What is the difference between an alternating current and a direct current?

A direct current flows only in one direction. An alternating current changes its direction of flow in a regular (cyclic) way, all the time and usually several times per second.

Other answers

An alternating current (AC) is an electrical current whose magnitude and direction vary cyclically, as opposed to direct current, whose direction remains constant.

The usual waveform of an AC power circuit is a sine wave, as this is the way the electricity is generated in the power plants. It also allows us to efficiently transmit the energy by transforming it to a higher voltage.

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Alternating current is an electrical current whose magnitude and direction vary cyclically. The direction of direct current remains constant. The frequency of the electrical system varies by country. Most electric power is generated at either 50 or 60 Hertz (cycles per second).

In electronic equipment, different waveforms are used, such as triangular, sawtooth or square waves, the latter form being the "heart and soul" of digital computers and modern communications.

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During just one cycle of a single-phase alternating supply, the "live" or "hot" wire of an alternating voltage (or current) supply goes from zero to its positive peak, then back to zero, then continues on to its negative peak and back to zero again.

These positive- and negative-going parts of each cycle are always voltages (or currents) relative to the "neutral" wire. A neutral wire is necessary to allow the currents to return to the alternator.

In a 60 hertz supply there are 60 such cycles in each and every second. Similarly, there are 50 such cycles in each and every second in a 50 hertz supply.

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The voltage in the "live" or "hot" wire usually follows a waveform very close to a perfect sine wave.

Depending on the voltages and currents required, alternators can be, for example, very large power generating machines installed at power stations or the much smaller type of alternators which are fitted into vehicles.

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AC, or Alternating Current, does exactly what the name implies. In USA, Canada - and in other countries which use the same service frequency - the direction of the current changes 60 times a second (also called a frequency of 60 Hertz) between the 'Hot' (also known as the 'Live') wire - often colored Black - and the Common (also known as the 'Neutral') wire, which is usually colored White. In Europe and many other areas of the world, the service frequency is 50 times a second (50 Hertz).

It is the service frequency which makes the typical "humming" sound you hear from transformers or electric AC motors when they are running.

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The good link below "AC, DC and Electrical Signals" gives a lot more information about the difference between alternating current and direct current.
Direct Current and Alternating Current:

The first and simpler type of electricity is called direct current, abbreviated "DC". This is the type of electricity that is produced by batteries, static, and lightning. A voltage is created, and possibly stored, until a circuit is completed. When it is, the current flows directly, in one direction. In the circuit, the current flows at a specific, constant voltage (this is oversimplified somewhat but good enough for our needs.)

The other type of electricity is called alternating current, or "AC". This is the electricity that you get from your house's wall and that you use to power most of your electrical appliances. Alternating current is harder to explain than direct current. The electricity is not provided as a single, constant voltage, but rather as a sinusoidal (sine) wave that over time starts at zero, increases to a maximum value, then decreases to a minimum value, and repeats.