Resistor / Resistance

Resistance is a property that opposes the flow of electric current, denoted by the symbol $R$.

Physical resistor characterized by 2 values:

• Resistance $R$
• Power rating $p_{max}$

A resistor ALWAYS absorbs power.

Resistor Colour Codes

Resistors have 4 or 5 colour bands printed on them to help you identify the value of the resistor. In a 4 band resistor the 1st and 2nd bands make up a two digit value (e.g. 63), the 3rd band is the multiplying factor, and the 4th band is the tolerance. If you have a 5 band resistor there is an extra band for the digits before the multiplying factor (e.g. 635).

Physical Resistance

$R=\frac{\rho L}{A}$ where

• $\rho$ is the Resistivity
• $A$ is the cross-sectional area
• $L$ is the length of the wire

Related

• Photoresistor
• Ohm’s Law
• Conductance