Electrical Properties of Engineering Materials

To finalize the material for an engineering product / application, we should have the knowledge of Electrical properties of materials. The Electrical properties of a material are those which determine ability of material to be suitable for a particular Electrical Engineering Application. Some of the typical Electrical properties of engineering materials are listed below-

Resistivity

It the property of material which resists the flow of electric current through material. It is the reciprocal of conductivity.
It is dented by ‘ρ’. Resistivity of a material of a conductor can be determined as below

Where, ‘R’ is the resistance of conductor in Ω.
‘A’ is the cross sectional area of conductor in m2
‘l’ is the length of the conductor in meter SI unit of resistivity of is Ω¦-meter. Resistivity of some materials is listed below

Sl. No. Element Resistivity at 20oC in Ω – m
1 Silver 1.59 × 10-8
2 Copper 1.7 × 10-8
3 Gold 2.44 × 10-8
4 Aluminum 2.82 × 10-8
5 Tungsten 5.6 × 10-8
6 Iron 1.0 × 10-7
7 Platinum 1.1 × 10-7
8 Lead 2.2 × 10-7
9 Manganin 4.82 × 10-7
10 Constantan 4.9 × 10-7
11 Mercury 9.8 × 10-7
12 Carbon (Graphite) 3.5 × 10-5
13 Germanium 4.6 × 10-1
14 Silicon 6.4 × 102
15 Glass 1010 to 1014
16 Quartz (fused) 7.5 × 1017

Conductivity

It is the property of material with allow the flow of electric current through material. It is a parameter which indicates that how easily electric current can flow through the material. It is denoted by ‘σ’. Conductivity of material is the reciprocal of resistivity. Conductivity of material can be determined by,

Its SI unit is 1/(Ω-meter) or ℧/meter.

Dielectric Strength

It is the property of material which indicates the ability of material to withstand at high voltages. Generally it is specified for insulating material to represent their operating voltage. A material having high dielectric strength can withstand at high voltages. Generally, it is represented in the unit of KV/cm. Dielectric strength of some insulating materials are listed below-

Sl. No. Material Dielectric Strength [KV(max.)/cm]
1 Air 30
2 Porcelain 80
3 Paraffin Wax 120
4 Transformer oil 160
5 Bakelite 220
6 Rubber 280
7 Paper 500
8 Teflon 600
9 Glass 1200
10 Mica 2000

Temperature Coefficient of Resistance

The temperature coefficient of resistance of a material indicates the change in resistance of material with change in temperature. Resistance of conductor changes with change of temperature.
The rise in resistance of a material with rise in temperature depends on following things,

  1. R2 – R1 ∝ R1
  2. R2 – R1 ∝ t2 – t1
  3. Property of material of conductor.

Where, R1 is the resistance of conductor at temperature of t1oC and R2 is the resistance of conductor at temperature of t2oC.
Hence, from above, R2 – R1 ∝ R1 (t2 – t1)
Or, R2 – R1 = α1 R1 (t2 – t1) ⇒ R2 = R1 [1 + α1 (t2 – t1)]
Where, α1 is temperature coefficient of resistance of material at temperature of t1oC. Its unit is /oC. Temperature coefficient of resistance of material is also depends on temperature. emperature coefficient of some materials are listed below,

Sl. No. Element Temperature Coefficient of Resistance in /oC
1 Manganin 0.00002
2 Constantan 0.00017
3 Nichrome 0.0004
4 Mercury 0.0009
5 Silver 0.0038
6 Copper 0.00386
7 Annealed copper 0.000393
8 Platinum 0.003927
9 Aluminum 0.00429
10 Carbon (Graphite) – 0.0005
11 Germanium – 0.05
12 Silicon – 0.07

Thermoelectricity

If the junction, formed by joining to two metals, is heated, a small voltage in the range of millivolt is produced. This effect is called thermoelectricity or thermoelectric effect. This effect forms the basis of operation of thermocouples and some temperature based transducers. This effect can be used to generate electricity, to measure the temperature and to measure the change is temperature of objects.

   
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