AC vs DC — Alternating and Direct Current Compared

Alternating Current (AC)

Electric current whose direction reverses periodically, usually following a sine wave. The current in wall outlets alternates — 50 or 60 times per second depending on the country.

$I(t) = I_0 \sin(2\pi f t)$

Unit: ampere (A)

Example: A 230 V, 50 Hz wall outlet delivers AC: the voltage swings from +325 V to −325 V and back, 50 times each second.

Direct Current (DC)

Electric current that flows in one constant direction. Batteries, solar cells and phone chargers (after conversion) supply DC.

$I = \frac{V}{R}$

Unit: ampere (A)

Example: A 1.5 V AA battery pushes DC through a flashlight bulb — electrons drift steadily from the negative to the positive terminal.

Key differences

Aspect	Alternating Current (AC)	Direct Current (DC)
Direction of flow	Reverses direction periodically	Flows in one direction only
Generation	Natural output of rotating generators in power plants	From batteries, solar cells, or rectified AC
Long-distance transmission	Easy: transformers step voltage up/down, so power travels at high voltage with low loss	Hard to change voltage; historically impractical for grids (modern HVDC is the exception)
Frequency	50 Hz or 60 Hz mains frequency	0 Hz — no oscillation
Storage	Cannot be stored directly	Stored in batteries and capacitors
Everyday source	Wall outlets, power grid	Batteries, USB chargers, car electrics

Why students confuse them

People say 'AC power' and 'DC power' as if they were different kinds of energy — they are the same energy carried differently. Your phone charger converts the wall's AC into the DC your phone's battery needs; the brick gets warm doing that conversion.