# What is Direct Current?

Instructor: Dina El Chammas Gass

Dina has taught college Environmental Studies classes and has a master's degree in Environmental and Water Resource Engineering.

Current is the flow of electric charge and exists in two forms: direct current and alternating current. In this lesson, you will learn what direct current is and what it is used for.

## What is Current?

Open any physics textbook on electric current, and the first thing you're asked to imagine is water flowing through a pipe. It's a good analogy, since it is hard to imagine incredibly small charged particles flowing through a wire, but it is very easy to imagine water flowing through a pipe. After all, we've seen that a million times. The point of the analogy is to ingrain in your brain that flow is to water as current is to electric charge.

The other useful aspect to the analogy to consider is that when you buy a pipe at the hardware store, you're just buying the pipe, not the water inside it. But when you buy an electric wire, you are buying the charged particles as well. What you supply to the wire when you get home is the pressure to move the charged particles. The pressure to move the charged particles is called voltage; the movement of the charged particles is called current.

## What is Direct Current?

Charged particles can move in two very distinct ways depending on their voltage source. The way you move charged particles is based on a universal law that states that like charges repel and opposite charges attract, known as Coulomb's law. We've likely all heard how 'opposites attract!'

We also need to consider what the charged particles move within; that is, electric wires. Electric wiring is made out of metals. The molecular structure of metals--that is, the way that everything is bound together within a metal--allows for electrons to flow freely within a metal. Protons are not so free to move. So if you want to create a current, you move the electrons within the wire. Since electrons are negatively charged, they will move towards anything that is positively charged and away from anything that is negatively charged.

To get those electrons moving, you provide a voltage source with one terminal that is very negatively charged and the other that is very positively charged. You connect a conducting wire between the terminals and the electrons start to move: away from the negative terminal and towards the positive terminal.

Now here is the trick. Direct current, or DC, is a current in which electrons always flow in one direction. If you want to create DC, you hook up a wire to a voltage source whose negative and positive terminals stay at the same location at all times. The electrons always know which direction to move, and it is always the same direction.

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