Follow the same current around a series circuit
Explain why one unbranched loop has the same steady current at every point, even across different loads.
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
- Describe current as charge-flow rate.
- Trace a complete conducting path.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does current describe?
What you will learn
- Identify an unbranched series path.
- Compare current readings before and after its loads.
Count paths rather than components
A series circuit has one conducting path through its components. In steady operation the current is the same at every point in that unbranched path. A current of 0.40 A before a lamp is also 0.40 A after it and through a following resistor. Different component resistance does not give a different current within the same series path.
If charge entered a section persistently faster than it left, charge would accumulate there. In the steady circuit model this does not occur. A lamp transfers energy to light and heating without consuming the charge that circulates through it.
Keep the comparison within one circuit
Changing a resistor can change the current in the entire loop under the same source conditions. The new current is still common to its series components. Do not confuse that before-and-after change with current decreasing as it travels around an unchanged loop.
An open contact anywhere in the sole path interrupts the simple loop current everywhere. A voltmeter measurement branch makes the diagram technically branched; treat its current as negligible only when that idealization is stated. Here, comparisons concern a steady unbranched path or ideal non-loading measurements.
Sources for this lesson
Pause and explain
Two unequal resistors are in one steady unbranched loop. Which current comparison is correct?
Worked example
An ammeter before two series loads reads 0.40 A. What would an ideal series ammeter after the loads read?
Show the worked solution
- Both meters are in the same unbranched path.
- Steady charge flow is the same around that path.
- The second meter reads 0.40 A.
Answer 0.40 A.
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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.
An ammeter reads 0.40 A before two series loads. State the current through each and in the return wire.
Give me a hint
Identify the single path before assigning any values.
Compare my reasoning
- The loads and return form one unbranched loop.
- The current is the same at each point in steady operation.
- Each load and the return carry 0.40 A.
0.40 A through each load and the return.
Look for these in your work
- Name all requested points.
- Use the same steady-path condition.
A series resistor is changed and the loop current falls from 0.60 A to 0.30 A. Does the lamp current remain 0.60 A? Explain.
Give me a hint
Compare the whole circuit before and after its change.
Compare my reasoning
- The changed circuit has a new common loop current.
- The lamp shares that series path.
- Its new current is 0.30 A, not its earlier 0.60 A.
No; the lamp current becomes 0.30 A in the changed series loop.
Look for these in your work
- Distinguish two operating states.
- Keep equality within each state.
A learner records 0.40 A before a lamp and predicts 0.10 A after it because light consumed charge. Correct the explanation.
Give me a hint
Use conservation of charge rather than conservation of useful light.
Compare my reasoning
- The lamp receives energy per charge.
- The circulating charge is not consumed as light.
- In the steady unbranched model, the current after it remains 0.40 A.
0.40 A after the lamp; energy is transferred while charge continues.
Look for these in your work
- Correct the predicted value.
- Separate charge and energy.
Common mistakes
- Energy is transferred; charge is not used up.
- Changing a load can change the current everywhere.
- A real measurement branch can invalidate an unbranched assumption.
What can you explain now?
How does steady series current compare before and after a lamp?
Compare with the explanation
It is the same.
The lamp transfers energy without consuming circulating charge.
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Make it stick
Use one common steady current in an unbranched series loop.
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.