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

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

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

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

5. 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

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

7. HIGH RESISTANCE