Direct Current: DC Electricity

Direct current is the continuous, one-way flow of electrons through a conductor such as a copper wire. This constant movement of electrons in one way is similar to the flow of water through a pipe. A DC (direct current) circuit is necessary for the current of electron to flow. This circuit consists of a source of electrical energy, such as a battery or solar cells, and a metal wire running from the positive end of the source to the negative terminal.

Direct current is used in nearly all electronic systems as their power supply and also to charge batteries, which are the primary sources of DC. Some railway propulsion use direct current, too, specially in urban areas. High voltage direct current is used to interconnect alternating current power grids.

Direct current is commonly found in many low-voltage applications, specially where these are powered by batteries, which can produce only DC, or solar power systems, since solar cells can produce only DC. Most automotive applications use DC, although the alternator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Applications using fuel cells, mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts, also produce only DC.

Resistance: Ohm's Law

Resistance is the opposition offered by a substance to the passage through it of a steady electric current. We can say that conductive materials such as aluminum offer the least resistance to the flow electrons (electrical current), whereas the insulating materials offer strong resistance. In physics the electrical unit of resistance is called Ohm (Ω).

The resistance of a wire is directly proportional to its length, and inversely proportional to its width. That is to say that the longer the wire is, the more resistance it will offer; and the wider it is, the less resistance it will offer. So, Ω=length/width.

Ohm's law: the Ohm's law states that the strength of an electric current through a conductor between two points is directly proportional to the potential difference (the voltage) across the two points, and inversely proportional to the resistance between them. The equation that describes this relationship is:

I= V/Ω, whereas I is the current in ampere, V the potential difference in volts, and Ω the resistance. The law was named after the German physicist Georg Ohm, who described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire.