In 1887 direct current (DC) was king. At that time there were 121 Edison power stations scattered across the United States delivering DC electricity to its customers. But DC had a great limitation -- namely, that power plants could only send DC electricity about a mile before the electricity began to lose power. So when George Westinghouse introduced his system based on high-voltage alternating current (AC), which could carry electricity hundreds of miles with little loss of power, people naturally took notice. A "battle of the currents" ensued. In the end, Westinghouse's AC prevailed. But this special feature isn't about the two electrical systems and how they worked. Rather, it's a simple explanation that shows the difference between AC and DC. When you receive a shock from static electricity, tiny particles called electrons actually move between your body and some other object. In a nutshell, that's what electricity is -- the movement of electrons.All matter is made of atoms, and all atoms have electrons. 

Electrons occupy a space that surrounds the atoms nucleus. Each electron resides in a "shell," and each shell has a maximum number of electrons that it can hold. For most atoms, the outermost shell does not contain its maximum number of electrons. Some atoms, such as copper, have only one electron in its outer shell. Because there's only one electron in the copper atom's outer shell, it's not strongly attached to the atom. In other words, it is easily pulled away.In a copper wire, electrons are able to move relatively freely from atom to atom.Not all materials allow electrons to move so freely, however. Carbon, for instance, puts up a resistance to the flow of electrons. Electrons can still move through the carbon, it just takes more energy to get them to move.You've no doubt heard the terms current and voltage. Current describes how many electrons are passing through a wire or some other object at any given moment. High current means lots of electrons are in motion. Voltage describes how much energy the electrons carry.

High voltage means lots of energy. You could view a battery as a kind of pump. But instead of pumping water through pipes, the battery moves electrons through a wire (and through the things that the wire is connected to).Here's how a battery works (the kind you buy at the checkout counter): The battery is made up of a zinc can, which acts as the battery's container (although it's usually covered by a shiny metal casing), and a carbon rod, which is at the battery's center, suspended in a pasty mixture that, in an alkaline cell, contains potassium hydroxide.A chemical reaction within the pasty mixture strips electrons from some of its atoms. These excess electrons collect on the zinc can, which acts as the negative terminal. At the carbon rod are atoms with a shortage of electrons. The electrons at the negative terminal want to go to positive terminal, they just need a way to get there. In our light bulb circuit, the way to get there is through the wire. The number of the electrons the battery can push through the circuit will depend on the resistance at the bulb's filament.

Because the electrons flow in one direction only, batteries produce direct current.With Edison's direct current system, electricity was produced not by batteries but by a DC generator. The generator actually produced alternating current, which was then converted to direct current with a commutator.how much does an ac unit weigh The purpose of a generator is to convert motion into electricity. does an ac unit have to be outsideThis wouldn't be possible if it wasn't for one fact: That a wire passing through a magnetic field causes electrons in that wire to move together in one direction.do i need two hvac unitsA generator consists of some magnets and a wire (usually a very long one that's wrapped to form several coils and known as an armature).

A steam engine or some other outside source of motion moves the wire or armature through the magnetic field created by the magnets.In the example to the left, a loop of wire is spinning within a magnetic field. Because it is always moving through the field, a current is sustained.But, because the loop is spinning, it's moving across the field first in one direction and then in the other, which means that the flow of electrons keeps changing. Because the electrons flow first in one direction and in the other, the generator produces an alternating current.One advantage that AC has over DC is that it can easily be "stepped up" or "stepped down" with a transformer. In other words, a transformer can take a low-voltage current and make it a high-voltage current, and vice versa.This comes in handy in transmitting electricity over long distances. Since AC travels more efficiently at high voltages, transformers are used to step up the voltage before the electricity is sent out, and then other transformers are used to step down the voltage for use in homes and businesses.

Imagine that you're holding a garden hose -- one with no nozzle attached. With nothing to obstruct the water, it pours out of the hose's end freely. But if you place your thumb over the end of the hose, the water's going to squirt out. The reason it does is because of the resistance created by your thumb.It works much the same way for a light bulb. Electrons move relatively freely through the wire, then they come to the bulb's filament, which resists the flow of electrons. The electrons can get through, but not as easily as they can through the wire. The work done overcoming the resistance causes the filament to heat up and to give off light.There are proponents on both sides of the DC vs. AC power debate. Here are a few answers specifically for data... The global power grid distributes electricity in the form of alternating current (AC) instead of direct current (DC). The choice of AC over DC dates back to the 1800s when Thomas Edison first touted the simplicity of DC, while notables like George Westinghouse and Nikola Tesla supported the use of AC.

Since AC proved easier to deliver commercially across great distances using thinner -- and far less expensive -- copper wiring, the industry ultimately adopted AC. However, AC is not necessarily the most efficient means of delivering power, and the use of DC to data center racks and systems has gained a following as power costs force organizations to watch their power budgets. Let's consider several key issues in DC delivery. What is direct current in the data center? The problem with AC is loss. AC initially leaves a power plant at very high voltages. As those voltages are carried to cities, towns, neighborhoods and individual buildings, those high voltages are divided down several times using transformers. And even once the AC voltage enters the building at a moderate 600 VAC or 480 VAC, it must be stepped down again to 240 VAC or 120 VAC to feed the rack servers' power supplies, which convert the AC into several DC voltages that power server components such as the processors, memory, disk drives and so on.

The AC-to-DC translation is not perfect, and a certain amount of loss occurs with each conversion. But you're paying for all of the electricity that enters your facility whether it's used or not, so those conversion losses cost the business money. DC proponents suggest that a single conversion from AC to DC would eliminate much of this loss and be far more efficient. The resulting DC would then be distributed to racks and systems throughout the data center, displacing traditional AC power cabling and subsystems. In the DC vs. AC power debate, what benefits should I look for? The general benefits are efficiency and cost savings. The concept is straightforward; you save money by eliminating points where power is lost during conversions. Lawrence Berkeley National Laboratory in California performed a demonstration back in 2006 which compared AC and DC power in the data center. The laboratory claimed data centers could save up to 20 percent on power costs using DC distribution.

In addition, power supplies in individual servers or other hardware systems would essentially be removed since power would already arrive at the rack in DC form, which need only be regulated down to lower voltages as needed. This would eliminate the need for redundant power supplies along with their noisy and failure-prone power supply cooling fans. The actual amount of savings through DC power distribution remains a matter of debate, and later testing performed by other groups -- such as The Green Grid -- questioned the ultimate difference between AC- and DC-powered data centers. For example, The Green Grid's report concludes that there are no significant differences between power distribution approaches -- mainly because no single AC or DC configuration is more efficient under every possible load condition and because servers and electrical distribution equipment are constantly getting more efficient with each new generation. Still, when savings are realized, the actual benefit will be greatest for the largest data center operators handling multimegawatt installations.

Today, giants such as Google and Swiss hosting company green.ch are among the organizations to deploy DC-powered data centers. What equipment or changes would be needed to support DC power in my data center? One of the biggest obstacles to DC adoption is that the technology is highly disruptive. It's not simply a matter of switching the electricity from AC to DC. A DC-driven data center would require an entirely different electrical distribution system and wiring to the racks. The electrical distribution would also need to integrate on-site generators so that backup generator power would be converted to DC for the facility. And the renovations go right to the servers and systems. Existing servers and other hardware systems cannot be retrofit for DC, so an entirely different suite of hardware would be needed. Uninterruptible power supply systems which depend on AC-to-DC conversion to charge internal batteries, and on voltage inverters to convert DC back to AC, would need to be replaced with DC-only units.