The Laws of Circuit - you can learn and practice by just reading

Copyright. Charles Kim 2006

Power Calculation Blue
Power calculation is sometimes confusing, especially when we calculate power supply/consumption amount by a voltage/current source, When power calculation is involved in a passive element (R), we apply the famous power formulae and its variations: P=V*I=I*I*R =V*V/(R). However, for a source, there is no R, therefore the only power equation is the basic formulae: P=V*I. Therefore, when you calculate power for a voltage source (V is given here), you have to find the current flowing through the voltage source to determine the power. Similarly, the voltage across a current source must be found to calculate the power supply/consumption for the current source. Remember, there is voltage developed across a current source, and current flows through a voltage source. Then power of a voltage source tells how much current it supplies to (or receives from) a circuit. Similarly. power of a current source tells how much voltage can be developed across the current source.
How to Find Voltage (including its polarity) when Power and Current are given
Now consider an example problem in which, for a box which can be anything (passive element like R or voltage/current souce), power (P) is given and current(I) is given, and the voltage across the box is sought. When the current (I) is given, the direction of the current is also given. Now first thing you have to check, to find the voltage value and the voltage polarity in the box, is to see if the power (P) is positive or negative. If the power is positive, then the box is a passive element (R), since passive element consumes power and this means that power must be positive. The voltage value (V) is given by P/I. The poraity of the voltage then must follow the passive convention which, in essense, is current flows from the high (+) polarity to the low(-) polairty of the voltage source. This means that the current flows through the voltage source from (+) to (-) polarity, and thus current flows in to the (+) polarity marked node.  In this situation, the voltage source consums power.  Surprised?  Take the voltage source as a battery, then you'd ve relived that the battery would be presently being charged.  If the power (P) is given with negative number, it tells you that the box delivers power. To have a negative number for the power (P), one of the two variables (i.e., V or I) must be a negative number. This means that, in this example case, the current should flow from low(-) polairty to high (+) polarity of the volatge across the box, meaning that current flows out of the (+) polarity marked node.  The value (without sign) of the voltage is the same P/I.

  • Now as a practice, can you find time to solve these 2 problems?