The electric power consumed by a device may be calculated by the using

1.	The electric power consumed by a device may be calculated by the using either of the two expressions P = I2R or P = V2/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.
Answer» Both the expressions are correct. In the first case where P = I²R current (I) remains constant. Whereas in the second expression where P = V² /R the Voltage (V) remains constant. Therefore P = I²R is used where the resistors are connected in series because in the series connection the current in each resistance is constant. P = V² /R is used where the resistors are connected in parallel because in the parallel connection the voltage in each resistance is constant. So power becomes inversely proportional to resistance.

The electric power consumed by a device may be calculated by the using either of the two expressions P = I2R or P = V2/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.

Answer»

Both the expressions are correct. In the first case where P = I²R current (I) remains constant. Whereas in the second expression where P = V² /R the Voltage (V) remains constant.

Therefore P = I²R is used where the resistors are connected in series because in the series connection the current in each resistance is constant.

P = V² /R is used where the resistors are connected in parallel because in the parallel connection the voltage in each resistance is constant. So power becomes inversely proportional to resistance.

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