The electric bulb converts electrical energy into Light and Heat energy. To convert electricity into Light and heat, it is necessary to have high resistance for any conductor. Tungsten is a metal which has high resistivity and high melting point. When a metal produces heat after passing electricity through it, which may lead to melting of the conductor. Due to this reason the conductors must have a high melting point when they are supposed to produce heat and light. Tungsten has a high melting point nearly 3422oC which protects the filament from melting.
The filament of the bulb is very thin and long to increase its resistance. The resistance decreases with increase in area of cross section(thickness) of a conductor and increases with increase in the length of the conductor. Generally the filament of a bulb spirals and is fitted in the bulb to increase in the length.
Resistance R ∝ 1/A (A= Area of cross section of the conductor)
R ∝ L/A
R= ρL/A (ρ= Specific resistance or Resistivity)
Resistance of a conductor depends on its Geometry (length and area of cross section) along with its nature and temperature.
Specific resistance or Resistivity only depend on the nature of the conductor but not on its Geometry. There is no change in the resistivity of a conductor if its length and area of cross section changes.
Units:
Resistance - Ohm ( Ω )
Specific resistance or Resistivity - Ohm - meter ( Ω-m )
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