Can anyone tell me why when I'm asked to find the voltage difference between point A and point B, my I8 has to be negative?

And is the answer given wrong since I'm supposed to add I9 but then on the next line of equation it is minus instead

P/s: the answer is given by my lecturer... but I don't think it is right.

 

Vab=14V is correct.

The orientation of the 80V source should give a clue as to the direction of I9 - i.e. opposite to that shown by its arrow on the schematic.

 

Can anyone tell me why when I'm asked to find the voltage difference between point A and point B, my I8 has to be negative?

And is the answer given wrong since I'm supposed to add I9 but then on the next line of equation it is minus instead

P/s: the answer is given by my lecturer... but I don't think it is right.

Again the same direction of current assumed versus the actual direction of current situation. Since they are both opposite, you attach a -ve sign to the actual current.

Solve the question yourself to see if your lecturer is wrong .