The power of circuit

Electricity and Magnetism Level 2

2 A of current is flowing through the above circuit. Find the power of the circuit (in Watts).

80 W 60 W 40 W 120 W

3 solutions

Chew-Seong Cheong
Aug 17, 2014

Power is only consumed by the resistor $R$ , and not the inductor $L$ or capacitor $C$ . Power consumed $P$ it is given by:

$P=I_RV_R=2A\times 40V=\boxed{80W}$

Sir power is consumed by inductor and capacitor that power is known as reactive power, we measure that in volt ampere reactive.

Here into this problem we got answer

From Vrms × Irms cos(36.86)=80 watts.

The reactive power here is Vrms ×Irms Sin(36.86)=60 volt ampere reactive.

Abhay Tiwari - 5 years, 10 months ago

Power as defined in Physics is the rate of doing work ( see here ). In SI units it is measured by W = J/s. Work = energy in the form of heat comes only from resistor. The term "reactive power" is only used for convenience and is not really power. Its unit is usually in kVAr by engineer.

Chew-Seong Cheong - 5 years, 10 months ago

Sir, the power which we are here talking about contains energy which we cannot recover like in resistor we can use its heat dissipation. Sir, for more see the physical significance here in this link https://en.m.wikipedia.org/wiki/Volt-ampere reactive#Physical significance of reactive_power

Abhay Tiwari - 5 years, 10 months ago

@Abhay Tiwari – Your understanding of power is similar to mind. But it was incorrect for you to say power is consumed or absorbed by inductors and capacitors. They don't consume or absorb energy but store it.

Chew-Seong Cheong - 5 years, 10 months ago

@Chew-Seong Cheong – Sir, please don't mind if I correct you there, this reactive power can be consumed by load/ generation elements. i.e. capacitor and inductor only.

Abhay Tiwari - 5 years, 10 months ago

Why not the inductor and capacitor? Can anyone please explain ?

Tammu Triveni - 6 years, 9 months ago

Tammu energy is absorbed by inductor, more elaborately it gets stored in the inductor and capacitor electromagneticallly and electrostatically respectively, which cannot be used by us or rather saying is not usefull for us but reduces efficiency of the circuit power handling capability. This is one of the main disadvantage of using inductor and capacitor in circuits.

Abhay Tiwari - 5 years, 10 months ago

Inductors and capacitors store energy not absorb power. Power is strictly "real" power in the form of energy/time. In electric circuits energy is only lost in the form of heat through resistance. Therefore we mention about power factor. For example. a power factor of 0.8 means only 0.8 of VA is power or W, the rest VAr is not power. Inductors can keep voltage and capacitors keep charges therefore they keep potential energy. Only some chemical reactions absorb energy hence power usually in the form of heat.

Chew-Seong Cheong - 5 years, 10 months ago

@Chew-Seong Cheong – Ya, thanks for correcting me there. And sir phase lag and phase lead can be proved by impedance triangle which in turn can be made from all the three powers I.e. total power(hypotenuse) , reactive power(height), active power ( base).

Abhay Tiwari - 5 years, 10 months ago

@Abhay Tiwari – Good, that you got it. I was trained an electrical engineer but I love math more.

Chew-Seong Cheong - 5 years, 10 months ago

@Chew-Seong Cheong – Sir, even I do love math more.

Abhay Tiwari - 5 years, 10 months ago

Ahmed Kamal
Jan 25, 2015

This capacitor acts as an open circuit after a given time .So at steady state there is no current flow on the circuit .

But we are talking about AC. There is a current flowing through inductor and capacity just $\pm 90^\circ$ out of phase.

Chew-Seong Cheong - 5 years, 10 months ago

Gacon Noname
Jun 19, 2014

$P = U.I.\frac{R}{Z} = 80W$