RRB ALP Stage-2 exam: Physics notes- CURRENT ELECTRICITY (PART 2)

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RESISTIVITY

The resistance of a uniform metallic conductor is directly proportional to its length (l) and inversely proportional to the area of cross-section (A). That is,
R ∝ l
and R ∝ 1/A
Combining both, we get
R ∝ l/A

or, R = ρl/A

where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor. The SI unit of resistivity is Ω m. It is a characteristic property of the material.

RESISTIVITY OF VARIOUS MATERIALS



TEMPERATURE DEPENDENCE OF RESISTIVITY

  • The resistivity of a material is found to be dependent on the temperature. 
  • The resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at high temperatures. For this reason, they are commonly used in electrical heating devices, like electric iron, toasters etc. Tungsten is used almost exclusively for filaments of electric bulbs, whereas copper and aluminium are generally used for electrical transmission lines.
  • Some materials like Nichrome (which is an alloy of nickel, iron and chromium) exhibit a very weak dependence of resistivity with temperature.
  • Unlike metals, the resistivities of semiconductors decrease with increasing temperatures.


TWO MAJOR TYPES OF RESISTORS:

(a) Wire bound resistors: They are made by winding the wires of an alloy, viz., manganin, constantan, nichrome or similar ones. The choice of these materials is dictated mostly by the fact that their resistivities are relatively insensitive to temperature. These resistances are typically in the range of a fraction of an ohm to a few hundred ohms.

(b) Carbon Resistors: Resistors in the higher range are made mostly from carbon. Carbon resistors are compact, inexpensive and thus find extensive use in electronic circuits. Carbon resistors are small in size and hence their values are
given using a colour code.

RELATION OF CURRENT AND DRIFT VELOCITY

  • When an electric field is applied, inside the conductor due to electric force the path of electron in general becomes curved (parabolic) instead of straight lines and electrons drift opposite to the field figure (B). 
  • Due to this drift the random motion of electrons get modified and there is a net transfer of electrons across a cross-section resulting in current.
  • Drift velocity is the average uniform velocity acquired by free electrons inside a metal by the application of an electric field which is responsible for current through it.




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