Alternating Current (AC) vs. Direct Current (DC)

What's the difference?

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

AC vs DC power
  • Alternates its polarity.

  • Example is electricity coming from a wall socket.

  • Comprised of voltage, current, and resistance.

  • Rapidly flows forward and backward.

  • Reverses its polarity between 50 and 60 times a second.

  • Powers electrical motors.

  • Remains the same; constant.

  • Example is electricity coming from a battery.

  • Comprised of voltage, current, and resistance.

  • Its constancy is essential for computers.

  • No change at all in voltage over time.

  • Powers all consumer electronics.

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 such as computers, which require a steady-state to compare the digital ones and zeros that make the system run.

Quick Primer on Electricity

Before we can understand AC and DC, it's important to know the basics of electricity. Electricity is the flow of electrons through a conducting material, such as 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: voltage, current, and resistance. In very simple terms, 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.

The Differences Between AC and DC

  • Current rapidly flows forward and backward.

  • Current reverses polarity between 50-60 times a second.

  • Current doesn't alternate at all.

  • Steady current with no changes in voltage over time.

AC and DC both have voltage, current, and resistance. The difference is in how the current flows.

Alternating current rapidly flows forward 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

How Are AC and DC Used?

  • Powers electrical motors.

  • Used for power transmission.

  • Power smaller devices.

  • Used for consumer electronics.

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. 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, on the other hand, 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.

These devices don't just 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.

AC can be changed to DC by an adapter that you might use to power the battery on your laptop.

Final Thoughts

Both AC and DC power are widely used in different types of devices, from refrigerators to computers. Some devices might even utilize both, using AC to power a motor and DC to power a touchscreen, for example. With AC and DC power, you don't have to choose and one is not better than the other. They are different and very necessary.