Electricity flows in two ways: either in an alternating current (AC) or in a direct current (DC). Electricity or "current" is nothing but the movement of electrons through a conductor, like a wire. The difference between AC and DC lies in the direction in which the electrons flow. In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forward" and then going "backward."
Alternating current is the best way to transmit electricity over large distances.

[B]Comparison:[/B]

[B]1. Amount of energy that can be carried[/B]

[I]Alternating Current:[/I] Safe to transfer over longer city distances and can provide more power.
[I]Direct Current:[/I] Voltage of DC cannot travel very far until it begins to lose energy.

[B]2. Cause of the direction of flow of electrons[/B]

[I]Alternating Current:[/I] Rotating magnet along the wire.
[I]Direct Current:[/I] Steady magnetism along the wire.

[B]3. Frequency[/B]

[I]Alternating Current:[/I] The frequency of alternating current is 50Hz or 60Hz depending upon the country.
[I]Direct Current:[/I] The frequency of direct current is zero.

[B]4. Direction[/B]

[I]Alternating Current:[/I] It reverses its direction while flowing in a circuit.
[I]Direct Current:[/I] It flows in one direction in the circuit.

[B]5. Current[/B]

[I]Alternating Current:[/I] It is the current of magnitude varying with time.
[I]Direct Current:[/I] It is the current of constant magnitude.

[B]6. Flow of Electrons[/B]

[I]Alternating Current:[/I] Electrons keep switching directions - forward and backward.
[I]Direct Current:[/I] Electrons move steadily in one direction or 'forward'.

[B]7. Obtained from[/B]

[I]Alternating Current:[/I] A.C Generator and mains.
[I]Direct Current:[/I] Cell or Battery.

[B]8. Passive Parameters[/B]

[I]Alternating Current:[/I] Impedance.
[I]Direct Current:[/I] Resistance only.

[B]9. Power Factor[/B]

[I]Alternating Current:[/I] Lies between 0 & 1.
[I]Direct Current:[/I] It is always 1.

[B]10. Types[/B]

[I]Alternating Current:[/I] Sinusoidal, Trapezoidal, Triangular, Square.
[I]Direct Current:[/I] Pure and pulsating.

[B]Origins of AC and DC current[/B]

A magnetic field near a wire causes electrons to flow in a single direction along the wire, because they are repelled by the negative side of a magnet and attracted toward the positive side. This is how DC power from a battery was born, primarily attributed to Thomas Edison's work.
AC generators gradually replaced Edison's DC battery system because AC is safer to transfer over the longer city distances and can provide more power. Instead of applying the magnetism along the wire steadily, scientist Nikola Tesla used a rotating magnet. When the magnet was oriented in one direction, the electrons flowed towards the positive, but when the magnet's orientation was flipped, the electrons turned as well.

[B]Use of transformers with Alternating Current[/B]

Another difference between AC and DC involves the amount of energy it can carry. Each battery is designed to produce only one voltage, and that voltage of DC cannot travel very far until it begins to lose energy. But AC's voltage from a generator, in a power plant, can be bumped up or down in strength by another mechanism called a transformer. Transformers are located on the electrical pole on the street, not at the power plant. They change very high voltage into a lower voltage appropriate for your home appliances, like lamps and refrigerators.

[B]Storage and Conversion From AC to DC and Vice Versa[/B]

AC can even be changed to DC by an adapter that you might use to power the battery on your laptop. DC can be "bumped" up or down, it is just a little more difficult. Inverters change DC to AC. For example, for your car an inverter would change the 12 volt DC to 120 Volt AC to run a small device. While DC can be stored in batteries, you cannot store AC.