Circuit with 2 voltage supplies
Circuit's Total Supply Voltage = 20 V
Notice (right side): Voltage starts (at the top) out at +10 V.
R1 drops 10 V (now at 0 V).
R2 drops 5 V (now at -5 V).
R3 drops 5 V (now at -10 V, which matches the lower voltage supply).
If the 2 resistors on the left are connected in parallel (to the + side of the upper voltage supply), none of the voltages on the right change. (Just the total current of the upper voltage supply changes.)
