Passive sign convention

Problem statement

I'm trying to understand how passive sign convention works, and how to read the voltages/currents direction in a circuit.

For different set of voltages and currents I'm trying to calculate power. I'm using the picture below as a reference.

a) i=10A v=125V

b) i=5a v=-240V

c) i=-12A v=480V

d) -25A v=-660V



Equations

p=vi or p=-vi









Attempt at solution

I don't understand how to get the polarities.

I'm having a hard time understanding how I can use the passive sign convention. My definition says if the current is in the direction of the reference voltage drop across the terminals, p=iv.



When I look at the picture below I'm confused how I could tell if there's a voltage drop or not? Whether the current arrow is pointing left or right, wouldn't it be pointing into the positive terminal regardless?



I have the solution from the book below but I don't understand. If you're standing at A, looking at B, why is the current going into the negative terminal? Also, if the current arrow was pointing right, does that mean the current is now going to the positive terminal?



Solution from book

Assume we are standing at box A looking toward box B. Then, using the

passive sign convention p = −vi, since the current i is flowing into the −

terminal of the voltage v. Now we just substitute the values for v and i into

the equation for power. Remember that if the power is positive, B is absorbing

power, so the power must be flowing from A to B. If the power is negative, B

is generating power so the power must be flowing from B to A.





Better picture: http://ift.tt/1zOeuSJ