Distinguish direct and alternating current
Identify whether current direction stays the same or reverses, without assuming every direct current has constant magnitude.
Section 4.2.2 · Electric current
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Describe current as charge flow.
- Read a graph above and below a zero line.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a negative current sign mean on a stated reference?
What you will learn
- Distinguish d.c. from a.c. by direction.
- Interpret signs on a stated current–time reference.
Use direction as the distinction
Direct current, d.c., flows in one direction. Its magnitude can be steady or vary while keeping that direction. Alternating current, a.c., repeatedly reverses direction. In a current–time graph, positive and negative signs indicate opposite directions relative to a stated reference.
A flat positive trace is steady d.c., but an always-positive wavy trace can also be d.c. A trace repeatedly crossing zero and taking both signs represents alternating direction. Reaching zero momentarily does not alone establish reversal; inspect the signs on either side.
Read the stated trace
For an ideal sinusoidal a.c. trace, opposite half-cycles have opposite current directions. Equal positive and negative charge passages can give zero signed net transfer over a full cycle while charge is moving during the cycle. Zero average is therefore different from no current at every instant.
A cell is a familiar d.c. source and the mains supply is a familiar a.c. supply. These are source examples rather than an instruction to measure mains directly. School current demonstrations use appropriate teacher-supervised isolated low-voltage apparatus; this written graph work does not demonstrate practical competence.
Pause and explain
A trace varies but always remains above zero. Which conclusion is supported?
Worked example
A current varies between +1 A and +3 A without becoming negative. Is it d.c. or a.c. relative to the stated reference?
Show the worked solution
- All non-zero values have the same sign.
- The direction does not reverse.
- Changing magnitude alone does not make it alternating.
Answer D.c.; the magnitude varies but the direction remains the same.
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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.
Classify a steady positive current and an always-positive varying current.
Give me a hint
Use direction rather than whether the trace is flat.
Compare my reasoning
- The steady positive current keeps one direction.
- The varying positive current also keeps one direction.
- Both are d.c. on the stated reference.
Both d.c.; only the first has constant magnitude.
Look for these in your work
- Classify both traces.
- Separate direction and magnitude.
A trace takes positive values, reaches zero, then takes negative values repeatedly. What does it show?
Give me a hint
Follow the sign on both sides of each zero crossing.
Compare my reasoning
- Positive and negative represent opposite directions.
- The trace repeatedly changes sign.
- It represents alternating current.
A.c.; the current direction repeatedly reverses.
Look for these in your work
- State the sign convention.
- Identify repeated reversals.
A full symmetric a.c. cycle has zero signed net transferred charge. Does that mean charge did not move?
Give me a hint
Distinguish net signed passage from non-zero flow during each half-cycle.
Compare my reasoning
- Charge moves one way during one part of the cycle.
- It moves the opposite way during another.
- Equal signed contributions can cancel while movement occurs.
No; opposite signed passages can cancel although current flowed during the cycle.
Look for these in your work
- Explain the two directions.
- Keep average and instantaneous values distinct.
Common mistakes
- D.c. need not have constant magnitude.
- A sign reversal represents a direction reversal.
- Zero average is not zero current at every instant.
What can you explain now?
What essential feature distinguishes a.c.?
Compare with the explanation
Repeated reversal of direction.
Changing magnitude without changing direction is still compatible with d.c.
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Make it stick
Classify current by direction and distinguish magnitude variation, zero crossings and zero signed averages.
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.