What is resistance?
Electricity flows through a material carried by electrons, tiny charged particles inside atoms. Broadly speaking, materials that conduct electricity well are ones that allow electrons to flow freely through them.
In metals, for example, the atoms are locked into a solid, crystalline structure (a bit like a metal climbing frame in a playground). Although most of the electrons inside these atoms are fixed in place, some can swarm through the structure carrying electricity with them. That’s why metals are good conductors: a metal puts up relatively little resistance to electrons flowing through it.
Plastics are entirely different. Although often solid, they don’t have the same crystalline structure. Their molecules (which are typically very long, repetitive chains called polymers) are bonded together in such a way that the electrons inside the atoms are fully occupied. There are, in short, no free electrons that can move about in plastics to carry an electric current. Plastics are good insulators: they put up a high resistance to electrons flowing through them.
This is all a little vague for a subject like electronics, which requires precise control of electric currents. That’s why we define resistance more precisely as the voltage in volts required to make a current of 1 amp flow through a circuit. If it takes 500 volts to make 1 amp flow, the resistance is 500 ohms (written 500 Ω). You might see this relationship written out as a mathematical equation:
V = I × R
Resistor Schematic Symbols and Values
The most common schematic symbol for a resistor is a zig-zag line:
Resistor values in ohms are usually shown as an adjacent number, and if several resistors are present in a circuit, they will be labeled with a unique identifier number such as R1, R2, R3, etc. As you can see, resistor symbols can be shown either horizontally or vertically:
Real resistors look nothing like the zig-zag symbol. Instead, they look like small tubes or cylinders with two wires protruding for connection to a circuit. Here is a sampling of different kinds and sizes of resistors:
In keeping more with their physical appearance, an alternative schematic symbol for a resistor looks like a small, rectangular box:
Resistors can also be shown to have varying rather than fixed resistances. This might be to describe an actual physical device designed to provide an adjustable resistance, or it could be to show some component that just happens to have an unstable resistance:
Any time you see a component symbol drawn with a diagonal arrow through it, that component has a variable rather than a fixed value. This symbol “modifier” (the diagonal arrow) is a standard electronic symbol convention.
Resistors have a wonderfully practical name – they literally resist the flow of electrons. In previous tutorials and videos, we talked about the nature of electricity and the way that electrons want to spread out and be as far apart from each other as possible. That desire for the electrons to be somewhere with less electron density is voltage and, without resistance, electrons would very quickly equalize everywhere. In reality, everything has resistance, which basically impedes, or limits, that flow of electrons from one place to another. The higher the resistance, the greater potential that needs to be overcome before the electrons start moving.
Resistors follow Ohm’s Law – a law that relates the voltage, current flow, and resistance.
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