Use fixed resistors variable resistors and heaters
Recognize the resistor family and distinguish adjusting resistance from choosing a useful heating output.
Section 4.3.1 · Circuit diagrams and circuit components
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use the qualitative voltage–current–resistance relation.
- Identify electrical and heating transfers.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a resistor do in a conducting circuit?
What you will learn
- Draw fixed and variable resistor symbols.
- Explain resistance control and heater energy transfer.
Recognize the resistor family
The usual rectangular fixed-resistor symbol has a wire from each end. It represents a component with a specified resistance under its stated operating conditions. A variable resistor adds a diagonal adjustment arrow. Used as a two-terminal variable resistor, it changes the resistance in a path rather than providing a separate source voltage.
At fixed source p.d., increasing the total positive resistance of a simple series load path decreases its current. Keep the supply and other relevant conditions unchanged before assigning a trend. A real component may change resistance as it heats, so fixed here means not manually adjustable rather than guaranteed temperature independence.
Identify the useful output
A heater uses electrical input to produce a heating effect as its useful purpose. A labelled resistive-element symbol identifies the heater in our diagrams; its resistor-like appearance does not make it an energy source. Heating can occur in other resistive components too, but in a heater it is the intended output.
Draw the heater or resistor within the load path and label its role when the symbol is shared. A three-terminal potential divider uses an adjustable track differently and is introduced separately. Do not join its wiper and end terminals by guesswork or infer a heater temperature without thermal information.
Pause and explain
At fixed p.d., a positive path resistance increases. Which trend follows?
Worked example
A fixed d.c. supply feeds a heater and a two-terminal variable resistor in series. Predict the initial current change when total resistance is increased.
Show the worked solution
- The supply p.d. is unchanged.
- The resistance of the conducting path increases.
- The current decreases; heater temperature changes require a thermal account.
Answer Current decreases under the stated fixed-p.d. conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw a fixed resistor and a variable resistor and explain the added mark.
Give me a hint
Start with a rectangle and two terminal wires.
Compare my reasoning
- Draw the fixed resistor rectangle in the wire.
- Draw a second rectangle with a diagonal adjustment arrow.
- Identify the arrow as resistance adjustment rather than current direction.
The variable resistor adds a diagonal adjustment arrow to the resistor symbol.
Look for these in your work
- Use recognizable terminals.
- Explain the arrow meaning.
Explain a heater in a cell circuit using an energy pathway.
Give me a hint
Name the source input and intended load output.
Compare my reasoning
- The cell chemical store supplies electrical transfer.
- The heater receives electrical input.
- Its useful purpose is heating the intended object or surroundings.
Cell chemical store → electrical transfer → heater heating output.
Look for these in your work
- Identify the heater as a load.
- Do not describe charge as consumed fuel.
A fixed-p.d. circuit has an adjustable resistor. Its resistance is increased but a learner predicts current rises and assigns an exact heater temperature. Assess both predictions.
Give me a hint
Separate the electrical trend from unspecified thermal behaviour.
Compare my reasoning
- At the same p.d., larger positive total resistance gives smaller current.
- The arrow only marks adjustment.
- An exact temperature also needs heat-loss, time and material information.
Current decreases under the stated conditions; an exact heater temperature is not determined.
Look for these in your work
- Use the fixed-p.d. condition.
- Avoid an unsupported temperature value.
Common mistakes
- A variable resistor is not a variable voltage source.
- State what is held fixed before predicting current.
- A heater is a load, not a source.
What can you explain now?
What does the diagonal adjustment arrow on a resistor indicate?
Compare with the explanation
Variable resistance.
The arrow indicates an adjustable resistance rather than charge-flow direction.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw resistor symbols and explain resistance adjustment and useful heater output.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.