1. Resistivity Definition Whiteboards

2. What determines the resistance of a piece of wire?

3. Resistivity: consider a piece of wire: L - Length of the wire in m A - Cross sectional area of the wire (πr 2 ?) R - Resistance of the wire in Ohms Demo: Different wires L A Some values of ρ in Ωm at 20 o C (From table 18-1 on p 535) Silver1.59E-8 Copper1.68E-8 Gold2.44E-8 Aluminium2.65E-8 Tungsten5.6 E-8 Iron9.71E-8 Platinum10.6E-8 Nichrome 100E-8 (alloy of Ni, Fe, Cr) R = ρL A ρ - Resistivity in Ωm

4. Resistivity: L - Length of the wire in m ρ - Resistivity in Ωm A - Cross sectional area of the wire (πr 2 ) R - Resistance of the wire in Ohms Some values of ρ in Ωm at 20 o C (From table 18-1 on p 535) Silver1.59E-8 Copper1.68E-8 Gold2.44E-8 Aluminium2.65E-8 Tungsten5.6 E-8 Iron9.71E-8 Platinum10.6E-8 Nichrome 100E-8 (alloy of Ni, Fe, Cr) R = ρL A Ex #1: A steel wire is 2.7 m long, and has a resistance of 7.3 ohms. What is its diameter? (0.21 mm)

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6. A copper wire is 1610 m long (1 mile) and has a cross sectional area of 4.5 x 10 -6 m 2. What is its resistance? (This wire is about 2.4 mm in dia) 6.0 Ω R = ρL A and A = πr 2 R = ?? ρ = 1.68E-8 Ωm L = 1610 m A = 4.5E-6 m 2 R = 6.010666667 = 6.0 Ω W Silver1.59E-8 Copper1.68E-8 Gold2.44E-8 Aluminium2.65E-8 Tungsten5.6 E-8 Iron9.71E-8 Platinum10.6E-8 Nichrome 100E-8

7. An Aluminium wire is 3.2 mm in diameter, and has a resistance of 142 ohms. What length is it? 43,000 m R = ρL A and A = πr 2 R = 142 Ω ρ = 2.65E-8 Ωm L = ?? A = π(1.6E-3) 2 L = 43095.53817 = 43,000 m W Silver1.59E-8 Copper1.68E-8 Gold2.44E-8 Aluminium2.65E-8 Tungsten5.6 E-8 Iron9.71E-8 Platinum10.6E-8 Nichrome 100E-8

8. A piece of wire has a diameter of 0.42 mm, and a length of 53 cm. What is its resistivity if it has a resistance of 4.9 ohms? (what kind of wire is it?) 130x10 -8 Ωm R = ρL A and A = πr 2 R = 4.9 Ω ρ = ?? L = 0.53 m A = π(0.21E-3) 2 ρ = 1.28088E-06 = 130x10 -8 Ωm W Silver1.59E-8 Copper1.68E-8 Gold2.44E-8 Aluminium2.65E-8 Tungsten5.6 E-8 Iron9.71E-8 Platinum10.6E-8 Nichrome 100E-8

9. Resistivity: Thermal effects. R = ρL A ρ – Resistivity will increase with temperature. (random motion, electrons) (demo) Some values of ρ in Ωm at 20 o C (From table 18-1 on p 535) Silver1.59E-8 Copper1.68E-8 Gold2.44E-8 Aluminium2.65E-8 Tungsten5.6 E-8 Iron9.71E-8 Platinum10.6E-8 Nichrome 100E-8 (alloy of Ni, Fe, Cr)