Calculate the power supplied by element A in the figure.
In this question, we are told that element A supplies power. When using our passive sign convention, we know that current leaves the positive terminal. In the figure shown to us, we see that current is actually coming in the opposite direction of what our sign convention would dictate, thus, we can write:
p=vi where v is voltage and i is current.
p=(20v)(-2A) (again, the 2A is negative because it’s in the opposite direction of what the current from element A is supplying)
This question can be found in Basic Engineering Circuit Analysis, 10th edition, chapter 1, question 1.22.