AC/DC: What's the Difference?

AC/DC: What's the Difference?

1880 light bulb

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.


Inside the Wire

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.


Inside the Battery

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.


Inside the AC Generator

The purpose of a generator is to convert motion into electricity. This 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.

A 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.


Inside the Light Bulb

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.

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