B.5.10—Direct and alternating current boundary

Feature	Direct current (DC)	Alternating current (AC)
Direction	remains one way	reverses direction
Magnitude	may be steady or vary	may vary
Example source	chemical or solar cell	alternating generator/mains supply

Defining distinction and boundary

A changing current remains DC if it never reverses. The syllabus requires this distinction only; waveform calculations, rms values, rectification and detailed AC-circuit analysis are outside this objective.

How it is assessed

The evidence includes full-wave rectification and an rms-current calculation from a time graph, so classify the waveform before selecting the relevant quantity.

Command terms

Identify / Calculate / Explain

What earns marks

Identify whether the current has a constant direction or reverses periodically. In waveform questions, distinguish peak from rms values and use the stated time variation; in rectification questions, explain how the diode arrangement changes the current direction or output waveform.

Watch for

Calling a pulsating or rectified output alternating simply because its magnitude varies, without checking whether its direction reverses.

Representative question

[Maximum number: 1]

The variation with time of the current in a resistor is shown.

What is the root mean square (rms) current?

$\frac{10 \sqrt{2}}{2} \mathrm{~A}$

10 A

$10 \sqrt{2} \mathrm{~A}$

B.5.10—Direct and alternating current boundary

Distinguish Direct and Alternating Current

B.5.10 Exam Analysis

Assessment in practice

Question 1