Resistance

Definition of an OHMAn ohm is a resistance in a conductor that

produces a potential difference of one volt when a current of one ampere is flowing through it.

R =

ResistivityElectrical resistivity (also known

as resistivity, specific electrical resistance, or volume resistivity) is an intrinsic property that

quantifies how strongly a given material opposes the flow of electric current.

Units: Ohm-Meters

Resistivity Coefficient =

ρ (ohm-meters)

Factors that Affects the Resistance of a Conductor

The resistance of any material is determined by the following factors: Material Length Cross-sectional Area Temperature

Resistance: Circular Wires Copper is the most widely used material because it is quite

malleable, ductile and available. Aluminum was tried for general wiring but because of its

thermal characteristics created difficulties. Silver and gold are used but because of cost, they have been

limited to places that justify the cost. Tungsten has a resistivity three times that of copper but there

are occasions when its physical characteristics (durability and hardness) are the overriding considerations.

Resistivity & Temp. Coeff. of Commonly Used

ConductorsMaterial

Resistivity ρ(ohm m)

Temperaturecoefficient αper degree C

Conductivity σ x 107 /Ωm

Silver 1.59 x10-8 .0038 6.29

Copper 1.68 x10-8 .00386 5.95

Copper, annealed 1.72 x10-8 .00393 5.81

Aluminum 2.65 x10-8 .00429 3.77

Tungsten 5.6 x10-8 .0045 1.79

Iron 9.71 x10-8 .00651 1.03

Platinum 10.6 x10-8 .003927 0.943

Manganin 48.2 x10-8 .000002 0.207

Lead 22 x10-8 ... 0.45

Mercury 98 x10-8 .0009 0.10

Nichrome(Ni,Fe,Cr alloy)

100 x10-8 .0004 0.10

Constantan 49 x10-8 ... 0.20

Carbon*(graphite)

3-60 x10-5 -.0005 ...

Germanium* 1-500 x10-3 -.05 ...

Silicon* 0.1-60 ... -.07 ...

Glass 1-10000 x109 ... ...

Quartz(fused)

7.5 x1017 ... ...

Hard rubber 1-100 x1013 ... ...

Calculating Resistance from Resistivity

Resistance: Circular Wires

in. 1000

1 mil 1

Example:

Example 2:

Temperature Effects Temperature has a significant effect on the resistance

of conductors, semiconductors and insulators. For good conductors, an increase in temperature will result

in an increase in the resistance level. Consequently, conductors have positive temperature coefficients.

For semiconductor materials, an increase in temperature will result in a decrease in the resistance level. Consequently, semiconductors have negative temperature coefficients.

Calculating Resistance from Temperature

Where:T – inferred absolute zero resistance temperature in °C

Annealed Copper = -234.5 °CAluminum = -236 °CHDC = -242 °C

Example 1:

Example 2:

SUMMARYThe higher the resistivity of a conductor, the higher

its resistance.The longer the length of a conductor, the higher its

resistance.The lower the cross-sectional area of a conductor,

the higher its resistance.The higher the temperature of a conductor, the

higher its resistance.