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Resistance is calculated with R = V ÷ I, giving ohms when volts are divided by amperes. Free GCSE AQA Physics notes with key definitions and 14 FAQs.
Measuring Resistance and Ohmic Conductors is part of Current, potential difference and resistance in GCSE AQA Higher Physics.
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Resistance is calculated with R = V ÷ I, giving ohms when volts are divided by amperes. Free GCSE AQA Physics notes with key definitions and 14 FAQs.
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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.
Worked example: Calculating Resistance from Measurements
A student measures 0.50 A flowing through a resistor when 6.0 V is applied across it. Calculate the resistance. [3 marks]
Answer: 12 Ω
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.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Resistance is calculated with R = V ÷ I, giving ohms when volts are divided by amperes. A resistor with 0.50 A flowing through it when 6.0 V is applied across it has a resistance of 6.0 ÷ 0.50 = 12 Ω.
An ohmic conductor is a component where current is directly proportional to potential difference at constant temperature, so its resistance remains constant. Fixed resistors and metal wires at constant temperature are ohmic, giving a straight-line I-V graph through the origin.
A variable resistor, or rheostat, allows the current in a circuit to be changed. In a resistance measurement circuit it allows multiple readings of current and potential difference to be taken, showing how the component behaves under different conditions.
An ammeter is an instrument that measures electric current in amperes (A). It must be connected in series with the component being measured, like a turnstile placed directly in the path of the current.
A voltmeter is an instrument that measures potential difference in volts (V). It must be connected in parallel across the component being measured, because it measures the difference between two specific points.
Examples of ohmic conductors are fixed resistors and metal wires at constant temperature. Their current is directly proportional to potential difference, giving a straight-line I-V graph through the origin.
To measure resistance, connect the component to a power supply with an ammeter in series and a voltmeter in parallel across it, often with a variable resistor to change the current. Then calculate resistance with R = V ÷ I for each pair of readings.
Examples of non-ohmic conductors are filament lamps, diodes and thermistors. Their resistance changes as conditions change, so they give curved I-V graphs rather than straight lines.
Connecting an ammeter in parallel creates a short circuit and could damage the ammeter. An ammeter must be connected in series with the component, directly in the path of the current.
One ohm is the resistance of a component that allows one ampere of current when one volt is applied across it. Volts divided by amperes gives ohms, so R = V/I.
An ammeter is connected in series because it must be directly in the path of the current, and current is the same everywhere in a series circuit. A voltmeter is connected in parallel because it measures the potential difference between two specific points across the component.
Measuring resistance needs a power supply (battery or power pack), the component being tested, an ammeter in series, a voltmeter in parallel and often a variable resistor to control the current. Resistance is then calculated with R = V/I.
A metal wire is only ohmic at constant temperature because resistance increases with temperature. Even metals become non-ohmic if they heat up significantly, so the key phrase for an ohmic conductor is 'at constant temperature'.
The gradient of an I-V graph shows how the resistance behaves: a constant gradient means constant resistance, as for a fixed resistor. A decreasing gradient, as for a filament lamp, means increasing resistance.