B) Resistance (ohms) Symbol - Ω

 

Ohm's Law

1) R = V/I (at constant T)

V - Potential difference (voltage) between ends of conductor (volts)

I - current (amperes)

 

 

Cross Multiply: V = IR

 

V directly related to I 

 

Since ....

R = V/I

Slope of V, I plot = ???

 


Slope = Resistance

 

 

Larger Version - St. Mary's Physics

 

 

2) The resistance in a wire depends on:

a) resistivity - ρ - metals structural resistance to electron flow

 

b) Length (L) of the wire

c) Thickness (Cross Sectional Area) of the wire (A)

R = ρL/A 

 

Larger Version

 

3) Resistance and Temperature

Metals - T decreases, R decreases

 

T up, R up

(many electrical devices have fans to decrease T)

Nonmetals – When temperature increases, resistance decreases 

 

Superconductor - conductor with no resistance

 

Achieved only at very low temperatures (expensive now)