RESISTANCE The opposition to the flow of electric current Unit: ohm (Ω) I love Nick Laing!

RESISTANCE OF A WIRE 1) Length- The resistance of a wire varies directly with its length R L

2) Cross sectional area- Resistance varies inversely with the cross sectional area of the wire. (A= π r 2) R area

3) Temperature – Resistance varies directly with temperature Superconductors- some metals have little to no resistance at very low temperatures

L- length (m) on reference table A = πr2 4)Resistivity- (ρ) the amount of resistance a 1m3 sample of a substance has at 20˚C. R = ρL A R- resistance (Ω) ρ- resistivity(Ω∙m) L- length (m) on reference table A = πr2 R ρ

EXAMPLE What will be the resistance of a silver wire having a length of 2m and a diameter of 0.3cm? A= π r2 R = ρL A=3.14(.0015)2 A A=7.07x10-6m2 R=1.59x10-8Ωm (2m) *Use ½ the diameter 7.07x10-6m2 R=0.0045Ω

HIGH RESISTANCE