Building It Up

Now that we know what electricity is, and we know we want some, we now have to set up a way we can obtain it.

Voltage

Obviously making the power of moving electrons somehow useful to us is different than say, from using the gravitation of water flowing downhill or by releasing the stored chemical energy in gasoline. However, the underlying idea remains the same. We may set up a system from which a potential energy can be stored and/or extracted to exert force. We will go into several different methods of doing this later, but for now we will say whenever there is an electric potential across two points in a circuit, regardless of how it is obtained, it is referred to as voltage.

Voltage: An existing electric potential between two points.

Current

So when there exists a potential difference between two points and they are connected by a conductor, electrons will travel between them.  In simple circuits like the one shown below, the terminal of the voltage source that has a higher potential is given the convention of +, while the lower voltage terminal is given -. This setup causes the flow of electrons, called current, to occur. Current is measured by how many electrons (due to the scale of electrons, they are collected into groups called coulombs)  pass through a given point of the conductor in a second.

Current: The rate of flow of electrons in a circuit

A side note on convention...

There are some potentially confusing points on the subject of the direction of electron flow. This confusion can be avoided if one understands and remembers that the "negative" aspect of the terminal and the electron, as well as the "current" associated with its movement, are arbitrary labels and have no inherent meaning in and of themselves. The meanings that we have attributed to them are in the form of a standard convention we still use from a time before the underlying theories behind electricity were fully discovered. Notice the diagram below, where the electrons actually move in the opposite direction of the described current flow.

In the days before electronics was the study of the movement of electrons, but was the study of moving charge, a positive voltage terminal was considered to be an excess of positive charge. Now we consider it to be a deficiency of negative charge, to which negatively charged particles(electrons) move towards. The two situations are in fact describing the same thing, but from a different perspective. The second perspective is more helpful because protons are stationary in electronics.

Resistance

Another concept that bears introduction is Resistance. It is defined as the ability of a circuit to impede current flow. It is important to understand that even conductors such as copper wire are resistors, but that resistance is negligible when compared to the other components in the circuit. For now we can just use the concept of resistance to model the behavior of our simple linear circuits.

Resistance: The amount components in a circuit slow the flow of current

Ohm's Law

We may state these in relation to each other, in the equation known as Ohms law:

V = IR

Where V is voltage, I is current and R is resistance. If we know two of these three variables at some point in the circuit, we may use this relation to find the third.

The next article will show the workings of the simple voltage source: a basic salt bridge battery.

Previous lesson: What is Electricity?