# Power Calculation: S=VI & V*I Meaning

• smruti
In summary, there are two notations used for calculating complex power, S=VI and S=VI*. The difference lies in the phase of the reactive power. The significance of the reactive power is important to understand in order to determine the correct notation to use. The equation S=VI* is derived from usual phasors. In an inductive circuit, the actual current lags the voltage and the reactive power is considered positive. In a capacitive circuit, the current leads the voltage and the reactive power is considered negative. Ultimately, both notations provide the same magnitude for components and the power delivered to the load.
smruti
in the calculation of complex power S=VI we sometimes use S=VI* and sometimes as S=V*I.what is the physical significance of these two notations and in which case these two expression has different meaning?

what is S ?

The difference is in the phase of the reactive power.
To understand that - you need to understand the significance of the reactive power.

The two relations get you the same magnitudes for the components and you normally only need the power delivered to the load anyway.
If you derive the equation for S from the usual phasors, then you get ##S=VI^\star##.

Thanks Simon

was wondering what the heck he was talking about

I agree with Simon, of course.In my opinion we use the conjugate current in formula S=VxI* since in an inductive circuit the actual current lags the voltage but the reactive power is considered positive[conventional].So if S[apparent power]= VxI=(Vre)x(Ire-jIim)=(VrexIre)+(-VrexIim)j then Q=-Vre*Iim.
If S[apparent power]= VxI*=(Vre)x(Ire+jIim)=(VrexIre)+(VrexIim)j and Q=+Vre*Iim.
If the current leads the voltage [capacitive circuit] then the reactive power is negative[conventionally].

smruti
S[apparent power]= VxI*=(Vre)x(Ire+jIim)=(VrexIre)+(VrexIim)j and Q=+Vre*Iim.
... erk: here let's tidy that up a bit:

##S = VI^\star = V_{re}\big(I_{re}+jI_{im}\big) = V_{re}I_{re} + j V_{re}I_{im}## and ##Q=+V_{re}I_{im}##.

... better? But did you take the conjugate properly? - I decided not to wade through all those letters to check.

LaTeX: worth the learning curve.

## 1. What is the formula for power calculation using S=VI?

The formula for calculating power using S=VI is simply multiplying the voltage (V) by the current (I). This formula is known as the "power equation" and is commonly used in electrical and electronic calculations.

## 2. What do the symbols V and I stand for in the power equation?

The symbol V represents voltage, which is the potential difference between two points in an electrical circuit. The symbol I represents current, which is the flow of electric charge through a conductor.

## 3. How is power related to voltage and current?

Power is directly proportional to both voltage and current. This means that as voltage and current increase, the power also increases. In other words, if either the voltage or current increases, the power will increase as well.

## 4. What is the significance of the V*I meaning in the power equation?

The V*I meaning in the power equation represents the product of voltage and current. This is the fundamental relationship in determining the amount of power in a circuit. It shows that power is dependent on both voltage and current.

## 5. Are there any units associated with the power equation?

Yes, the units for power are watts (W), which is equal to one joule per second (J/s). This means that one watt of power is equivalent to one joule of energy being transferred per second.

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