# Difference Between AC and DC Current

## Main Difference – AC vs. DC Current

Electric power could be supplied either as an alternating current (AC) or as a direct current (DC). The main difference between AC and DC current is that, in DC current, electrons continuously flow in one direction whereas, in AC current, electrons oscillate back and forth periodically.

## What is DC Current

In DC current, electrons flow in one direction only. A DC current can be formed by connecting two points at different electric potentials with a conductor. Electrons will then flow from the more negative potential towards the less negative potential, as long as the potentials are maintained. For example, if we connect two points at electric potentials -2 V and -5 V, electrons will flow from the -5 V end to the -2 V end.

For historical reasons, the direction of current is taken to be in the opposite direction to the direction of electron flow. The direction of current in the above example is from -2 V to -5 V. There is nothing flowing in this direction: it is just a convention.

## What is AC Current

In AC current, electrons oscillate back and forth. Homes are usually powered by AC currents. Here, a conductor is connected between a potential that periodically changes its value and a potential which stays at 0 V. The varying potential changes its value between positive and negative values, so that the electrons in the conductor are made to travel back and forth. The potential difference applied across the conductor then varies sinusoidally:

How voltage varies with time in AC (blue) and DC (red) circuits

Most electronic equipment operate with DC currents. Often, the AC current from the supply needs to be converted to DC current before the device can make use of the current. The main reason for using AC current for transmission (rather than DC) is that historically, it was easy to change the voltage of AC currents using a transformer. This meant that electricity could be transferred over large distances at a high voltage and small current. When electricity is transmitted with a smaller current, the power loss during transmission is significantly lower. When supplying electricity into homes, a transformer could be used to  easily convert the smaller current with a high voltage in the transmission lines into a larger current with a smaller voltage used in homes.

Since the current is continually oscillating, the power dissipated across any device connected to an AC current will also change periodically. However, for alternating currents the voltage can be characterized by a single number called the root mean square (RMS) voltage. For a sinusoidal AC current, the RMS voltage can be given in terms of the maximum voltage($V_{max}$) as:

$V_{rms}=\frac{V_{max}}{\sqrt{2}}$

Root mean square (RMS) voltage and maximum (peak) voltage for a sinusoidal voltage

Often, the power dissipated by a component is calculated using the RMS voltage. The RMS voltage and the frequency (how many times the current reverses direction per second) vary from country to country. Typically, RMS voltages of 230 V are used, with a frequency of 50 Hz. In the USA, the power is supplied with an RMS voltage of 120 V at a frequency of 60 Hz.

## AC vs. DC: The War of Currents

In the late 1800s, Thomas Edison advocated using DC current to transmit electricity. However, Nikola Tesla and George Westinghouse were convinced of AC current’s advantages for long-distance transmission. The competition between the two groups was dubbed the “War of Currents”. It is said that Edison went to great lengths to make AC current unpopular, including killing animals with AC current to make people feel that it is dangerous. However, in the end AC current triumphed and the majority of transmission today is done using AC current. However, transmitting DC can be relatively much cheaper and nowadays, it is not difficult to change the voltage of DC current. Therefore, high-voltage DC currents are also occasionally used to transmit power.

The War of Currents: Edison (left) wanted to distribute electricity using DC, while Tesla (right) wanted to distribute electricity using AC.

## Difference Between AC and DC Current

### Flow of Charge Carriers

In DC current, charge carriers flow along one direction only.

In AC current, charge carriers flow back and forth.

### Variation of Power

In DC current, the power dissipated across a load remains nearly constant.

In AC current, the power dissipated across a load varies continuously.

### Cabling

DC current could be transmitted using only two cables.

AC current (3-phase) needs 3 cables to transmit.

Image Courtesy:

“Thomas Alva Edison, three-quarter length portrait, seated, facing front” by Louis Bachrach, Bachrach Studios, restored by Michel Vuijlsteke ( United States Library of Congress’s Prints and Photographs division digital ID cph.3c05139) [Public Domain], via Wikimedia Commons

“A photograph image of Nikola Tesla (1856-1943) at age 34.” by Napoleon Sarony (postcard (radiographics.rsna.org)) [Public Domain], via Wikimedia Commons