Physics doubt

[IBdoingnothing]

hey guys, I've got a doubt regarding dissisipation of power.

How do we know when to use P  = I^2 R

P = V^2 / R

P = VI

 

Is there some rule I'm not aware of? For some reason, P  = I^2 R is used for power lost, and P = VI is used for total power. Hope to hear form u guys soon!

[kw0573]

All three are equivalent, and is the power dissipated through use or storage of electrical energy. The only difference is that based on what you are given, you would use different equations. If you only know current and resistance, but not voltage, then P = I²R; similarly if you don't know current, then V²/R, and finally if doesn't know resistance, then VI. Note that if you are given one, you can derive the other two from the definition of resistance, R = V/I. What you are noting about power lost or total power is just coincidental that certain values were given by the test-makers. 

These 3 equivalent equations above are analogous to the equivalent kinematic equations, which each relates 4 out of 5 quantities, and are derived from definition of velocity and acceleration. As a general advice, you should roughly know how some equations are derived, especially for the electromagnetic phenomena equations, so at least you know what each variable is. The angle in many equations, for example, are often not given directly in a question so examiners try to "trick" students and see if they really know which angles to use. 

[Guest SNJERIN]

Also, it is often that you are given all of the three values, V I and R. In such cases it is safer to use P=RI^2. Because if you use the equation P= V^2/R for determining the power losses during the transmission of electrical power using transformers you get a wrong answer, because even though you know the voltage output of a step-up transformer that would not tell you what the pd across the wire is. So it is important in such cases to use  P=RI^2.