Q17 of 72 Page 12

Derive the formula for electrical power which is used only when current I and resistance R are known to us.

We know that P = V × I ----(1)
From Ohm's law = R
or V = I × R ----(2)
Substituting the value of V in equation (1), we get
P = I × R × I
  = I2 × R

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15 Derive the expression for the resistivity of a substance and give the meaning of each symbol which occurs in it. 16 (a) What happens to the electrical resistance when mercury is cooled to 4.12 K? 
(b) What name is given to this phenomenon? 18 State Coulomb's law and give the unit of force constant. 19 Derive the formula for electrical power, which is used only when voltage V and resistance R are known to us.