Resistors

Resistance and Resistivity
Resistance (R) is the measure of how much an object - a resistor - opposes the flow of electric current. The resistance of an object is measured in Ohms ($$\Omega$$)

Resistivity ($$\Rho$$) is a measure of how strongly a material opposes the flow of electric current. Resistivity is measured in ohm metres ($$\Omega$$m). Related wiki page: table of resistivities

The relationship between resistance and resistivity is:


 * $$R = {l \cdot \rho \over A} \,$$

where
 * "l" is the length of the object
 * "A" is the cross sectional area of the object (assuming constant), and
 * "ρ" is the resistivity of the material

Resistors
A resistor is a passive electrical device that has been manufactured specifically to add resistance to a circuit, to effect a voltage drop.

The mathematical equation that describes the relationship between resistanece, voltage drop and current through a resistor is known as Ohms Law


 * $$I = \frac{V}{R}$$

where
 * "I" is the current in amperes,
 * "V" is the potential difference between the ends of the resistor in volts, and
 * "R" is the resistance of the resistor, measured in ohms

Resistor colour bands
The resistance of a resistor is coded into a series of coloured bands along its length:
 * band A is first significant figure of component value
 * band B is the second significant figure
 * band C is the decimal multiplier
 * band D if present, indicates tolerance of value in percent (no color means 20%)