Question

Please point where I am wrong .The formula I am using is P=VI , and getting the wrong 50 watts . The formula used in all solutions are P = v^2/R.
But why am I wrong instead of using the correct formula ?

Answer 1

• V = 220 V

• P = 100 W

P = V²/R

⇒100 = (220)²/R

⇒100 = 48400/R

⇒R = 48400/100

⇒R = 484 Ω

$\rule{100}2$

• V = 110 V

• R = 484 Ω

P = V²/R

⇒P = (110)²/484

⇒P = 12100/484

⇒P = 25 W

$\rule{200}2$

• P = 100 W

• V = 220 V

P = VI

⇒100 = 220 I

⇒I = 100/220

⇒I = 10/22

⇒I = 5/11

$\rule{100}2$

• V = 110 V

• I = 5/11 A

P = VI

P = 110 × (5/11)

⇒P = 50 W

Reason for such mistakes :-

• V^2/R is used when the potential in the circuit is same. i.e. if the two resistance are connected parallel then we will use V^2/R to calculate the power.

• Similarly if resistances are connected in series then current will be constant in the circuit. Then, we will use I^2R formula to calculate power.

• If we know the current and potential then VI is use to find out the power.