I'm currently studying AC circuits and I find it difficult to understand phase differences between voltage and current on different components. For example, let's look at this circuit: enter image description here

And here is the time domain simulation in PSpice:
enter image description here source - green, resistor - blue, capacitor - red

Why does resistor voltage preceeds source voltage, shouldn't they be in phase? If voltage on the capacitor lags current by \$\pi/2\$, it also lags voltage on the resistor by \$\pi/2\$, since resistor voltage and current are in phase. What am I missing? Thank you for you time.


2 Answers 2


Memory mnemonic: ELI the ICE man.

L = Inductor, C = Capacitor. E = voltage, I = current.

In ELI, E comes before I. In a circuit with an ideal inductor, I always lags \$V_S\$ by \$90^{\circ}\$.

ICE, I comes before E. In a capacitor, I always leads \$V_S\$ by \$90^{\circ}\$.

With a resistor, I is in phase with \$V_S\$.

For a series RC circuit, you have a combination of the resistor and capacitor.

I leads \$V_C\$ by \$90^{\circ}\$ and I is in phase with \$V_R\$. Note change in subscripts.

I will lead \$V_S\$ by phase angle \$\theta\$, some where between \$0^{\circ}\$and \$90^{\circ}\$. In your case: \$\theta = 25.7^{\circ}\$.

\$V_C\$ first, \$V_S\$ second (\$64.3^{\circ}\$) (largest) and \$V_R\$ last (in phase with current \$90^{\circ}\$ behind).

This is NOT what you have. You have an issue with polarities. Would you believe you have to pull invert your \$V_C\$ (not sure how you do it) and reverse resistor polarity.

Your source (green) is the largest. \$V_R\$ will be larger (blue) than \$V_C\$ (red), but sequence is wrong. Blue waveform is closet to green, so that is correct.


In a capacitor, current leads voltage by 90 degrees and this is what you have. Blue is voltage across the resistor and that voltage is in phase with the current through it (and the capacitor) so, you can totally expect blue to lead red by 90 degrees and it looks like it is.

Why does resistor voltage preceeds source voltage, shouldn't they be in phase?

They won't be in phase because the current drawn by R and C in series isn't in phase with the supply voltage (green).


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