DIRECT CURRENT AND ALTERNATING CURRENT

Most students of electricity begin their study with what is known as direct current (DC), which is electricity flowing in a constant direction, and/or possessing a voltage with constant polarity. DC is the kind of electricity made by a battery (with definite positive and negative terminals), or the kind of charge generated by rubbing certain types of materials against each other.

As useful and as easy to understand as DC is, it is not the only “kind” of electricity in use. Certain sources of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages alternating in polarity, reversing positive and negative over time. Either as a voltage switching polarity or as a current switching direction back and forth, this “kind” of electricity is known as Alternating current (AC )

 

Direct vs alternating current

Whereas the familiar battery symbol is used as a generic symbol for any DC voltage source, the circle with the wavy line inside is the generic symbol for any AC voltage source.

Alternating Current vs Direct Current

Electricity flows in two ways; either in alternating current (AC) or in direct current (DC). Electricity or 'current' is nothing more than moving electrons along a conductor, like a wire, that have been harnessed for energy. Therefore, the difference between AC and DC has to do with 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 "forwards" and then going "backwards."

Comparison chart

 

Origins of AC and DC current

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 magnet that was rotating. 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.

 

Use of transformers with Alternating Current (AC)

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.

Storage and conversion from AC to DC and vice versa

AC can even be changed to DC by an adaptor 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.