الجمعة، 20 مايو 2011

Resistance

        It is the property of the material by which it opposes the flow of current through it. The resistance of element is denoted by the symbol 'R'. Resistance is measured by ohms ( Ω ).
       The resistance of a given material depends on the physical properties of that material and given by,
 l = Length in meter.
a = Cross section area in square meter.
ρ = Resistivity in ohms-meter 
R = Resistance in ohms
       We can define unit ohms as below,
1 ohm : The resistance of a circuit, in which a current of 1 ampere generates the heat at the rate of 1 joules per second is said to be 1 ohm.
Now  4.186 Joules = 1 Calorie
Hence                 1 Joule =  0.24 Calorie
       Thus unit 1 ohm can be defined as that resistance of of circuit if it develops 0.24 calories of heat, when one ampere current flows through the circuit for one second.
       The unit ohm also can be defined as, one ohm resistance is that which allows one ampere current to flow through it when one volt voltage is impressed across it. 
       The relation between voltage and current for a resistance is given by Ohm's low as,
             v = R i
     
       The power absorbed by a resistance is give by :
       While the amount of energy converted to heat energy in terms t is given by,
As i2 term is always positive, the energy absorbed by the resistance is always positive.
       If the voltage across resistance is constant V and the current through it is constant I then the energy for t  ≥ 0 is given by,
While,

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