Wednesday, June 8, 2011

Light bulb and resistance

Diagram shows a bulb labelled 12 V, 24 W.
a. What is meant be 12 V, 24 W.
b. Why filament is made of high resistance conductor.
c. State 2 factors that affected the resistance of the conductor (filament).

Suggested answer :


a. 12 V, 24 W means that the bulb will consume 24 J of electrical energy every second if it is connected to 12 V mains supply.

b. To give out more heat energy and the bulb will become brighter. 
(If you connect a bulb to a constant current source, then its brightness increases with increasing resistance, because the power I2 R is proportional to the resistance)
c.
i) Length of  conductor
The resistance of a conductor increases with length.


ii) Cross sectional area ( thickness) of  conductor
The thicker the conductor, the lower the resistance.

iii) Temperature of conductor 
Filament light bulbs are non Ohmic conductors,it resistance increases with temperature.
It is a known fact that as an filament light bulb gets hotter, it gets brighter as well.

iv) Material
For fixed length and thickness of a conductor, its resistance is affected by the type of material.
Example resistivity of tungsten is higher than aluminium.

more info : http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity




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