Current, Resistance and Potential Difference
Master Current, Resistance and Potential Difference for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Potential difference, current and resistance are related by V = IR, where resistance opposes the flow of charge.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The current through a component depends on the resistance of the component and the potential difference across it. The equation is potential difference = current × resistance, or V = IR.
Resistance is measured in ohms. The greater the resistance, the smaller the current for a given potential difference, because resistance opposes the flow of charge.
For some components the resistance is constant, and these are called ohmic conductors. A fixed resistor at constant temperature is ohmic, so current is directly proportional to potential difference and its I–V graph is a straight line through the origin. A filament lamp is not ohmic, because its resistance increases as it heats up.
Revision notes
The equation
Potential difference = current × resistance, V = IR.
Potential difference in volts, current in amperes, resistance in ohms. Rearranged: I = V ÷ R and R = V ÷ I.
What resistance does
Resistance opposes the flow of charge.
For a fixed potential difference, a greater resistance gives a smaller current. This is why a variable resistor can be used to control the current in a circuit.
Ohmic conductors
An ohmic conductor has a constant resistance, so the current through it is directly proportional to the potential difference across it.
Its I–V graph is a straight line through the origin. A fixed resistor at constant temperature is ohmic; a filament lamp is not, because heating increases its resistance.
Key points
- Potential difference = current × resistance.
- Resistance is measured in ohms.
- Greater resistance gives a smaller current.
- An ohmic conductor has constant resistance.
- Its I–V graph is a straight line through the origin.
- A filament lamp is not ohmic.
Worked examples
Example 1
A current of 2 A flows through a resistor of 6 Ω. Calculate the potential difference across it. [2 marks]
Model answer
Potential difference = current × resistance = 2 × 6 (1 mark) = 12 V (1 mark).
Example 2
A potential difference of 12 V produces a current of 0.5 A. Calculate the resistance. [2 marks]
Model answer
Resistance = potential difference ÷ current = 12 ÷ 0.5 (1 mark) = 24 Ω (1 mark).
Example 3
Explain what is meant by an ohmic conductor. [2 marks]
Model answer
An ohmic conductor is one whose resistance remains constant (1 mark), so the current through it is directly proportional to the potential difference across it (1 mark).
Common mistakes
Rearranging the equation incorrectly.
V = IR, so R = V ÷ I and I = V ÷ R.
Saying a filament lamp is ohmic.
Its resistance increases as it heats up, so its graph curves.
Forgetting the constant temperature condition.
A resistor is ohmic only while its temperature stays constant.
Using the wrong unit for resistance.
Resistance is measured in ohms, symbol Ω.
Exam tips
- Write V = IR before rearranging.
- State ohms as the unit for resistance.
- Say directly proportional when describing an ohmic conductor.
- Mention constant temperature as the condition.
Key terms
- Resistance
- The opposition to the flow of charge, in ohms.
- Potential difference
- The energy transferred per coulomb of charge, in volts.
- Ohmic conductor
- A component with constant resistance.
- Ohm
- The unit of resistance, symbol Ω.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.