Describe a variable potential divider wiper
Trace the two parts of a resistive track and predict output changes with a specified reference end.
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Identify track ends and wiper.
- Use series voltage shares.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What are the three divider terminals?
What you will learn
- Explain variable-divider action.
- Interpret endpoints and uniform-track assumptions.
Split the track at the wiper
The supply is connected across a resistive track. The wiper divides the track into an upper resistance Rupper and a lower resistance Rlower. With negligible output loading, the same current passes through both parts, and their p.d.s sum to the supply p.d. The output here is defined from the wiper to the lower reference end.
Moving the wiper toward the upper supply end increases the lower track portion and its voltage share, so this defined output rises. Moving toward the lower reference end decreases that share, so output falls. Measure instead across the upper section and the trend reverses. Name the reference end before saying up or down.
Use endpoints and model conditions
For an ideal unloaded uniform track, the output is proportional to the fraction of total track resistance below the wiper. Across 12 V, a quarter of the resistance below it gives 3 V and three quarters gives 9 V. Equal distance fractions imply equal resistance fractions only for a uniform track; some practical tracks are deliberately non-linear.
At the lower end the ideal defined output is 0 V; at the upper end it is the whole supply p.d. An attached low-resistance load can change the shares because it draws current from the wiper. A divider is not an independent ideal voltage source whose output stays fixed under every load.
Pause and explain
An unloaded uniform 12 V track has one quarter of resistance below the wiper. What is the lower-reference output?
Worked example
An unloaded uniform divider is across 12 V. One quarter of its resistance is below the wiper. Find output from wiper to lower end.
Show the worked solution
- The lower resistance fraction is 1/4.
- Its voltage share is (1/4) × 12.
- The output is 3 V.
Answer 3 V in the ideal unloaded uniform-track model.
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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.
Describe the lower-reference output trend when an unloaded divider wiper moves from its lower end toward the upper end.
Give me a hint
Follow the resistance portion between wiper and reference.
Compare my reasoning
- At the lower end that portion is zero.
- Moving upward increases that portion.
- Its voltage share rises toward the full supply p.d.
Output rises from 0 V toward the supply p.d. in the ideal unloaded model.
Look for these in your work
- Name the reference end.
- State the loading assumption.
An unloaded uniform track spans 12 V. Three quarters of its resistance is below the wiper. Find lower and upper shares.
Give me a hint
Use complementary fractions of the total.
Compare my reasoning
- Lower share is 12 × 3/4 = 9 V.
- Upper fraction is 1/4, giving 3 V.
- The shares sum to 12 V.
Lower-reference output 9 V; upper share 3 V.
Look for these in your work
- Use resistance fractions.
- Check the sum.
A non-uniform track has its wiper halfway along its physical length. Can an exact half-supply output be inferred, especially with a load attached?
Give me a hint
Distinguish distance, resistance fraction and loading.
Compare my reasoning
- Half length need not be half resistance for a non-uniform track.
- A load can also change the lower effective resistance.
- Exact output requires resistance distribution and load data.
No; resistance distribution and output loading must be known.
Look for these in your work
- Identify both model conditions.
- Do not infer equal voltage from appearance alone.
Common mistakes
- Name the output reference end.
- Distance fractions need a uniform track.
- A load can alter the divider output.
What can you explain now?
For output measured to the lower end, what happens as the wiper moves toward the upper end?
Compare with the explanation
The output rises in the unloaded model.
The lower resistance portion and its voltage share increase.
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Make it stick
Trace the wiper resistance shares, identify the reference and state uniform-track and loading assumptions.
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