電路(2電池)電路(2電池)電路(2電池)

Option A

Current = (6+12)/(2+4) A = 18/6 A = 3 A
The current flows counter-clockwise.
Hence, when going from P to Q along the current direction (i.e. counter-clockwise), we have,
volage across the 4-ohm resistor = -(3 x 4) v = -12 v
The -ve sign indicates a drop in voltage
But there is a rise of +12 v from the -ve pole to +ve of the 12v battery
Therefore, voltage between P and Q = (-12 + 12) v = 0 v