Monday, October 29, 2012

Alternating Current and Direct Current

AC/DC (electricity)

Electricity is the most widely used form on energy because it is most efficiently distributed and transported. Also it can be easily converted to other forms of energy like mechanical energy, heat energy etc. Also since it is one of the fastest forms of usable energy it is used in logic digital circuits for computation since it gives the fastest results. It is this versatility of electricity that makes it so useful for modern life.
Of course electrical energy has various forms as well. It can be stored charge, alternating current or direct current. Each of these forms of energy have different advantages. Now we know that most  of power that we get in our homes is in the form of AC. However it is not necessarily the most used form of energy. In fact most electronic equipment use DC power and converts AC from the supply to DC. Then why is it made AC in the first place. Let us discuss that shall we?

DC (Direct Current)
This is the most simplest form electrical energy.
Simply put a direct current is the flow of electrons in one direction when a voltage difference exists between two terminals forming a closed circuit. The electrons flow in one direction the current is obtained by dividing the voltage with the resistance. It causes heat dissipation in the resistance given by the product of the square of the current with the resistance.  (I2R)
Here given a voltage and a closed circuit the current depends on voltage alone. Although with capacitances and inductances the current would change with time at steady state they would give a constant current.

AC (Alternating Current)

Although the term AC means alternating current it generally refers to any alternating quantities like voltage or flux. It is as it sounds , it is something whose value changes sinusoidally with time. That means that if a quantity has a maximum value of K it’s value over time would be K*sin(wt).
Like DC AC current  also produces a heating effect in the resistors having a value of the product of the max value of the current squared with the value of the resistance divided by 2.
(I2R/2). Of course the difference in the ac quantity is that the current depends not only on the voltage but also on the frequency of the alternating voltage applied. Here the value does not alternate with time like with capacitors and inductances in dc current. However they do yield impedances whose value changes with frequency.

Where AC scores over DC in ?
Well if you think about it i might have given a hint. In case of dc the heat lost is I2R. While in case of AC it is I2R/2. Yes in case of AC the power lost due to heating can be reduced considerably. The another thing to consider is that AC power is that it can be transmitted to other places with very little loss of power. With the help of transformers the voltage can be stepped up and the current reduced during transmission keeping power constant.(P=VI)

Also in case of DC the resistance is the only thing which can be used to limit current while causes heating by I2R. In case of AC however capacitances and inductances can be used to limit current and they don’t add to heating power loss. In fact in case of AC there are two components to power. The active power as well as the reactive power. Adding capacitances adds to reactive power which is not lost. Active power is lost due to heating. Hence by adding capacitive and inductive components to limit the current we can reduce power loss to a very small value.

Plus the biggest advantage of AC power is the three phase power supply. what’s that?

Well in case of the power supplies we see normally they provide two wires(Not counting the earth connection)called single phase. In case of three phase supply they use three wires. In case of three phase supply the power delivered depends on the current drawn from the supply as voltage decreases with increased current drawn. However this is the advantage of three phase supplies that the power delivered is same irrespective of the current drawn from the supply. Also most rotating electrical machines use alternating current as they produce much less heating.

Where DC scores over AC?
Well the problem with ac is that it flows in both directions. It might not be a problem in case of resistors , capacitors etc. whose values are same no matter in which direction the current flows, but in case of non- linear elements whose values of resistance changes depending on which direction the current flows. (e.g.diodes) ac is not of much use. In fact all computational devices use DC power. They have special devices to convert AC into DC after which it is used in the system.  Also in case of systems which would actually require heating like water heaters DC is the preferred choice.
Plus industries use motors whose speeds are to be controlled. DC motors much better speed control and regulation,

Both AC and DC have their own distinct advantages. But AC is what makes electricity the preferred mode of distribution of energy and DC is what gives your computer the ability to read this article.

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