Transcript Resistance and Resistivity

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:
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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