AC vs DC


There are two types of electrical current, AC and DC. One flows back and forth alternating directions: alternating current. The other flows consistently in the same direction direct current. This page discusses the differences between the two, along with the advantages each carries along with it.

Brief summary of AC and DC

For more information on the specific currents, see their respective pages: AC and DC
  • Alternating current is produced within most types of power plants by spinning generators. The direction of current reverses, or alternates, 50-60 times per second depending on a country's standards.[1] Alternating current is the current that travels through power lines and comes through the power outlets found throughout a home or building. There are various reasons why AC was the current of choice to perform this task, which will be discussed below.
  • Direct current is produced by power sources like batteries, fuel cells, and solar panels. Such power sources have two terminals that are positive and negative respectively, which creates a relatively constant voltage for electrons to flow through. Current always flows in the same direction between these two terminals.[1]

Advantages of each

Alternating current uses of varying voltage and flow of electrons within a conductor. Direct current flows in one direction and with a relatively constant current (charge flowing by a point per unit time). The way each can be manipulated, however, is what is important, and provides clear advantages for certain applications among the two.

Advantages of AC

AC is the current of choice for power plants and the electrical grid as a whole. When a plug is connected to an electrical outlet, alternating current comes out, providing power to countless devices like light bulbs and refrigerators. AC is preferred for this application because:

There are cheap and reliable ways of increasing or decreasing the voltage using transformers, which minimizes power loss in electrical transmission.

Resistance reduces the energy transmitted in a wire. By increasing the voltage on the wires to very high voltages for long distance transmission, this loss can be reduced. The loss of power ([math]P_{lost}[/math]) is given by the equation:[4]

[math]P_{lost}=I^2\times R[/math]

where:

  • [math]I[/math] is the current in amperes
  • [math]R[/math] is the resistance in ohms

Increasing the voltages the grid transmits electricity reduces this lost power. As the voltage gets higher, the current decreases proportionally because the transmitted electrical power (energy per unit time) remains the same. For example, if the voltage is increased by a factor of 100, the current must decrease by a factor of 100 and the resulting power lost will be decreased by 1002 = 10000. However there is a limit, being that at extremely high voltages (2000 kV) the electricity begins to discharge resulting in high losses.[4]

Efficient transmission saves power companies and consumers a lot of money, which helps reduce pollution since power plants do not need to make up for lost electricity by using more fuel.

Other advantages of AC include:[5]

  • Low maintenance costs of high speed AC motors.
  • Easy to interrupt the current (ie. with a circuit breaker) due to the current going to zero naturally every 1/2 cycle. For example, a circuit breaker can interrupt about 1/20th as much DC as AC current.

Advantages of DC

A big advantage of direct current is that it is easier to change the speed of a DC electric motor than it is for an AC one. This is useful in many applications, such as electric and hybrid cars.[5]

Direct current is used in essentially all consumer electronics, since transistors (the building blocks of modern electronics) rely on it to operate. Devices that use DC current include cell phones, laptops, TVs and much more.

Direct current may also be used to transmit electricity with even greater efficiency than alternating current over extremely large distances by use of HVDC transmission (high-voltage direct current). Along with higher efficiency, HVDC also allows for different AC systems (ie. 50 Hz and 60 Hz) to be connected.[6]

For Further Reading

For further information please see the related pages below:

References

  1. 1.0 1.1 How Stuff Works. (Accessed December 30, 2015). Direct Current Versus Alternating Current [Online], Available: http://science.howstuffworks.com/electricity8.htm
  2. http://phet.colorado.edu/sims/circuit-construction-kit/circuit-construction-kit-ac_en.jnlp
  3. http://phet.colorado.edu/sims/circuit-construction-kit/circuit-construction-kit-ac_en.jnlp
  4. 4.0 4.1 R. Paynter and B.J. Boydell, "Transmission Lines and Substations" in Introduction to Electricity, 1st ed., Upper Saddle River, NJ: Pearson, 2011, ch.25, sec.3, pp.1102-1104
  5. 5.0 5.1 Private communication with M. Pigman power engineer for Tacoma Power, Sept. 17th, 2015.
  6. Spark Fun. (Accessed December 30, 2015). Alternating Current vs. Direct Current [Online], Available: https://learn.sparkfun.com/tutorials/alternating-current-ac-vs-direct-current-dc

Authors and Editors

Jordan Hanania, Kailyn Stenhouse, Jason Donev
Last updated: April 28, 2020
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