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

What's the difference between these two types of electricity?

There are two types of electricity we use to power the devices in our lives. With direct current (DC), the electric charge flows in only one direction, while alternating current (AC) flows back and forth. We looked at both types of electricity to help you better understand the differences between DC and AC and how we use them in our everyday lives.

AC vs. DC power

Overall Findings

AC
  • Alternates its polarity.

  • An 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.

DC
  • Remains the same; constant.

  • An example is electricity coming from a battery.

  • Comprised of voltage, current, and resistance.

  • Its constancy is essential for computers.

  • No change in voltage over time.

  • Powers all consumer electronics.

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.

When you plug in a device such as a TV set into a wall socket, the electricity used is alternating current. The corded appliances in your home use AC power, receiving electricity that changes direction and voltage from higher to lower currents.

When electricity comes from a battery, it's a direct current. Since DC power has consistent and constant voltage, it's essential for powering electronics such as a tablet or smartphone. DC power has a smooth and steady electrical current that always flows in the same direction, between positive and negative terminals.

Electricity has three main components: voltage, current, and resistance. Voltage indicates how powerful the electrical flow is, current is how fast the electricity flows, and resistance shows how hard it is for the electrons to flow along a conductor.

AC and DC Currents: Constancy vs. Rapid Flow

AC
  • The current rapidly flows forward and backward.

  • The current reverses polarity between 50 and 60 times a second.

DC
  • The current doesn't alternate.

  • A 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.

AC 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 these electrons power anything?

It's not the accumulation of electrons that creates energy, however. Electrons don't have to reach a destination before power is created. It's the movement of the electrons 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
SparkFun / CC BY-SA 4.0

DC, on the other hand, doesn't alternate. 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 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
SparkFun / CC BY-SA 4.0

AC and DC Uses: Different Types Mean Different Jobs

AC
  • Powers electrical motors.

  • Used for power transmission.

DC
  • Powers smaller devices.

  • Used for consumer electronics.

Because of the differing natures of AC and DC, each has a different use.

Most of the world's electrical motors run on alternating current. In these motors, the rapid voltage reversal of the current flips 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

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 little loss on its way to the customer.

Upon arrival, the voltage is reduced from around 765,000 volts to a manageable 110 to 220 volts and sent into your home.

Direct current, on the other hand, cannot achieve such dramatic voltage transformations without larger power losses.

Direct current is typically used to power small, delicate devices. All consumer electronics, from tablets to PCs, run on direct current, as does anything that's battery-powered.

These devices don't only benefit from DC. These devices can't function on AC. Devices that work on 1s and 0s (like computers) need a 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 can't operate. Since AC continually changes, it can't provide a stable comparison level for electronics.

AC can be changed to DC by an adapter that might be used to power the battery on a laptop.

Final Verdict: Both Are Essential to Modern Life

AC and DC power are widely used in different types of devices, from refrigerators to computers. Some devices might 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. These are different and both are necessary.