Distinguish conventional current and electron flow
Draw conventional current from positive to negative externally and electron flow in the opposite direction in metallic wires.
Section P4.2.2 · Electric current
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
- Explain metallic current using electrons.
- Identify positive and negative source terminals.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What sign does an electron carry?
What you will learn
- State conventional-current and metallic electron-flow directions.
- Keep external circuit arrows, charge signs and source processes distinct.
Use the conventional reference
Conventional current is defined in the direction of positive charge flow. In the external circuit of a d.c. source, it runs from the positive terminal through the components toward the negative terminal. This convention is used for standard current arrows even though the mobile carriers in metal wires are negative electrons.
Free-electron flow in the external metallic path is from the negative terminal toward the positive terminal, opposite to conventional current. Draw and label the two arrows separately; do not assign a new positive charge to an electron to make the arrows agree.
Keep the path boundary explicit
The terminal-to-terminal statements here refer to the external conducting path. Processes inside an energy-supplying source maintain the charge movement around the complete circuit and are not described by simply extending an external downhill arrow through the source. The source transfers energy; it does not create the circuit's carrier population from nothing.
An ammeter polarity uses a chosen conventional reference. A negative reading means the measured conventional direction is opposite to that reference. For a.c., the directions reverse with the source drive, so do not label one physical wire end as the permanent starting point for electron flow in all conditions.
Sources for this lesson
Pause and explain
Electrons move externally from terminal X to terminal Y in a d.c. metallic path. Which conventional arrow applies?
Worked example
A cell's positive terminal is connected to a metal path returning to its negative terminal. Compare conventional current and electron flow externally.
Show the worked solution
- Conventional current follows the positive-charge reference from + to −.
- Electrons carry negative charge.
- Their directed flow runs externally from − to +.
Answer Conventional current: + to −; electron flow: − to + in the external metal path.
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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.
State both external directions for a d.c. source with labelled + and − terminals and metal wires.
Give me a hint
Use a positive-charge conventional arrow and reverse it for electrons.
Compare my reasoning
- Conventional current goes externally from + to −.
- Electron flow goes externally from − to +.
- The arrows describe the same metallic conduction with different references.
Conventional + to −; electrons − to + externally.
Look for these in your work
- Label the conventions.
- State the external-path condition.
Electrons move right through a section of metal wire. Draw the conventional current arrow there.
Give me a hint
Reverse the movement direction of the negative carriers.
Compare my reasoning
- Electrons are negative.
- The conventional direction is positive-charge flow.
- The conventional arrow therefore points left.
Leftward conventional current.
Look for these in your work
- Use the carrier sign.
- Do not relabel the electrons as positive.
A learner draws + to − conventional current externally, then extends it unchanged through the energy-supplying cell. Explain the boundary mistake.
Give me a hint
Separate the external path from the source's energy-supplying processes.
Compare my reasoning
- The terminal direction statement describes the external circuit.
- The source maintains movement around the complete path and supplies energy.
- Its internal processes cannot be described by blindly extending the external terminal rule.
The external rule must not be applied unchanged to processes inside the source.
Look for these in your work
- Name the path boundary.
- Retain the source's energy-supplying role.
Common mistakes
- State which arrow convention is used.
- Keep electrons negative.
- Terminal statements refer to the external path.
What can you explain now?
Do external conventional-current and electron-flow arrows agree in a metal wire?
Compare with the explanation
No; they are opposite.
The conventional arrow uses positive charge direction while the moving electrons are negative.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw and label opposite conventional and electron arrows in the external d.c. metallic path.
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.