AC vs DC

Two very different ways current can flow.

DC (Direct Current) flows in one constant direction — the kind you get from batteries, USB power, and solar panels. Nearly everything you'll build early on runs on DC.

AC (Alternating Current) reverses direction many times per second — the kind that comes out of a wall outlet. AC is efficient to transmit over long power lines, which is why homes are wired with it; appliances then convert it to DC internally where needed.

• DC has a fixed positive and negative terminal — polarity matters.
• AC's "frequency" (50 or 60 Hz in most countries) is how many direction-reversals happen per second.
• Beginner projects almost always use DC — it's safer and far simpler to reason about.