All about finding or solving for the resistance

1. Resistance

2. Definition of an OHM
An 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 =

3. Resistivity
Electrical 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)

4. 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

5. 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.

6. Resistivity & Temp. Coeff. of
Commonly Used Conductors
Material
Resistivity ρ
(ohm m)
Temperature
coefficient α
per 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

7. 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
... ...

8. Calculating Resistance from
Resistivity

9. Resistance: Circular Wires
in.
1000
1
mil1 =

10. Example:

11. Example 2:

12. 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.

13. Calculating Resistance from
Temperature

14. Where:
T – inferred absolute zero resistance temperature
in °C
Annealed Copper = -234.5 °C
Aluminum = -236 °C
HDC = -242 °C

15. Example 1:

16. Example 2:

17. SUMMARY
The 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.