Resistance (R, measured in ohms, Ω) describes how easily current travels through a material. Not all materials have the same resistance.
- Low Resistance:
Copper is 75% less "resistant" than aluminum, which is three times less "resistant" than iron. Wet humans also have low resistance.
- High Resistance:
Glass, ceramics, wood. Dry humans have a higher resistance than wet humans.
If you ever look up at electric power lines, you will notice that they are made of metal. There is a reason for this - metals tend to have much lower resistances than other materials, such as wood. Copper is one of the best (lowest-resistance) conductors that is available in large quantities on the planet. If you have ever looked at the wiring in your house or in an electric appliance, you may have seen some copper wiring. Copper, however, is very expensive. The high-voltage power lines that move electricity over hundreds of miles tend to be made out of an aluminum alloy or similar type of material. While aluminum has a higher resistance than copper, its low cost makes it worthwhile for electric power lines.
The mathematics of resistance are particularly important for this class. Resistive materials can be connected in series (back to back) or in parallel (side to side), or in various combinations. Using some fairly simple formulas we can figure out the total resistance of any combination of resistors connected in series or parallel.