# Calculating the voltage drop in AC circuit

I'm supposed to calculate the voltage drop (Ux). Where R=R=R=1kΩ, C=1µF, I=1mA, E= 2sin(ωt+pi/2), ω = 10^3 rad/s.

I know I have to use superposition method, but I dont know how to calculate impedance in both situations.

• Which voltage drop do you want to calculate? Where does the voltage Ux occur? – TomS Feb 10 '16 at 9:49
• Oh god, I forgot, added to the picture. – Vendace Feb 10 '16 at 10:19

• 1 Substitute current source with it's output impedance. Ideally it is infinite, so leave it open.
• 2 Calculate currents and voltages due to voltage source.
• 3 Substitute voltage source with it's output impedance, Ideally, zero, so replace it with a short circuit to ground.
• 4 Calculate currents and voltages due to current source. (Hint: it is DC, so what does that mean for the capacitor?)
• 5 Add Corresponding currents and voltages from step 2 and 4.

Where is the top resistor connected to? It seems to float. If it is not connected at the other side, it has no function in this diagram.

• I get the steps. But that top resistor makes no sense to me, I just redrew it from the task as it was shown there. Do I just ignore it when counting impedance? – Vendace Feb 10 '16 at 12:14
• If the other terminal is not connected to anything, it carries no current. And neither does it have a voltage drop. Then yes, ignore it. – Bart Feb 10 '16 at 12:16

simulate this circuit – Schematic created using CircuitLab

simulate this circuit

Superpositioning seems definitely right.

1st part: - Treat the DC current source as open circuit and calculate the AC impedance of the capacitor. Then calculate the voltage on X2.

2nd part: - Treat the AC voltage source as shortcut and the capacitor as an open circuit. Then calculate the potential of the node between R1 and R3