Can a single capacitor circuit have phase shift?

Click For Summary

Discussion Overview

The discussion centers around whether a single capacitor in a circuit with an AC sinusoidal voltage source can experience a phase shift, particularly in relation to the voltage across the capacitor compared to the source voltage. Participants explore the conditions under which phase shifts can occur, the role of additional circuit elements, and the implications of circuit configurations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that a capacitor alone cannot have a phase shift relative to the voltage source, as the phase of the voltage source is fixed.
  • Others argue that phase shifts can only be discussed when there are multiple points in the circuit, suggesting that at least three points are necessary to measure phase differences accurately.
  • A participant mentions that phase shifts may occur due to the presence of resistance in the circuit, either from an external resistor or the internal resistance of the voltage source.
  • One participant shares their experience from a lab where they needed to explain the presence of phase shift in a circuit, indicating that a single capacitor could not account for observed phase shifts.
  • Another participant questions the validity of simulation results, suggesting that logic and circuit theory should guide understanding rather than relying solely on simulations.
  • Some participants discuss the mathematical relationships involving impedance and reactance, emphasizing the importance of complex numbers in analyzing phase shifts.
  • There are mentions of specific capacitor values and their impact on phase shifts, with suggestions to measure voltages at different points in the circuit to observe phase differences.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the conditions necessary for phase shifts in circuits involving capacitors. There is no consensus on whether a single capacitor can exhibit a phase shift without additional components.

Contextual Notes

Participants express uncertainty about the definitions and measurements of phase shifts, particularly in relation to voltage and current waveforms. The discussion also highlights the potential confusion between resistance and impedance in circuit analysis.

Evales
Messages
53
Reaction score
0
Hi guys,

Short question here, just wondering if it's possible for a capacitor, alone in a circuit with an AC sinusoidal voltage source to experience phase shift?

Meaning can the voltage over a capacitor have a different phase to the voltage from the voltage source.

Thanks!
 
Engineering news on Phys.org
If you have a 'Voltage Source' then its phase cannot be different from what it 'wanted' to be (that's the definition). A capacitor, connected across it must follow that voltage. To get any phase shift, there has to be some resistance in series - either one you soldered in or the 'real' internal resistance of your source. You have to measure phase relative to some reference, of course.
 
Thanks so much for the reply!

So phase shift only occurs over the element? I'm having some trouble understanding what causes the phase shift, is it because there is another element in the circuit so there is a place for a phase shift to exist without touching the source?
 
The phase shift is not "over the element" it is the difference in the voltage that appears across the element COMPARED with the voltage that appears across two other points in the cicruit. ergo - you need at least three points in a circuit before you can discuss 'phase shift / phase difference.
Alternatively, you can discuss the phase difference between the Voltage and Current waveforms associated with just one element. But you need to specify which you are doing before making any definite statement.
BUT, for a capacitor connected to a voltage source, the current will be infinite all the time (except at max and min) because any charge imbalance will be rectified in zero time if there is no R involved. Such models are not much use to the student, actually!
 
It's true, however I had a lab where I needed to identify an unknown circuit. One of the possible circuits given was a capacitor by itself and I needed to explain why it could not have been a single capacitor as there was phase shift present.

I just simulated something in pSpice as well, out of curiosity, does the phase difference only happen if the resistor is before the capacitor in the circuit? When I tested it in pSpice a circuit with a resistor after the capacitor had no difference in phase compared to the source.
 
Why would there be a difference between source and capacitor if there were no resistor in between?
MAybe you should rely on the logic of the circuit in itself rather than on a simulation which may not be doing what you think it's doing?
Simulations are great for complex circuit analysis (no-one would bother to do a complex circuit by hand these days) but, for one or two components, does it really help?

But what is the phase that you are calculating? is it between two voltages of a voltage and a current. If you give the information here that you gave spice, then it would be more clear.
 
t0N5X.png

Voltage source:
Voff: 0
Vamp: 240
Freq: 50
TD: 0
TR: 0
Phase: 90

Traces here:
ux7Vu.png


Gonna be honest, out of all the subjects this one is the hardest for me, I just don't think in circuit logic. I've spent more hours on this subject than any but it's still fail city haha.

It's my last circuits unit though, going into Computer Systems engineering.
 
Is that capacitor 1 Farad? If so, you won't get much of a phase shift.

Try 1 μF.

Measure from ground to a point between the resistor and the capacitor. You should get a clear phase shift compared with the input voltage.
 
Thanks, I tried that, you're absolutely right. It did cause a phase shift. It was confusing me quite a bit how I could have gotten those results.
 
  • #10
Seeing that the resistance of a capacitor is 1/Jωc...or 1<-90/(ωc)...

You're damn skippy that you will get a phase shift between the voltage and current once you plug it into your V=IR!

Your max phase shift of 90 degrees of the current will occur with the least amount of real resistance. As real resistance goes up in series with your capacitor...the phase shift will get less and less.

Again...it's all in the math.

OH sorry...you are talking about voltage vs voltage...see sophie.
If your interested in current vs voltage shift...see my speel.
 
  • #11
psparky said:
Seeing that the resistance of a capacitor is 1/Jωc...or 1<-90/(ωc)...

You're damn skippy that you will get a phase shift between the voltage and current once you plug it into your V=IR!

Your max phase shift of 90 degrees of the current will occur with the least amount of real resistance. As real resistance goes up in series with your capacitor...the phase shift will get less and less.

Again...it's all in the math.

OH sorry...you are talking about voltage vs voltage...see sophie.
If your interested in current vs voltage shift...see my speel.

Not just being picky here, but . . . . The word is Impedance. The word Resistance had other connotations and we mustn't confuse the two.
We then say that the Impedance, Z is R+jX, where X is the Reactance. For a Capacitor, the Reactance is 1/ωC.

You can't afford to neglect the use of Complex Numbers for this or you can't arrive at a phase shift.
V=IZ gives you it.
 
  • #12
sophiecentaur said:
Not just being picky here, but . . . . The word is Impedance. The word Resistance had other connotations and we mustn't confuse the two.
We then say that the Impedance, Z is R+jX, where X is the Reactance. For a Capacitor, the Reactance is 1/ωC.

You can't afford to neglect the use of Complex Numbers for this or you can't arrive at a phase shift.
V=IZ gives you it.

Exactly what I was trying to say. How silly of me.
 

Similar threads

Need a voltage divider for a capacitor source voltage
  • · Replies 78 ·
3
Replies
78
Views
6K
Phase Current Sensing Circuit: Q&A
  • · Replies 1 ·
Replies
1
Views
2K
Phasor diagrams for RLC circuit and resonant frequency
  • · Replies 3 ·
Replies
3
Views
5K
Equation that describes positive/negative feedback
  • · Replies 20 ·
Replies
20
Views
2K
Does wire length difference create phase shift?
  • · Replies 5 ·
Replies
5
Views
4K
Basic question about RLC circuits
  • · Replies 32 ·
2
Replies
32
Views
4K
Alternator torque when connected to a phase-shifted AC source
  • · Replies 3 ·
Replies
3
Views
2K
Build Your Own AC Generator: How to Create 2V for an LED
  • · Replies 10 ·
Replies
10
Views
2K
How to introduce a phase shift in an AC circuit
  • · Replies 17 ·
Replies
17
Views
7K
Metering a Single Phase 208 VAC Circuit
  • · Replies 77 ·
3
Replies
77
Views
9K