AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Current, resistance and potential difference: V = I R
This is the symbolic form of the relationship between potential difference (V), current (I) and resistance (R).
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This is the symbolic form of the relationship between potential difference (V), current (I) and resistance (R). V is measured in volts, I in amperes and R in ohms. The equation allows any one quantity to be found when the other two are known. For example, a 12 V supply driving 0.50 A through a resistor gives R = V ÷ I = 12 ÷ 0.50 = 24 Ω. Rearranged forms are I = V ÷ R and R = V ÷ I. The equation describes ohmic behaviour at constant temperature; for a filament lamp the resistance rises as the current increases, so V and I are not directly proportional across the whole range. When using the equation, convert mA to A and kΩ to Ω first, then substitute and give the answer with the correct unit.
What earns the marks
- Write the equation as V = I R and identify V, I and R with their units volts, amperes and ohms.
- Substitute values into V = I R correctly, including conversion of mA to A and kΩ to Ω.
- Rearrange to I = V ÷ R or R = V ÷ I and carry out the calculation accurately.
- Explain that the equation represents Ohm's law for an ohmic conductor at constant temperature.
- Use the equation to compare components, for example showing that doubling resistance halves current at constant potential difference.
Common mistakes and how to put them right
Treating V = I R as V = I ÷ R.
Put it right by remembering that potential difference is the product of current and resistance.
Using inconsistent units such as mA with kΩ without converting.
Put it right by converting all values to A and Ω first.
Applying a single constant resistance to a filament lamp across a range of currents.
Put it right by stating that its resistance changes as temperature changes.
Exam tips
- Quote the equation, substitute the known values, then rearrange only if needed so your working is clear.
- Give the final answer to an appropriate number of significant figures and always include the unit.
- In explain questions, link a change in resistance to the effect on current at constant potential difference using the equation.
Try the lesson
The start of the Current, resistance and potential difference: V = I R lesson, made from checked teaching notes for this specification point.
Have a go first
A resistor has a resistance of 6 Ω. A current of 3 A flows through it. Using V = I R, what is the potential difference across the resistor?
Key words
Say what each word means before you turn the card.
Ohm's law relationship
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Equation manipulation and substitution
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Units and conversions
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Ohmic versus non-ohmic behaviour
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See every definition
- Ohm's law relationship
- The equation V = I R describes ohmic behaviour at constant temperature, where V is potential difference, I is current and R is resistance.
- Equation manipulation and substitution
- Rearranged forms are I = V ÷ R and R = V ÷ I, allowing any one quantity to be found when the other two are known.
- Units and conversions
- V is measured in volts, I in amperes and R in ohms, so convert mA to A and kΩ to Ω first, then substitute and give the answer with the correct unit.
- Ohmic versus non-ohmic behaviour
- The equation describes ohmic behaviour at constant temperature, but for a filament lamp the resistance rises as the current increases, so V and I are not directly proportional across the whole range.
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