Electricity

Electricity is the movement and distribution of electric charge.

Charge is a fundamental property of matter much like mass. Unlike mass, charge has a polarity or sign. Fortunately for humans, charge can be moved around and manipulated much easier than mass can.

Charge is carried by charge carriers. Often times these are electrons, but they don't have to be. Charge can only move well in conductive materials. It is important to know that charge propagates through a conductor via the charge carriers, but the flow of charge carrier is not equivalent to the flow of charge. When current flows in a wire it moves very quickly, but the individual electrons do not.

Current is the amount of charge crossing a cross section per unit time. The more charge is flowing through a constant area, the more current there will be. Current is the rate of change of charge with respect to time (derivative) through a constant area.

Voltage is the pushing pressure that is applied to charge in a circuit. Voltage is not a measure of potential energy, it is a measure of potential energy per unit charge. And remember, potential energy is always relative to some location. Given this, voltage is always measured between two points.

Resistance is a property of materials that determines how easy it is for charge to move through them. It takes work to move charge through a resistance, or to make current flow through a resistance (same thing).

When we design and build circuits it is helpful to represent them as components connected together. This idea is also key to how circuits are built and circuit components are sold. This approach has a number of advantages:

1. It is not necessary to reinvent the wheel (components) every time a new circuit is built.
2. There is an abstraction layer between the circuit design and the physical circuit that is built. I can add a resistor to my circuit that has a certain resistance without having to know anything about the actual physical resistor that I am going to use. Then later, when I move to the physical realization of the circuit, I can select the actual resistor component.
3. The mathematics involved when solving for voltages and currents in a lumped element model is significantly easier.