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What is Electric Current? - Definition, Unit & Types

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  • 0:05 The Flow of Electrons
  • 2:31 Direct and Alternating Current
  • 5:37 Units of Current
  • 6:30 Lesson Summary
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Lesson Transcript
Instructor: Jim Heald

Jim has taught undergraduate engineering courses and has a master's degree in mechanical engineering.

Like a river current is the flow of water molecules, electrical current is the flow of charged particles. In this lesson, we're going to explore what electrical current is, what causes it, and that, unlike a water current, electrical current doesn't always flow in one direction.

The Flow of Electrons

When you hear the word 'current,' what does it make you think of? Perhaps water flowing down a river? That's a good association, because that's precisely the reason electrical current was given its name. Electrical current is very similar to a water current, only instead of water molecules moving down a river, charged particles move down a conductor. In this lesson, we're going to explore what exactly current is, what causes it, and find out that, unlike a water current, electrical current doesn't always flow in one direction.

Current is the flow of charged particles through a conducting medium, such as a wire. When we talk about electricity, the charged particles we're referring to are almost always electrons. You see, the atoms in a conducting material have lots of free electrons that float around from atom to atom and everywhere in between. The motion of these electrons is random, so there is no flow in any given direction. However, when we apply a voltage to the conductor, all of the free electrons will move in the same direction, creating a current.

A curious thing about electric current is that while the electrical energy transfers through the conductor at nearly the speed of light, the electrons themselves move much, much slower. In fact, if you were to walk leisurely alongside a current carrying wire, you would be traveling more than 100 times faster than the electrons!

Electrons move slowly because they do not move very far to transfer energy to each other.
Why Electrons Move Slowly

To see why this is, we can visualize a current carrying wire like a tube filled with marbles. The marbles represent the electrons and the tube represents the wire. If we put a marble into one end of the tube, it pushes on the first marble, which pushes on the next marble, and so on down the line. If we were standing at the other end of the tube, we would see a marble exit at the same time the other marble entered. In other words, the motion, and therefore the energy, was transmitted nearly instantaneously. However, each individual marble only moved a tiny distance in the tube to transfer that energy. Because the electrons in a wire don't have to travel very far to transfer their energy to the next electron, their overall progress through the wire is relatively slow.

Direct and Alternating Current

There are two different types of current in widespread use today. They are direct current, abbreviated DC, and alternating current, abbreviated AC. In a direct current, the electrons flow in one direction. Batteries create a direct current because the electrons always flow from the 'negative' side to the 'positive' side.

In a direct current, the electrons travel in one direction.
Direct Current Image

Alternating current, abbreviated AC, pushes the electrons back and forth, changing the direction of the flow several times per second. In the United States, the current changes direction at a rate of 60 hertz, or 60 times in one second. The generators used in power plants to produce electricity for your home are designed to produce alternating current. You've probably never noticed the lights in your house actually flicker as the current changes direction because it happens too fast for our eyes to detect.

So, why do we need two types of current, and which one is better? Well, that's a good question, and the fact that we're still using both types of current should tell you that they both serve a purpose. Back in the 19th century, it was understood that to send power efficiently over the long distance between a power plant and a home, it had to be transmitted at a very high voltage. The problem was that sending really high voltage into a home was extremely dangerous for the people living there.

The solution to this problem was to reduce the voltage right outside the home before sending it inside. With the technology that existed at the time, it was much easier to reduce the voltage of AC than it was of DC, so AC won out as the preferred type of current. To this day, we still use AC for all of our long-distance power transmission, largely because of its ability to easily transform to other voltages.

So, why do we need DC at all? Well, first and foremost, it's important to realize that we currently don't have any way to store electrical energy. 'But, wait a minute!', you may say. 'What about batteries? Don't they store electrical energy?' Well actually, batteries convert electrical energy and store it as chemical energy. As we mentioned earlier, batteries only create DC, and in turn, can only be charged with DC. That means that AC must first be converted to DC before it can be used with a battery. Until an AC battery is invented, DC will always be a necessity.

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