Resistance and Resistivity

Electrical Resistance Electrical resistance: describes how well a circuit component resists the passage of electric current 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 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 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 thin wires resist electrical current more than thicker wires long wires offer more electrical resistance long wires offer more electrical resistance materials of wire: 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: temperature: higher temperature (greater jostling of atoms), greater resistance

Resistance factors

Resistivity  = resistivity Unit:  m Metals  small resistivity (10x10 -8  m) Insulators  large resistivity (1x10 15  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 = 5.2x10 -7 m 2 and 16-gauge wire has A = 13x10 -7 m 2, find the resistance per meter of each if they are copper (  = 1.58 x  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 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 cross- sectional area of 2x10 -7 m 2 and is 1.3 m long. The material has resistivity of 4x10 -5  m at 200°C and a temperature coefficient of 3x /°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 Transmission of electricity MRI MRI Maglev Maglev Powerful, small electric motors Powerful, small electric motors Faster computer chips Faster computer chips

Superconductors