ohmic Conductors is part of Current, potential difference and resistance in GCSE AQA Higher Physics.
4 min
Reading time
24
Lessons in this topic
GCSE AQA Higher
Pathway
Expert reviewed
Content review
Measuring Resistance and Ohmic Conductors: Learn how to measure resistance using a circuit, draw circuit diagrams, and understand the behaviour of ohmic…
Use this lesson to stay inside Current, potential difference and resistance while connecting the detail back to the wider Electricity topic.
Read the core explanations from Measuring Resistance and Ohmic Conductors before testing yourself from memory.
Every electrical component fights against the flow of current - this opposition is called resistance. But here's the puzzle: some components have fixed resistance whilst others change their resistance as conditions vary.
Understanding resistance is crucial for designing safe circuits and predicting how components will behave. In your phone, computer, or any electronic device, engineers need to know exactly how much current will flow through each component.
The key question we'll answer is: how do we actually measure resistance, and why do some components follow predictable patterns whilst others don't?
Think of an ammeter like a turnstile that counts people passing through - it has to be placed directly in the path of the current. If you connect it in parallel, you'll create a short circuit and potentially damage the ammeter.
A voltmeter measures the 'electrical pressure' difference between two points. Like measuring the height difference between two floors, you need to connect to both points simultaneously - hence the parallel connection.
To measure resistance, we need to find both the current through a component and the potential difference across it. This requires a carefully designed circuit with the right connections.
The basic circuit needs: a power supply (battery or power pack), the component being tested, an ammeter in series, and a voltmeter in parallel. We often add a variable resistor to control the current.
Here's the crucial part: the ammeter goes in series because current is the same everywhere in a series circuit. The voltmeter goes in parallel because it measures the potential difference between two specific points.
The variable resistor (rheostat) allows us to change the current and take multiple readings. This is essential because we need to see how the component behaves under different conditions.
The key insight from this practical is that different components behave very differently. Some give straight-line graphs (ohmic), others give curves (non-ohmic).
Once we have current and potential difference measurements, we can calculate resistance using Ohm's law. This fundamental relationship connects the three key electrical quantities.
We can rearrange this formula to find resistance: $R = \frac{V}{I}$. This tells us that resistance equals potential difference divided by current.
The units work out perfectly: volts divided by amperes gives ohms. One ohm is the resistance of a component that allows one ampere of current when one volt is applied across it.
Not all components follow Ohm's law. Understanding the difference between ohmic and non-ohmic conductors is crucial for predicting circuit behaviour.
Examples of ohmic conductors include fixed resistors and metal wires at constant temperature. When you plot current against potential difference, you get a straight line through the origin.
The key phrase is 'at constant temperature'. Even metals become non-ohmic if they heat up significantly, because resistance increases with temperature.
Non-ohmic conductors include filament lamps, diodes, and thermistors. These components have resistance that changes as conditions change, giving curved I-V graphs.
For a filament lamp, as current increases, the filament gets hotter. Higher temperature means higher resistance, so the current doesn't increase as much as you'd expect from Ohm's law.
Notice how the fixed resistor gives a straight line (constant gradient = constant resistance), whilst the filament lamp curves (decreasing gradient = increasing resistance).
Resistance questions appear frequently in GCSE physics papers. Understanding the common pitfalls will help you avoid losing valuable marks.
Circuit diagram questions test whether you can draw the correct symbols and connections. Remember: ammeter in series (same current path), voltmeter in parallel (measuring across the component).
Calculation questions usually follow the pattern: state formula, substitute values with units, calculate answer with units. Each step typically earns one mark.
Graph interpretation questions ask you to identify ohmic behaviour (straight line) or explain why components are non-ohmic (resistance changes with temperature or other factors).
The most common mistake is confusing series and parallel connections for meters. Practice drawing circuits until this becomes automatic - it's tested in almost every electricity paper.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Learn how to measure resistance using a circuit, draw circuit diagrams, and understand the behaviour of ohmic conductors.
By the end of this lesson, you should be able to Explain the design and use of a circuit to measure the resistance of a component; Draw appropriate circuit diagrams using correct circuit symbols; Calculate resistance using current and potential difference measurements; Explain the difference between ohmic and non-ohmic conductors.
Read each section, then close the notes and explain the idea from memory. Check your explanation against the page and finish with an exam question from Electricity.