What is the difference between alternating current (AC) and direct current (DC) and what are their individual pros and cons? These are questions that are fairly common being that virtually every electronic device in existence today is marked with one, the other, or both. Perhaps we know as much as what they stand for and their general uses, but not much more than that.
Direct current, as per its name, is an electric current that travels in one direction only, as opposed to an alternating current, which periodically reverses direction. Positive and negative poles in a magnetic or electric field create a condition in which electrons flow, called polarity. A direct current system has a fixed polarity where one pole is always positive and the other is always negative.
An alternating current on the other hand, reverses polarity continuously, at a rate determined by its frequency. Frequency is measured in hertz (Hz), which is equal to one cycle per second. The US uses 60 Hz while Europe uses 50 Hz. Japan for instance uses both – the eastern half uses 50 Hz while the western half uses 60 Hz. Most appliances are manufactured to accommodate both frequencies.
Back in the late 1880s when modern electricity began being produced on a commercial scale, a fierce rivalry between those who supported DC and those who supported AC ensued, known as the War of Currents. Thomas Edison was a staunch advocate of direct current while George Westinghouse, backed by Nikola Tesla, promoted alternating current.
Why AC won the War of Currents
It became clear at the time, that AC was far superior in terms of transmission of large amounts of electricity over large distances due in large part to its ability to easily step voltage up or down. As electricity travels up and down a conductor (wire), it loses energy in the form of heat due to resistance, expressed by Joule’s first law.
This means that energy is lost in proportion to its current. The obvious solution to this was to step up the voltage in order to lower the current – higher voltage equals the ability to lower the current for the same power. But then it became necessary to transform the voltage both before the long-distance lines and after, when distributing it to the consumer.
The problem with DC at the time was that there was no viable way to do this, while it was relatively simple to do so with AC. Today, technology has advanced to where long-distance DC is now not only viable, but in some ways superior to AC, in the form of high-voltage direct current (HVDC). However, there are still pros and cons to both AC and DC.
Pros to Alternating Current (AC)
- Cheaper and easier to transform voltage.
- Because of the alternating current, plugs can be inserted in either direction, as opposed to DC which must have one prong larger than the other to ensure proper circuit.
Pros to Direct Current (DC)
- About 1.4 times more efficient due to not being a sine wave. As an AC generator produces voltage, it traces out a shape known as a sine wave, due to the continually alternating polarity. The crest of the wave is the peak voltage, but not the actual “usable” voltage. The usable voltage, or the voltage that comes out of the wall socket is equal to the RMS or root-mean-square of the peak voltage.
The RMS is the square root of the mean of the squares of the values. If you were to measure the height of the sine wave at an arbitrary number of random points, the square root of the average of the squares of the measured heights is your approximate RMS. The more points on the sine wave you measure the closer your answer will get to the actual RMS, which is the peak voltage times the reciprocal of the square-root of 2, approximately 0.7.
You can also divide the peak voltage by the square-root of 2, which roughly equals 1.4, and arrive at the same answer. Thus, if an AC and DC circuit both have equal voltage and wiring capacity, the DC circuit will output 1.4 times more usable voltage.
- DC does not lose power to reactive losses. Reactive losses are those suffered by AC lines via heat, due to the natural capacitance of conducting material and the back and forth motion of alternating current.
To sum it up in light of today’s technology, AC systems are more economic for short distances, whereas the tables are now turned for long-distance transmission, with high-voltage direct current (HVDC) coming out on top. AC has the advantage of cheaper transformers while DC has the advantage of cheaper wire and insulators, making it the obvious winner for long distances.