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Resistivity Definition Whiteboards

Resistivity Definition Whiteboards. What determines the resistance of a piece of wire?. Resistivity: consider a piece of wire:. R = ρ L A. ρ - Resistivity in Ω m. Some values of ρ in Ω m at 20 o C (From table 18-1 on p 535) Silver 1.59E-8 Copper 1.68E-8 Gold 2.44E-8

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Resistivity Definition Whiteboards

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  1. Resistivity • Definition • Whiteboards

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

  3. Resistivity:consider a piece of wire: • R = ρL • A ρ - Resistivity in Ωm Some values of ρ in Ωm at 20oC(From table 18-1 on p 535) Silver 1.59E-8 Copper 1.68E-8 Gold 2.44E-8 Aluminium 2.65E-8 Tungsten 5.6 E-8 Iron 9.71E-8 Platinum 10.6E-8 Nichrome 100E-8 (alloy of Ni, Fe, Cr) A L • L - Length of the wire in m • A - Cross sectional area of the wire (πr2) • R - Resistance of the wire in Ohms Demo: Different wires

  4. Resistivity: Some values of ρ in Ωm at 20oC (From table 18-1 on p 535) Silver 1.59E-8 Copper 1.68E-8 Gold 2.44E-8 Aluminium 2.65E-8 Tungsten 5.6 E-8 Iron 9.71E-8 Platinum 10.6E-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) • L - Length of the wire in m • ρ - Resistivity in Ωm • A - Cross sectional area of the wire (πr2) • R - Resistance of the wire in Ohms

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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 m2. What is its resistance? (This wire is about 2.4 mm in dia) • R = ρL • A • and • A = πr2 • R = ?? • ρ = 1.68E-8 Ωm • L = 1610 m • A = 4.5E-6 m2 • R = 6.010666667 = 6.0 Ω Silver 1.59E-8 Copper 1.68E-8 Gold 2.44E-8 Aluminium 2.65E-8 Tungsten 5.6 E-8 Iron 9.71E-8 Platinum 10.6E-8 Nichrome 100E-8 W 6.0 Ω

  7. An Aluminium wire is 3.2 mm in diameter, and has a resistance of 142 ohms. What length is it? • R = ρL • A • and • A = πr2 • R = 142 Ω • ρ = 2.65E-8 Ωm • L = ?? • A = π(1.6E-3)2 • L = 43095.53817 = 43,000 m Silver 1.59E-8 Copper 1.68E-8 Gold 2.44E-8 Aluminium 2.65E-8 Tungsten 5.6 E-8 Iron 9.71E-8 Platinum 10.6E-8 Nichrome 100E-8 W 43,000 m

  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?) • R = ρL • A • and • A = πr2 • R = 4.9 Ω • ρ = ?? • L = 0.53 m • A = π(0.21E-3)2 • ρ = 1.28088E-06 = 130x10-8Ωm Silver 1.59E-8 Copper 1.68E-8 Gold 2.44E-8 Aluminium 2.65E-8 Tungsten 5.6 E-8 Iron 9.71E-8 Platinum 10.6E-8 Nichrome 100E-8 W 130x10-8Ωm

  9. Resistivity: Thermal effects. Some values of ρ in Ωm at 20oC(From table 18-1 on p 535) Silver 1.59E-8 Copper 1.68E-8 Gold 2.44E-8 Aluminium 2.65E-8 Tungsten 5.6 E-8 Iron 9.71E-8 Platinum 10.6E-8 Nichrome 100E-8 (alloy of Ni, Fe, Cr) • R = ρL • A ρ – Resistivity will increase with temperature. (random motion, electrons) (demo)

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