AC vs DC

DC: the push goes one way

A battery gives direct current, or DC. Its push always points the same way, so the electrons drift one way around the loop: out of the negative end, through the wire and the bulb, and back into the positive end. On a graph of current over time, DC is a flat line.

AC: the push swings back and forth

A wall outlet gives alternating current, or AC. Its push keeps reversing. In North America it goes through 60 full cycles every second (60 hertz); in much of the rest of the world it's 50. On a graph, AC is a wave that rises, falls below zero and comes back.

Because the push keeps flipping, the electrons don't travel around the circuit. They just wiggle back and forth in place, while energy still flows to the bulb and lights it.

Why the grid uses AC

AC's voltage is easy to change with a transformer: two coils of wire wrapped around an iron core. Power plants use transformers to step up the voltage very high before sending power across long transmission lines. Higher voltage means less current for the same power, and less current means less energy lost as heat in the wires. Near your home, more transformers step down the voltage to a safer level for your outlets (about 120 V in North America).

Your charger is a converter

Phones and laptops need steady DC. So the charger you plug into the wall is a converter: AC goes in from the wall, DC comes out to your device. Inside, it lowers the voltage and turns the back-and-forth current into one-way current.

Try it yourself

Look at the small print on a phone charger or laptop power brick. You'll find an Input line with a wave symbol (~) or "AC", often "100-240V ~ 50/60Hz", and an Output line with DC, such as "5V ⎓". The ⎓ symbol (a solid line over a dashed one) means DC. Read only the label: never open a charger or touch mains wiring.

Good to know

• North America (US, Canada, Mexico) uses about 120 V at 60 Hz; most of Europe and much of the world uses 230 V at 50 Hz.
• 60 Hz means 60 full cycles per second, so the direction reverses 120 times a second.
• "Electrons wiggle in place" is simplified: in AC they oscillate tiny distances while energy moves through the circuit via the electromagnetic field. In DC their net drift is also slow, millimeters per second.
• The animation shows AC in slow motion; the real swing is far too fast to see.
• Electrons move from − to +; conventional current is drawn the other way.
• High-voltage DC (HVDC) is also used for some very long or undersea links today, so AC isn't the only option for transmission.
• A phone charger usually steps the voltage down and rectifies it to about 5 V DC (USB); modern chargers can negotiate higher voltages.
• The "war of the currents" of the late 1880s (Edison's DC vs Westinghouse and Tesla's AC) is left out of the video for time.
• The 0.5 A on the meter in the video is just an example value.