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Resistance and Resistivity Electrical Resistance Electrical resistance: • describes how well a circuit component resists the passage of electric current • defined as the ratio of energy-source voltage to the current moving through the energy receiver • measured in ohms after 19th century German physicist Georg Simon Ohm Electrical Resistance Factors affecting electrical resistance: • • • • thin wires resist electrical current more than thicker wires long wires offer more electrical resistance materials of wire: copper has a low electrical resistance, so it is used to make connecting wires rubber has an enormous resistance, so it is used in electrical insulators temperature: higher temperature (greater jostling of atoms), greater resistance Resistance factors Resistivity = resistivity Unit: m Metals small resistivity (10x10-8 m) Insulators large resistivity (1x1015 m) Semi-conductors medium resistivity (0.5 m) Example – Why are long wires thick? Wire thicknesses are measured in gauges. 20gauge wire is thinner than 16-gauge wire. If 20gauge 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 (= 1.58 x 10 -8 m). 20-gauge .0331 /m 16-gauge .0132 /m THICKER WIRE OFFERS LESS ELECTRICAL RESISTANCE Resistivity and Temperature = resistivity at temperature T 0 = resistivity at temperature T0 = temperature coefficient of resistivity Unit: 1/°C (or 1/K) Resistivity and Temperature Metals Resistivity increases with temperature is positive Semiconductors Resistivity decreases with temperature is negative Resistance and Temperature R = resistance at temperature T R0 = resistance at temperature T0 = temperature coefficient of resistivity Unit: 1/°C (or 1/K) Example A heating element is a wire with crosssectional 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 3x102 1/°C. Find the resistance of the element at 350°C. R = 1430 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 Superconductors