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?
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