Direct comparison
AC vs DC current
Alternating current (AC) periodically reverses direction; direct current (DC) flows steadily in one direction only.
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Side-by-side comparison
| Dimension | Alternating current (AC) | Direct current (DC) |
|---|---|---|
| Direction of flow | Reverses direction periodically. | Flows in one direction only. |
| Waveform | Usually a sine wave that rises and falls. | A steady, constant level. |
| Typical source | Mains supply and rotating generators. | Batteries, solar cells and power supplies. |
| Voltage change | Easily stepped up or down by a transformer. | Needs electronic converters to change voltage. |
| Long-distance transmission | Efficient at high voltage; the basis of the grid. | Used for some very long high-voltage links (HVDC). |
| Frequency | Typically 50 Hz (UK/Europe) or 60 Hz (North America). | Zero — it does not alternate. |
| Common uses | Homes, appliances, motors and the power grid. | Electronics, phones, cars, LED lighting. |
| Powers electronics? | Must be converted to DC first by an adapter. | Directly — most electronics run on DC internally. |
| Discovered/championed by | Associated with Tesla and Westinghouse. | Associated with Edison’s early systems. |
Why mains electricity is AC
Mains power is delivered as alternating current chiefly because AC voltage can be changed easily and efficiently with a transformer. Electricity travels long distances with far less wasted energy at high voltage, so the grid steps voltage up for transmission and back down for safe use in homes. Direct current cannot be transformed so simply, which is why AC won out for the grid — even though most of the gadgets you plug in quietly convert that AC back into DC to run.
Common questions
FAQ
Is my home electricity AC or DC?+
Mains electricity supplied to homes is alternating current (AC), reversing direction 50 times a second in the UK and Europe and 60 times a second in North America. Many devices then convert it to direct current internally, which is why phone chargers and laptop adapters contain a small power supply.
Why do batteries supply DC, not AC?+
A battery produces electricity through a chemical reaction that pushes current steadily in one direction from its negative to its positive terminal, so its output is direct current. There is no mechanism inside a simple battery to reverse the flow, which is why batteries always supply DC.
Can you convert between AC and DC?+
Yes. A rectifier converts AC into DC, which is what a phone charger does, while an inverter converts DC into AC, which is how a solar system or car inverter powers mains appliances. These conversions are routine and underpin much of modern electronics and renewable energy.
Going deeper
