Resistance
When a voltage is applied across a conductor the electrons starts
moving across a particular direction, due to this movement these electrons collide
with the atoms as well as the ions present in the conductor and due to this phenomena
heat is generated in the conductor. These atoms or molecules opposes the flow
of the electrons this opposition is considered as the resistance of the
particular conductor.
This resistance is a property of the material of the conductor,
higher the obstruction higher the resistance.
Resistance of a conductor can be obtained by the following formula;
R : Resistance
Ρ : Resistivity
l : Length of the conductor
A : Cross-sectional area of the
conductor.
R = ρ(l/A)
Resistance is measured in Ohms (Ω).
Through this relationship the following can be derived.
- Resistance is proportional to the length of the wire, when the
length increases the resistance increases.
- Resistance is inversely proportional to the cross-sectional area,
that is when the cross-sectional area is high the resistance is low.
- Resistivity is a component which depends on the material therefore
resistance also depends on the type of material.
- Moreover, resistance also depends on the temperature as well.
Resistivity.
This is the resistance with specifics, that is, resistivity of a
material is known as the resistance of that material with 1m length and 1
square meter area.
Resistance of a conductor can be obtained by the following formula;
R : Resistance
Ρ : Resistivity
l : Length of the conductor
A : Cross-sectional area of the
conductor.
ρ = R(A/l)
Resistivity is measured in Ohms (Ωm).
Resistivity is directly proportional to the temperature and the
nature of the material.
Inverse of resistivity is known as conductivity.
σ
= 1/ρ
Resistivity and Temperature Coefficient at 20-degree C
Material
|
Resistivity ρ
(ohm m) |
Temperature
coefficient α per degree C |
Conductivity σ
x 107 /Ωm |
Ref
|
|
Silver
|
1.59
|
x10-8
|
.0038
|
6.29
|
3
|
Copper
|
1.68
|
x10-8
|
.00386
|
5.95
|
3
|
Copper,
annealed
|
1.72
|
x10-8
|
.00393
|
5.81
|
2
|
Aluminum
|
2.65
|
x10-8
|
.00429
|
3.77
|
1
|
Tungsten
|
5.6
|
x10-8
|
.0045
|
1.79
|
1
|
Iron
|
9.71
|
x10-8
|
.00651
|
1.03
|
1
|
Platinum
|
10.6
|
x10-8
|
.003927
|
0.943
|
1
|
Manganin
|
48.2
|
x10-8
|
.000002
|
0.207
|
1
|
Lead
|
22
|
x10-8
|
...
|
0.45
|
1
|
Mercury
|
98
|
x10-8
|
.0009
|
0.10
|
1
|
Nichrome
(Ni,Fe,Cr alloy) |
100
|
x10-8
|
.0004
|
0.10
|
1
|
Constantan
|
49
|
x10-8
|
...
|
0.20
|
1
|
Carbon*
(graphite) |
3-60
|
x10-5
|
-.0005
|
...
|
1
|
Germanium*
|
1-500
|
x10-3
|
-.05
|
...
|
1
|
Silicon*
|
0.1-60
|
...
|
-.07
|
...
|
1
|
Glass
|
1-10000
|
x109
|
...
|
...
|
1
|
Quartz
(fused) |
7.5
|
x1017
|
...
|
...
|
1
|
Hard rubber
|
1-100
|
x1013
|
...
|
...
|
1
|
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