Re: How do you tell the direction of power transmission?

You will only expend/create power where you have a voltage drop, following
P = current x voltage.  If the device has +5 volts on terminal 1, and
+1 volts on terminal 2, and the current is +2 amps flowing from 1 to 2,
then this device is _using_ 2*(5-2) = 6 Watts.  If the current were
flowing the other way, from 2 to 1, or to put it another way, if the
current from 1 to 2 were -2 amps, then the device would be creating
6 watts in the circuit.  If you had only an unbroken power line, you
woud have to look at the potentials on each side of the two wires,
and the current in the circuit.  Suppose one wire was at 100 V, and
the other was at 0 volts (ground).  Then the power would be being used
at the end where the current was flowing into, that is the power would be
"flowing" in the same direction as the current.  If the situatuion
were different, and the "hot" wire had -100 V potential, then the power
would be flowing in the opposite sense to the current.