Track junctions and complete branches
Parallel: branch currents add at junctions, while every complete branch has the supply potential difference across it.
Continue where you stopped
The core idea
Electricity · Lesson 4 of 6 · about 25–35 min
What you need to understand
Parallel: branch currents add at junctions, while every complete branch has the supply potential difference across it.
Definitions
- parallel
- connected on separate branches across the same two junctionsFor example: Home appliances are connected in parallel.
Key idea
- LookTwo parallel branches carry 0.20 A and 0.10 A from a 3.0 V supply. State the source current and potential difference across each branch.
- ThinkParallel: branch currents add at junctions, while every complete branch has the supply potential difference across it.
- DoExplain household parallel wiring using independent operation and full supply potential difference across every complete branch.
Explanation
At a junction, current splits between branches and rejoins later. The source current equals the sum of branch currents, but branch currents need not be equal.
Each complete branch connects across the same two supply junctions, so the full supply potential difference is across every complete branch. Adding a branch provides another path and lowers effective resistance. Household parallel wiring lets appliances operate independently at the supply potential difference.
Pause and say it: Parallel: branch currents add at junctions, while every complete branch has the supply potential difference across it.
Common mistake
Tempting wrong idea: Current always divides equally between parallel branches.
Why it fails: Each parallel branch has the full supply voltage across it, so a branch with lower resistance carries a larger current. Branch currents are equal only when the branches are identical, but they always add up to the supply current.
Use this instead: The source current is the sum of branch currents, but the branch currents need not be equal.
Practical work
What to show: Explain household parallel wiring using independent operation and full supply potential difference across every complete branch.
Before you finish: Adding a parallel component does not have the same effect as adding one in series.
Practical link: The topic investigation is taught in “Choose the meter job before connecting it”.
6. Worked Examples
Modelled example 1
Complete a parallel-branch record
Problem
Branches carrying 0.20 A and 0.10 A are connected across a 3.0 V supply. State the source current and potential difference across each complete branch.
Study the worked solution
Use junctions and shared endpoints
Method
Add the branch currents and assign the supply potential difference to each complete branch.
Reason
Current splits and rejoins, while every complete branch connects across the same two source junctions.
Working
I = 0.20 A + 0.10 A = 0.30 A; each branch has 3.0 V across it.
Guided practice 2
Find a missing branch current
Problem
A 6.0 V source supplies 0.55 A to two parallel branches. One branch carries 0.20 A. Find the current in the other branch and state its potential difference.
Use the junction and branch rules
Hints
Hint 1: at the junction
The two branch currents add to the source current.
Hint 2: across the branch
Each complete branch spans the source junctions.
View solution step by step
Conserve current at the junction
Method
Subtract the known branch current from the source current.
Reason
The branch currents must add to the current entering the junction.
Working
missing current = 0.55 A − 0.20 A = 0.35 A
Use the complete-branch connection
Method
Assign the supply potential difference to the complete branch.
Reason
The branch connects across the same two junctions as the source.
Working
Its potential difference is 6.0 V.
Challenge 3
Switch on another household branch
Problem
One room lamp is operating on a household parallel circuit. A second lamp on another branch is switched on. Explain the supply potential difference across each active branch, the change in effective resistance and why the first lamp can remain on if the second is switched off.
Connect all three consequences to the branch layout
Hints
Hint 1: shared junctions
Each active branch remains connected across the same two supply junctions.
Hint 2: added path
Relate the extra complete path to effective resistance and independent operation.
View solution step by step
Reason from independent complete paths
Method
Each active branch has the supply potential difference across it, and adding the second branch lowers effective resistance.
Reason
The extra branch adds another complete path between the supply junctions.
Working
Opening the second branch does not break the first branch’s complete path.
Guided practice
Try it with support
Two parallel branches carry 0.20 A and 0.10 A from a 3.0 V supply. State the source current and the potential difference across each branch.
- Branch currents add.
- Each complete branch spans the supply junctions.
Check the guided answer
Answer: The source current is 0.30 A and each complete branch has 3.0 V across it.
Check: The branch potential difference is not a fraction of the supply potential difference.
Practise and continue
Practise this
What happens to effective resistance when another complete parallel branch is added? Explain from paths.
Need a hint?
- Ask whether current has another possible route.
Check your answer
Answer: Effective resistance decreases because the added branch provides another path for current.
Check: Adding a parallel component does not have the same effect as adding one in series.
Think like a scientist
Why can one household lamp fail without switching off every other lamp?
Check the reasoning
Each lamp has an independent complete branch across the supply, so opening one branch does not break the others and every intact branch retains the supply potential difference.
Remember: Household parallel wiring supports independent operation at the supply potential difference.
One-minute check
- Hide the page and explain track junctions and complete branches in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “Current always divides equally between parallel branches.”