Use the same potential difference across parallel branches
Apply a shared endpoint p.d. to each complete branch while distinguishing whole-branch and component voltages.
Section P4.3.2 · Series and parallel circuits
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Identify two common junctions.
- Add series p.d.s.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What defines a potential difference measurement?
What you will learn
- Use equal p.d. across parallel arrangements.
- Calculate internal series shares within one branch.
Compare the same endpoint pair
Every parallel branch connects across the same two junctions, so the p.d. across the arrangement equals the p.d. across one whole branch. If VAB = 6 V, each A-to-B branch has 6 V across it. Ideal connecting wires within a junction are treated as having negligible p.d. drop.
A branch may contain more than one series component. Its whole p.d. is 6 V, but one component can have 2 V and another 4 V. Assigning 6 V to every component inside the branch would confuse whole-branch endpoints with individual component endpoints.
Do not infer equal currents
At 6 V, a 3 Ω resistor branch carries 2 A and a 6 Ω resistor branch carries 1 A under a constant-resistor model. Their p.d.s are equal while their currents differ. Together they draw 3 A from the ideal source.
Non-negligible connecting-wire drops or source changes require a revised model. An ammeter or load placed in a shared feed is not automatically itself a parallel branch. Label A and B and check both connections before applying the shared-p.d. rule.
Sources for this lesson
Pause and explain
A 6 V branch contains two series components, one at 2 V. What is the other drop?
Worked example
Two branches span A and B at 6 V. One branch contains series drops of 2 V and Vx. Find the branch total and Vx.
Show the worked solution
- The whole branch p.d. is 6 V.
- Its series drops sum to that total.
- Vx = 6 − 2 = 4 V.
Answer Whole branch 6 V; remaining component 4 V.
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.
State the whole-branch p.d.s for three complete branches between A and B when VAB = 6 V.
Give me a hint
All branches share the same endpoint pair.
Compare my reasoning
- Identify A and B for each branch.
- The p.d. between these points is 6 V.
- Each whole branch therefore has 6 V.
6 V across each whole branch.
Look for these in your work
- Use both endpoints.
- Distinguish whole branches from internal loads.
One 6 V branch contains two series loads with 2 V across the first. Find the other drop.
Give me a hint
Apply the series sum within that branch.
Compare my reasoning
- The branch total is 6 V.
- 2 + Vother = 6.
- Vother = 4 V.
4 V.
Look for these in your work
- Keep the branch total.
- Check the two drops sum to 6 V.
At 6 V, branches contain 3 Ω and 6 Ω constant resistors. Find branch and source currents and assess an equal-current claim.
Give me a hint
Apply I = V/R separately, then the junction balance.
Compare my reasoning
- The currents are 6/3 = 2 A and 6/6 = 1 A.
- The source supplies 2 + 1 = 3 A.
- Equal p.d. does not require equal current.
2 A and 1 A; source 3 A; the branch currents are unequal.
Look for these in your work
- Use the common p.d.
- Add the branch currents only at the junction.
Common mistakes
- Equal branch p.d. is not equal branch current.
- A whole branch can contain smaller series shares.
- Check both endpoints before using the rule.
What can you explain now?
Why do parallel branches have the same p.d.?
Compare with the explanation
They connect to the same two junctions.
Each whole branch measures the same endpoint potential difference.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Apply equal p.d. to whole branches sharing the same junction pair, then inspect their internal components.
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.