Describe that in alternating current (a.c.) the movement of charge changes…

The idea

Alternating current means the charge carriers in a circuit repeatedly reverse their direction of travel. In a metal wire the charge carriers are electrons; in an a.c. supply they drift one way, then the other, then back again, completing one full cycle each time they return to their starting pattern. The supply voltage drives this reversal, so the current alternates about zero rather than staying at a steady value. Contrast this with direct current (d.c.), where charge moves one way only, as from a battery. A useful picture is a pendulum swinging left and right rather than a ball rolling steadily downhill. The frequency tells you how many complete reversals happen each second, and the direction of conventional current is opposite to the electron drift.

What earns the marks

• States that in a.c. the direction of charge movement reverses repeatedly.
• Distinguishes a.c. from d.c., where charge moves in one direction only.
• Identifies that the current alternates about zero rather than staying constant.
• Links the reversal of charge movement to the alternating supply voltage.

Common mistakes and how to put them right

• Claims that charge stops moving in a.c.

  Put it right by saying the charge keeps moving but reverses direction.

• Treats a.c. as a steady one-way current like a battery.

  Put it right by stressing the repeated reversal.

• Says the charge itself changes into a different type.

  Put it right by explaining that only the direction of movement changes.

Exam tips

• Read the question stem for the word 'direction' and make the reversal explicit in your answer.
• Contrast a.c. with d.c. in one short sentence to show the difference clearly.
• Avoid saying the current is 'zero'; say it alternates about zero, reversing direction.