At the heart of all things electrical is a fundamental property of matter called electric charge. It's carried by subatomic particles. Protons have a positive charge (+), and electrons have a negative charge (-). You've seen the effects of charge in everyday life. For example, the static shock you get from a doorknob is a tiny transfer of charge.

The basic rule is simple: opposite charges attract, while like charges repel. A positive charge and a negative charge will pull toward each other. Two positive charges, or two negative charges, will push each other away.

Charge is measured in a unit called the coulomb, named after the French physicist Charles-Augustin de Coulomb. A single coulomb is a huge amount of charge. To give you some perspective, the charge of a single electron is incredibly small.

It takes about $6.24 \times 10^{18}$ electrons to make up just one coulomb of negative charge.

When electric charges are in motion, we get an electric current. Think of it like water flowing through a pipe. The water molecules themselves are like the individual charges. The flow of all those molecules together is the current.

We measure electric current in amperes (A), often just called "amps." One ampere is defined as one coulomb of charge passing a specific point in one second. If you could count the electrons flowing past a point in a wire, and you saw one coulomb's worth of them go by in one second, you would be measuring a current of one amp.

The relationship can be written as an equation, where $I$ is current, $Q$ is charge, and $t$ is time.

This means current is the amount of charge ($"\Delta Q"$) that flows past a point over a certain amount of time ($"\Delta t"$).

Here's where things can get a little confusing. We know that in a typical metal wire, it's the negatively charged electrons that are actually moving. They flow from the negative terminal of a battery, through the circuit, and to the positive terminal. This is called electron flow.

However, long before scientists understood the role of electrons, they established a different standard. Early experimenters, like Benjamin Franklin, assumed that electricity was a flow of positive charge. Based on this assumption, they defined the direction of current as the direction a positive charge would move. This means it flows from the positive terminal to the negative terminal.

Why stick with a convention that seems backward? It's largely a matter of tradition, and thankfully, it doesn't affect how circuits work mathematically. For all calculations, we use conventional current, assuming a flow from positive to negative. It's an important distinction to know, but in practice, you'll almost always work with conventional current.

Ready to test your knowledge on charge and current?

With charge and current covered, you now have the first two building blocks for understanding electricity.