state the properties of high resistivity and low resistivity
Answers
Answer:Material having low resistivity or high conductivity are very useful in electrical engineering for manufacturing electrical engineering machines or equipment’s. These material used as conductors for all kind of winding required in electrical machines, apparatus and devices. These material are also used as conductor in transmission and distribution of electrical energy.
Required Properties in high Low Resistivity or High Conductivity Conducting Material
This following properties are required in high conductivity materials:
Highest possible conductivity (ideally zero).
Least possible temperature coefficient of resistance (ideally zero).
High melting point.
High mechanical strength.
High ductility, so that can be drawn in the form of wire easily.
High corrosion resistance (free from oxidation).
Solder ability, so that can be soldered easily to join the conductors.
Low cost.
Long life or durable.
High flexibility.
The above required properties varies with the purpose for which material is being used. Any impurity whether metallic or non metallic increase the resistivity of metals. Even an impurity of low resistivity will increase the resistivity of metal. The reason behind this is that the addition of slight impurity creates imperfections in the crystal lattice which disturb the flow of electrons through metals.
Some of low resistivity or high conductivity materials and their resistivity with temperature coefficient of resistance are given in table below –
Sl No. Metals Chemical Symbol Resistivity (µΩ -cm) Temperature Coefficients of Resistance (/oC)
1 Silver Ag 1.58 0.0038
2 Copper Cu 1.68 0.00386
3 Gold Au 2.21 0.0034
4 Aluminum Al 2.65 0.00429
Silver (Ag)
Silver is the best conductor of electricity. It is having highest conductivity. It is moldable and weld able. The main drawback of it that it is very costly, which limits its practical use in electrical machines / equipment. However, it is still used in precious equipment used for research where cost of equipment does not matter.
Properties
Resistivity : 1.58 µΩ -cm
Temperature coefficient of resistance at 20oC: 0.0038/oC
Melting point: 962oC
Specific gravity: 10.49 gm /cm3
Copper (Cu)
The extensively used, high conductivity material as conductor for electrical machines or equipment, is copper. Malleability, weld ability and solder ability are most important properties of copper. Copper in pure form is having good conductivity. But the conductivity of standard grade copper is reduced due presence of impurities.
Properties
Resistivity : 1.68 µΩ -cm
Temperature coefficient of resistance at 20oC: 0.00386 /oC
Melting point: 1085oC
Specific gravity: 8.96gm /cm3
Gold (Au)
Gold is a precious and costly metal. It is having good conductivity. Gold is having highest malleability and ductility among all metals. Due to high cost, its practical use is limited to precious instruments used for research.
Properties
Resistivity : 2.21 µΩ -cm
Temperature coefficient of resistance at 20oC: 0.0034 /oC
Melting point: 1064oC
Specific gravity: 19.30gm /cm3
Aluminum (Al)
Aluminum is an element which is a silver-white, light weight, soft, non-magnetic and ductile metal. Aluminum is the third most abundant element (after oxygen and silicon) and most abundant metal found in earth’s crust. The main ore of aluminum is bauxite. Aluminum is having low density, high ductility, good corrosion resistance and good conductivity, which makes it suitable to use as electric conductor for transmission and distribution of electricity.
Properties
Resistivity: 2.65 µΩ -cm
Temperature coefficient of resistance at 20oC: 0.00429 /oC
Melting point: 660oC
Specific gravity: 2.70 gm /cm3