Resistance and Resistivity Electrical Resistance Electrical resistance describes

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 19 th 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 (10 x 10 -8 m) Insulators large resistivity (1 x 1015 m) Semi-conductors medium resistivity (0. 5 m)

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. 2 x 10 -7 m 2 and 16 -gauge wire has A = 13 x 10 -7 m 2, 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 = 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 R 0 = resistance at temperature T 0 = temperature coefficient of resistivity Unit: 1/°C (or 1/K)

Example A heating element is a wire with crosssectional area of 2 x 10 -7 m 2 and is 1. 3 m long. The material has resistivity of 4 x 10 -5 m at 200°C and a temperature coefficient of 3 x 102 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 n Transmission of electricity n MRI n Maglev n Powerful, small electric motors n Faster computer chips

Superconductors