What is a Resistor?
Special components called resistors are made for the express purpose of creating a precise quantity of resistance for insertion into a circuit. They are typically constructed of metal wire or carbon and engineered to maintain a stable resistance value over a wide range of environmental conditions.
Unlike lamps, they do not produce light, but they do produce heat as electric power is dissipated by them in a working circuit. Typically, though, the purpose of a resistor is not to produce usable heat, but simply to provide a precise quantity of electrical resistance.
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.
or
So, voltage equals the resistance times current. But, for understanding intuitively what is going on, I prefer to think of it as current is equal to voltage over resistance. So, as voltage increases (the electron density differential increases) or the resistance decreases (the difficulty of the electrons to move) the current increases (amount of electrons flowing).
We also have another tutorial that uses a different example to help you understand the relationship between voltage, current, and resistance, if this is still not clear.
The simplest form of this equation is that if voltage is equal to 1 volt and resistance is equal to 1 ohm, there will be 1 amp of current. To get 2 amps of current, you can either increase the voltage to 2 volts or decrease the resistance to 0.5. Or increase the voltage to 1.5 and decrease the resistance to 0.75, if you want to. Either way – it’s a very linear relationship and extremely easy to use. But, the takeaway is, in some situations you can change the voltage, in others, you can change the resistance. In more uncommon cases, you can even change the current. The world is your oyster! But, typically you change the resistance to make whatever current you need.
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