Page 190 - WCS - Electrical

WORKSHOP CALCULATION & SCIENCE - CITS

Ohm 4 depends on the temperature of the conductor

If the potential difference across the two ends of conductor 1 L L
is 1 volt and the current through it is 1 ampere, then the R α L ; R α A ; R α A ; R = ρ A
resistance of the conductor is 1 Ohm.
Specific resistance
Laws of resistance The specific resistance of a material is the resistance
Specific resistance
The resistance offered by conductor depends on the offered to a current it passed between the opposite faces
following factors. The specific resistance of a material is the resistance offered to a current it passed between the opposite faces of
of the unit cube of the material. Specific resistance is
measured in Ohm - m or micro ohm - cm.
The resistance of the conductor the unit cube of the material. Specific resistance is measured in Ohm - m or micro ohm - cm.
Each mateiral has its own specific resistance or resistance or
1 is directly proportional to the length of the conductor Each mateiral has its own specific
resistivity.
(R  L) resistivity.
2 Varies inversely proportional to its cross sectional area E.g. : Copper - 1.72  cm, Silver - 1.64  cm,
E.g. : Copper - 1.72 mW cm, Silver - 1.64 mW cm,
Eureka - 38.5  cm, Iron - 9.8  cm,
⎛ 1 ⎞ Eureka - 38.5 mW cm, Iron - 9.8 mW cm,
of the conductor ⎜ R α ⎟ Aluminium - 2.8  cm, Nickel - 7.8 cm.
⎝ A ⎠ Aluminium - 2.8 mW cm, Nickel - 7.8 mW cm.
3 Depends on the material with which it is made. R = ohm cm
ρl
R=
A ohm cm
R = Resistance in ohms
R = Resistance in ohms
l = Length of the conductor in cm
l = Length of the conductor in cm
r = Specific Resistance in ohm cm
r = Specific Resistance in ohm cm
(symbol pronounced as rho)
(symbol pronounced as rho)
A = Area of cross - section in cm 2
A = Area of cross - section in cm 2
Conductor, insulator, types of connections - series and parallel
Conductors
Some materials and metals readily allow passage for electric current to flow. In such materials, called conductors,
electrons are able to pass readily from atom to atom.
Properties of conductors
A good conductor should have the following properties.
Electrical properties

• The conductivity must be good.
• Electrical energy spent in the form of heat must be low.
• Resistivity must be low (to reduce voltage drop and loss).
• Increase in resistance with temperature must be low.
Mechanical properties

• Ductility (the property of being drawn into thin wires).
• Solderability: the joint should have minimum contact resistance.
• Resistance to corrosion: should not get rusted when used outdoors.
• Should withstand stress and strain.

• It should be easy to fabricate.

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CITS : WCS - Electrical - Exercise 18

100 Workshop Calculation & Science : (NSQF) Exercise 1.7.33