So by our convention, if the current is flowing downhill from plus to minus,

and you have a positive voltage, positive current,

then we have a positive power, positive power is consumed.

In the other case, power is flowing from minus to plus, so

we end up with a negative power.

Remember that negative power is generated.

Another interesting fact about power, is that the sum of power generated and

consumed in a system is zero, because power is the rate

of change of energy and energy is always conserved,

the sum of power generated and consumed in a system is zero.

We know that energy is conserved, we're neither creating or destroying energy,

and since power is simply the rate of change in that energy,

the sum of power also has to be zero.

So take a look at this system.

You have an unknown power with three other sources

that are generating and consuming 3 watts, -7 watts, and 6 watts.

What is Px?

Think about it.

We have plus 3, plus 6, which is a plus 9, and

a -7 which means that PX must be -2,

because -2 watts and -7 watts totals -9 watts, and

plus 3 watts plus 6 watts equals plus 9 watts.

So, 3 and 6 is 9, 2 and 7 is 9.

PX must be a -2.