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Resistance and Resistivity

Resistance and Resistivity. 20.3. Another way to find resistance. The resistance varies directly with length and inversely with width (or cross-sectional area) Short, thick wire  small resistance Long, skinny wire  large resistance. Resistivity.  = resistivity Unit:  m

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Resistance and Resistivity

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  1. Resistance and Resistivity 20.3

  2. Another way to find resistance • The resistance varies directly with length and inversely with width (or cross-sectional area) • Short, thick wire  small resistance • Long, skinny wire  large resistance

  3. Resistivity •  = resistivity • Unit:  m • Table 20.1 lists resistivities of some materials • Metals  small resistivity (10x10-8  m) • Insulators  large resisitivity (1x1015 m) • Semi-conductors  medium resistivity (0.5  m)

  4. Example – Why are long wires thick? • Wire thicknesses are measured in gauges. 20-gauge wire is thinner than 16-gauge wire. If 20-gauge wire has A = 5.2x10-7m2 and 16-gauge wire has A = 13x10-7 m2, find the resistance per meter of each if they are copper. • 20-guage  .0331 /m • 16-guage  .0132 /m

  5. Resistivity and Temperature •  = resistivity at temperature T • 0 = resistivity at temperature T0 •  = temperature coefficient of resistivity • Unit: 1/°C (or 1/K)

  6. Resisitivity and Temperature • Metals • Resistivity increases with temperature •  is positive • Semiconductors • Resistivity decreases with temperature •  is negative

  7. Resistance and Temperature • R = resistance at temperature T • R0 = resistance at temperature T0 •  = temperature coefficient of resistivity • Unit: 1/°C (or 1/K)

  8. Example • A heating element is a wire with cross-sectional area of 2x10-7m2 and is 1.3 m long. The material has resistivity of 4x10-5 m at 200°C and a temperature coefficient of 3x10-2 1/°C. Find the resistance of the element at 350°C. • R = 1430 

  9. Superconductors • Materials whose resistivity = 0 • Metals become superconductors are very low temperatures • Some materials using copper oxide work at much higher temperatures • No current loss • Used in • Transmission of electricity • MRI • Maglev • Powerful, small electric motors • Faster computer chips

  10. Practice Problems • Resistance is futile • 611 CQ 3, 4, P 10 – 15 • Total of 8 Problems

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