Alternating Current: Start Here
Alternating Current: Start Here
- Electric current has a direction. Direct current keeps one direction. Alternating current repeatedly reverses.
Start with direction, not wave shape
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- The letters d.c. mean direct current. Its value can change, but its direction does not reverse.
- The letters a.c. mean alternating current. A positive value and a negative value represent opposite chosen directions.
- An alternating current does not have to be sinusoidal. Any repeated reversal of direction makes the current alternating.
“Alternating” describes direction. “Sinusoidal” describes one possible wave shape.
Your turndirection check
A current is +2.0 A for 4 ms, then −2.0 A for 4 ms, and this pattern repeats. Is it a.c. or d.c.? Explain.
Show worked answer
It is a.c. because its sign changes repeatedly, so the current reverses direction every 4 ms. The square shape does not make it d.c.
Why the power grid uses a.c.
The syllabus assumes one earlier idea about electrical transmission: an alternating voltage can be changed by a transformer.
- For the same transmitted power, a higher voltage allows a smaller current.
- A smaller current gives much less resistive heating in transmission cables because cable loss depends on current squared.
- Topic 21 does not require transformer equations. The examinable work here is the waveform, r.m.s. value, power, rectification and smoothing.