What is a Direct Current?
A direct current is a form of electrical current used in direct current (DC) electricity and consists of electrons that move continuously in a single direction. This is in contrast to alternating current (AC) electricity in which electrons move first in one direction, then in the other direction, alternating rapidly between each. DC electricity is often used in batteries and similar forms of electrical power supplies since it is usually simple to create a circuit using DC electricity, though over long distances AC electricity has often been preferred. A direct current is one of the easiest forms of electrical current to understand and is often used to explain how electricity works.
Electricity is created through the flow of electrical current through a system, called a circuit, which includes some form of conductor and a source of the current. The current consists of a stream of negatively charged electrons through the system, and in a direct current these electrons flow through the circuit in a single direction. Electrical charges flow from one charge to the other, “opposites attract” as they say, and so the negatively charged electrons naturally flow toward a positively charged area.
A direct current flows through a circuit using a conductor, which is a material, often metallic, that naturally provides a condition in which electrons can easily move. Conductive materials often work to transfer energy of any type, be it heat or electricity, so a metal surface will typically become hot faster than a nonconductive surface, and will also conduct electricity. This type of circuit is created by joining a negative and positive part of a power source, such as a battery, to either end of the circuit.
Direct current flows from the negative end, through the circuit, and ultimately reaches the positive end. By placing something that can utilize the charge, such as a light bulb, circuit board, or automobile engine, on the circuit, the charge is transferred to the item as it passes toward the positive end. Many circuit diagrams show charge flowing from the positive end toward the negative end; however, this is not accurate and is often done as part of an old convention still used despite its inaccuracy.
An alternating current, on the other hand, still flows from negative to positive, but the polarization is changed and switched repeatedly throughout the use of the circuit. This means that at one moment the flow may happen from left to right, but then will be switched to go from right to left. The voltage of alternating current is more easily changed through the use of transformers, making AC electricity often better for long distances such as between a power plant and a home or business. Direct current is typically used in batteries and power supplies that travel a shorter distance, since it does not require polarization changes and can be more easily incorporated into a smaller circuit.
@post 1: Root mean square?
actually can we define it in any other way? what is the thinking?
While going through the basic rhetoric of DC and AC current, some discussion should be included of the 'effective power' delivered by a specific AC voltage as related to the same as DC voltage.
In other words, what is the effective power of 120 V AC when compared to the power delivered to the load by 120 V DC. There is a general multiplier, but I don't remember. Do you?
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