Use fixed resistors variable resistors and heaters
Recognize the resistor family and distinguish adjusting resistance from choosing a useful heating output.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
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.
Sources for this lesson
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.