Alternating Current (AC) vs. Direct Current (DC): What to Know

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Electricity sparks between two wires

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Electricity is divided into two types of current: alternating and direct. Alternating current alternates its polarity many times a second, while direct current remains constant and unchanging.

The electricity that comes from your wall is alternating current, while the electricity from a battery is direct current. But it's not just battery-powered devices that use direct current: nearly all electronic devices convert the AC from your wall into DC using a device called a rectifier.

The constancy of direct current is essential for running devices like computers, which require a steady state to compare the digital ones and zeros that make the system run.

What is Electricity, Anyway?

Electricity is the flow of electrons through a conducting material like a metal wire. Electrons bump into each other in a long chain, which causes an overall movement of electrons down the wire. This movement of the electrons through the conductor creates electricity, as well as a magnetic field. That electrical energy powers everything in your life with a plug or an “on” switch.

Electricity has three main components that tell us how powerful the current is. These three attributes are voltage, current, and resistance. Voltage tells us how powerful the electrical flow is, current tells us how fast the electricity is flowing, and resistance tells us how hard it is for the electrons to flow along our conductor. This generalized definition isn’t accurate enough for a textbook, but it’s complete enough for the purposes of this article.

The Difference Between AC and DC

Alternating current (AC) and direct current (DC) both have voltage, current, and resistance. It’s how the current flows that makes the difference.

Alternating current rapidly flows forwards and backward, reversing its polarity between 50 and 60 times a second. This immediately clashes with an intuitive understanding: If the electrons are going in and then coming right back out, how can they power anything?

It’s not the accumulation of electrons that creates energy, however. Electrons have no destination that they need to reach before power is created. It’s the movement of the electrons themselves that creates electrical energy. Just as water flowing through a pipe creates a force regardless of direction, electrons flowing through a wire create electricity.

Ideal sine wave of alternating current
The ideal sine wave of alternating current. SparkFun / CC BY-SA 4.0

DC, on the other hand, doesn’t alternate at all. Under ideal conditions, it’s a steady current with no changes in voltage over time. While DC converted from AC with a rectifier is often an approximation of this steady line, it definitely doesn't flip around like AC. If we visualize DC as water flow, it creates a constant rate of movement in only one direction.

Linear plot of direct current voltage
A linear plot of direct current voltage. SparkFun / CC BY-SA 4.0

What are AC and DC Used for?

Thanks to their differing natures, AC and DC have different uses.

Most of the world’s electrical motors run on alternating current. In these motors, the rapid voltage reversal of the current is used to flip a magnet’s polarity back and forth rapidly. This rapid reversal of polarity causes a wire inside the magnets to rotate, creating a spinning force that powers a motor.

Visualization of AC-powered motor with armature and magnets
Visualization of AC-powered motor with armature and magnets.

AC is also used for power transmission. The voltage of AC is comparatively easy to change, making it a better choice for long-range transmission than DC current. AC can be sent at enormous voltages through the wires, resulting in very little loss on its way to the customer.

Upon arrival, the voltage is dramatically reduced from something like 765,000 volts to a more manageable 110-220 volts and sent into your home. Direct current cannot achieve such dramatic voltage transformations without much larger power losses.

Direct current is typically used to power smaller, more delicate devices. All consumer electronics, from your tablet to your PC, run on direct current, as does anything that's battery-powered.

Not only do these devices benefit from DC: they simply cannot function on AC. Devices that work on 1s and 0s (like computers) need a rock-solid voltage level to distinguish between a high signal, which represents a one, and a low signal, which represents a zero. With the continually flipping current of AC, electronic devices don’t have a steady state to use for comparison. Without a stable current, those devices wouldn’t be able to operate. Since AC is constantly changing, it just cannot provide a stable comparison level for electronics.

Both AC and DC power are widely used in different types of devices, from refrigerators to computers. Some devices might even use both, using AC to power a motor and DC to power a touchscreen. One is not better than the other, but simply different.