Compare conductor p d when current is held constant
Apply the resistance trend under the specific constant-current condition and distinguish it from fixed supply voltage.
Section 4.3.3 · Action and use of circuit components
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Use V = IR.
- Distinguish a controlled quantity and a changing quantity.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which relation connects p.d., current and resistance?
What you will learn
- State the constant-current resistance–p.d. trend.
- Identify when the condition is not met.
Keep current fixed
For a conductor, p.d. increases as resistance increases when current is held constant. V = IR makes the condition explicit: with the same I, a larger R requires a larger V. At 0.20 A, resistances of 10 Ω and 20 Ω require 2 V and 4 V respectively.
A constant-current comparison is a stated operating condition, not an automatic property of every cell circuit. The source or experimental control must provide the required p.d. while maintaining that current within its range. A diagram label cannot guarantee unlimited output voltage.
Distinguish a fixed-p.d. comparison
If a conductor instead remains across a fixed 4 V supply and its resistance rises from 10 Ω to 20 Ω, its p.d. stays 4 V while current falls from 0.40 A to 0.20 A. The same resistance increase therefore gives different quantity changes under different controls.
A thermistor, LDR or adjustable resistor can change resistance, but the resulting voltage trend depends on its surrounding circuit and control conditions. Identify the conductor and what is held constant before predicting an output. This lesson establishes the Core relation; divider redistribution is separately labelled Supplement.
Pause and explain
A conductor remains across a fixed 4 V supply while its resistance doubles. What is its p.d.?
Worked example
Current is held at 0.20 A while a conductor resistance rises from 10 Ω to 20 Ω. Find its p.d. in each state.
Show the worked solution
- Use the same current for both states.
- First V = 0.20 × 10 = 2 V.
- Second V = 0.20 × 20 = 4 V.
Answer P.d. increases from 2 V to 4 V at constant 0.20 A.
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.
At constant 0.20 A, compare p.d.s for 10 Ω and 20 Ω.
Give me a hint
Use the same current in both V = IR calculations.
Compare my reasoning
- Vfirst = 0.20 × 10 = 2 V.
- Vsecond = 0.20 × 20 = 4 V.
- The p.d. doubles when resistance doubles at fixed current.
2 V and 4 V.
Look for these in your work
- Keep current fixed.
- State both operating states.
At constant 0.10 A, a conductor changes from 30 Ω to 50 Ω. Find the p.d.s and increase.
Give me a hint
Calculate each state before taking the difference.
Compare my reasoning
- Vfirst = 0.10 × 30 = 3 V.
- Vsecond = 0.10 × 50 = 5 V.
- The increase is 5 − 3 = 2 V.
3 V to 5 V; increase 2 V.
Look for these in your work
- Use the stated current.
- Distinguish value and change.
A 10 Ω conductor changes to 20 Ω across fixed 4 V. Find current in both states and assess a claim that its p.d. must rise.
Give me a hint
Identify which quantity this comparison actually holds fixed.
Compare my reasoning
- Current initially is 4/10 = 0.40 A.
- Current finally is 4/20 = 0.20 A.
- P.d. stays 4 V because the source condition fixes it.
0.40 A to 0.20 A; p.d. remains 4 V, so the claimed rise is incorrect.
Look for these in your work
- Use the fixed-p.d. condition.
- Contrast it with constant current.
Common mistakes
- The trend requires constant current.
- Fixed voltage gives a different comparison.
- A real source has an operating range.
What can you explain now?
At constant current, what happens to p.d. when resistance increases?
Compare with the explanation
P.d. increases.
V = IR links a larger resistance to a larger p.d. when the current stays fixed.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
State the constant-current condition before applying the increasing-resistance increasing-p.d. trend.
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.