The electric power consumed by a device may be calculated by using either of the two expressions P = I2R or P = V 2/R . The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality. How can the seemingly different dependence of P on R in these expressions be explained.

The expression P=I²R IS USED ONLY WHEN RESISTANCES ARE CONNECTED IN SERIES BECAUSE IN SERIES COMBINATION CURRENT IN EACH RESISTANCE IS CONSTANT

FOR EXAMPLE : IF R & R' ARE CONNECTED IN SERIES & I CURRENT IS PASSING THROUGH BOTH OF THEM THEN TOTAL POWER P=I²R+I²R'

P=P+P' THIS IS THE EXPRESSION FOR POWER WHEN RESISTANCES ARE CONNECTED IN SERIES

SIMILARLY WHEN RESISTANCES ARE CONNECTED IN PARALLEL THEN POWER BECOMES INVERSELY PROPORTIONAL TO R BECAUSE VOLTAGE IS CONSTANT IN THIS CASE