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A resistor is an element that causes opposition to the flow of the electric current. This causes that a voltage appear in the terminals of the resistor
The following graph shows an incandescent light bulb, located in the path of an electric current.
This current leaves the positive terminal of the battery and returns to the negative terminal, passing through the ligh bulb.
This light bulb behaves like a resistor.
Resistors are represented with the capital letter R and its values (resistance) are measured in Ohms.(Ω)
 Resistors are manufactured in a wide variety of values. There are resistors with values of KiloOhms (KΩ), MegaOhms (MΩ).
The last two units are used to represent very big resistors. In the following chart we see the equivalences among them.
1 MicroOhm = 1 x 10-6Ohms (Ω)
1 KiloOhm (KΩ) = 1 x 103Ohms
1 MegaOhm (MΩ) = 1 x 106Ohms
1 MegaOhm (MΩ) = 1 x 103 KiloOhms (KΩ)
In order to know the value of a resistor without measuring it, there is a resistor color code that helps us to easily obtain its resistance.
To obtain the resistance of any element of a specific material, it is necessary to know some details of the material such as: its lenght, cross area, specific resistance or resistivity of the material that is made.
Resistance and Conductance
Conductance is the ability of one material to pass electrical current. The reciprocal or inverse of the resistance it is the conductance, and it is generally represented by the capital letter G. A circuit with high conductance has low resistance, and vice versa.
A resistance of 1 Ohm has a conductance of 1 mho
A resistance of 1000 Ohms have a conductance of 0.001 mho ("ohm" spelled backwards).
Related Links
Wire wound resistor
Photoresistor - LDR
Resistors in series and parallel
Energy dissipation in resistors (Joule's Law)
Electrical Resistance
Impedance
Resistivity
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