OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall and apply the relationship between I, R and V, and that for some…
Full specification wording
Simple circuits: recall and apply the relationship between I, R and V, and that for some resistors the value of R remains constant but that in others it can change as the current changes
This criterion asks you to recall and use the relationship linking current I, resistance R and potential difference V, and to recognise that resistance is not always fixed.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 7 of 14
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This criterion asks you to recall and use the relationship linking current I, resistance R and potential difference V, and to recognise that resistance is not always fixed. The relationship is V = I × R, rearranged as I = V ÷ R or R = V ÷ I. Apply it with consistent units: current in amperes, potential difference in volts, resistance in ohms.
For example: A potential difference of 6 V across a 12 Ω resistor gives I = 6 ÷ 12 = 0.5 A. A fixed resistor keeps the same R whatever current flows, so doubling V doubles I. A filament lamp or thermistor behaves differently: its resistance changes as current changes, so the V–I graph curves rather than staying straight.
What earns the marks
- Recalls the relationship V = I × R and rearranges it correctly to find any one of the three quantities.
- Applies the relationship with current in amperes, potential difference in volts and resistance in ohms.
- States that a fixed resistor keeps a constant resistance as the current through it changes.
- Recognises that the resistance of some components, such as a filament lamp, changes as the current changes.
Common mistakes and how to put them right
Adds current and resistance instead of multiplying them; use V = I × R.
Uses milliamperes directly in the calculation without converting to amperes; convert first.
Assumes every resistor has a constant resistance.
Put it right: Recognise that some components change resistance as current changes.
Exam tips
- Write the equation, substitute the values with units, then give the answer with its unit.
- Check which quantity the question asks for and rearrange before substituting.
- Read the graph or data carefully to decide whether resistance is constant or changing.
Try the lesson
The start of the Simple circuits: recall and apply the relationship between I, R and V, and that for some resistors the value of R remains constant but that in others it can change as the current changes lesson, made from checked teaching notes for this specification point.
Have a go first
A fixed resistor of resistance 20 Ω has a potential difference of 4 V across it. What is the current through the resistor?
Listen
This criterion asks you to recall and use the relationship linking current I, resistance R and potential difference V, and to recognise that resistance is not always fixed. The relationship is V = I × R, rearranged as I = V ÷ R or R = V ÷ I. Apply it with consistent units: current in amperes, potential difference in volts, resistance in ohms. For example, a potential difference of 6 V across a 12 Ω resistor gives I = 6 ÷ 12 = 0.5 A. A fixed resistor keeps the same R whatever current flows, so doubling V doubles I. A filament lamp or thermistor behaves differently: its resistance changes as current changes, so the V–I graph curves rather than staying straight. A strong answer earns marks for these points. Recalls the relationship V = I × R and rearranges it correctly to find any one of the three quantities. Applies the relationship with current in amperes, potential difference in volts and resistance in ohms. States that a fixed resistor keeps a constant resistance as the current through it changes. Recognises that the resistance of some components, such as a filament lamp, changes as the current changes. Exam tip: Write the equation, substitute the values with units, then give the answer with its unit.
See it
Key words
Say what each word means before you turn the card.
Ohmic and non-ohmic behaviour
Try answering aloud or in your head before turning the card.
Interpreting V–I characteristics
Try answering aloud or in your head before turning the card.
See every definition
- Ohmic and non-ohmic behaviour
- A fixed resistor keeps the same R whatever current flows, but a filament lamp or thermistor has a resistance that changes as current changes.
- Interpreting V–I characteristics
- For a fixed resistor doubling V doubles I, while a filament lamp or thermistor gives a V–I graph that curves rather than staying straight.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Simple circuits: recall and apply the relationship between I, R and V, and that for some resistors the value of R remains constant but that in others it can change as the current changes
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge