Compare source and branch currents in parallel
Explain how positive conducting branches share the total current and what an open branch changes.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Identify common parallel junctions.
- Describe charge flow through a path.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What defines parallel branches?
What you will learn
- Compare source current with each positive branch current.
- State why the comparison needs conducting branches.
Follow the split and join
In a parallel circuit the current from the source splits between the branches and joins again on return. When two or more branches carry positive current in the same source-to-return sense, the source current is larger than the current in each single branch because it supplies the other conducting branches too.
The branches need not carry equal currents. Their component behaviour and common driving conditions determine their currents. An equal split requires suitable equal branch behaviour, not merely the word parallel. Numerical junction balances are a separately labelled Supplement requirement.
Keep the stated conditions
If one of two branches is open and carries zero current, the source current can equal the current in the remaining branch. This is consistent with the split-and-join account; the usual strict larger-than statement assumes more than one positive conducting branch. A zero branch is not a smaller positive contribution.
An ammeter in the common source path and one within a branch measure different paths. A common interruption stops every branch, while a single branch interruption can leave others operating. The comparison is for a steady idealized circuit, not a guarantee about sources with additional internal or reverse-current paths.
Sources for this lesson
Pause and explain
One of two branches is open. Which source-current comparison is possible?
Worked example
Two lamp branches both conduct from source to return. Compare the source current with either branch current.
Show the worked solution
- The source feeds both positive branch flows.
- Each single branch is only part of that flow.
- The source current is greater than either branch current.
Answer Source current is larger than either individual conducting branch current.
Try a topic test
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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.
Two positive lamp branch currents flow from source to return. Rank the source current relative to each.
Give me a hint
Account for both conducting contributions qualitatively.
Compare my reasoning
- The source current reaches the junction.
- It supplies both branch flows.
- It is larger than each individual branch flow.
Source current is larger than either individual branch current.
Look for these in your work
- Compare both branches.
- State that both conduct positively.
The upper branch opens while the lower remains conducting. Compare the source current with lower branch current in the ideal two-branch model.
Give me a hint
The open branch has no current contribution.
Compare my reasoning
- The upper contribution is zero.
- The source supplies only the lower path.
- Source current equals the lower branch current.
They are equal under this model.
Look for these in your work
- Use the zero-current condition.
- Preserve the complete lower path.
A learner says every parallel circuit splits current equally and always has source current strictly larger than a branch even if all others are open. Correct both claims.
Give me a hint
Separate matching branch behaviour and positive contributions.
Compare my reasoning
- Unequal loads can carry unequal branch currents.
- Only other positive conducting flows make the source current strictly larger.
- With a single conducting branch it can equal that branch current.
Equal division is not automatic; strict inequality requires other positive conducting branches.
Look for these in your work
- Correct both generalizations.
- Use the stated operating conditions.
Common mistakes
- Parallel does not guarantee equal branch currents.
- A zero-current open branch changes a strict comparison.
- Place meters in the intended paths.
What can you explain now?
With two positive conducting branches, how does source current compare to either branch?
Compare with the explanation
It is larger.
The source supplies the current in the other conducting branch as well.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Compare source and branch currents with the positive-conducting-branch condition explicit.
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.