Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Voltage and current phase shifting/Current without voltage?

  1. Jun 28, 2017 #1
    Hello, I've recently been learning about capacitance and inductance causing the current to lead or lag. But how can the current possibly maintain the same waveform? Shouldn't the zeros of voltage and current always align? How can there be current without voltage?
     
  2. jcsd
  3. Jun 28, 2017 #2

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    No they don't. They align only for purely resistive elements. Inductors and capacitors are different.

    We have a PF Insights article on just that subject here. Please read that first, then post again if you have questions.

    https://www.physicsforums.com/insights/ac-power-analysis-part-1-basics/
     
  4. Jun 28, 2017 #3
    Thanks for the quick reply.
    Although interesting, I didn't really get the answer to my question from that article.
    I understand how the power in the circuit is 0 at any time the voltage or current is 0. But I'm not too interested in the power for this specific question.

    Perhaps it would be easier (for me at least) to think in terms of the movement of electrons.

    I'm glad you sent me to that article actually, as it shows exactly the graph I'm having trouble with where the current is shifted 90 degrees.
    Have I been taught an untruth at some point? Can there be current without voltage?
     
  5. Jun 28, 2017 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Anorlunda hit it. Check his article..
    Remember you're being taught about sinewaves.

    Go back to capacitor basics
    i = c dv/dt

    only for sinewaves do the function and its derivative have the same shape.
    d(sine) = cosine , and a cosine is just a sine shifted by 90 degrees.
    When one is at zero the other is at peak.

    Try it for a triangle wave.

    It's good that you question. Bouncing the basics against one another to resolve such apparent conflicts molds your thinking processes and you arrive at robust mental models. Keep up the good work.
     
    Last edited: Jun 28, 2017
  6. Jun 28, 2017 #5

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    No, that is seldom a helpful way to do it.

    Have you studied derivatives, sines and cosines? If yes, and I say ##I=C\frac{dV}{dt}## for a capacitor, and V=cos(wt), what is I? What is V when I is zero?
     
  7. Jun 28, 2017 #6

    cnh1995

    User Avatar
    Homework Helper

    No.
    When input voltage is zero, the current in the circuit is maintained by the energy stored in the inductor or capacitor. The phase difference is a result of the energy storing ability of the reactive elements and their V-I relationship.

    As jim said, derivatives and integrals of a sine-wave signal are also sine waves with some phase difference. Write the KVL equation (integro-differential equation) of an RLC circuit and solve it for sine wave as well as triangular wave.
     
  8. Jun 28, 2017 #7
    Unfortunately, I'm not at liberty to be doing mathematical proofs as I'm at work, however, you may have shed some light on the situation.

    So we're basically fudging the graphs by ignoring the voltage across the capacitor when the source voltage is 0?
     
  9. Jun 28, 2017 #8

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Not at all.
    We're aligning our thinking so our imagination comes into agreement with how Mother Nature built the real world.
    We can imagine all sorts of things that aren't right.

    I use this quote from Lavoisier a lot.
    https://web.lemoyne.edu/giunta/EA/LAVPREFann.HTML
    Build your mental picture models slowly and frequently cross checking them with math.
    Especially in electrical phenomena for analog electronics is really well described by the equations.
    When your mental model leads you to the right formula you're getting someplace.

    You have to form the mental habit of that differential-integral relation and it's not easy.
    Sine and cosine at any instant are equal* to one another's slope'. That picture won't let you down.
    *(well make that proportional related by ω to one another's slope)


    That Laviosier article is quite interesting. Print a copy and take it home? I kept that snippet framed above my desk for years.

    old jim
     
    Last edited: Jun 28, 2017
  10. Jun 28, 2017 #9

    cnh1995

    User Avatar
    Homework Helper

    Which graphs are you talking about? Is it a purely capacitive circuit or an RC circuit?
     
  11. Jun 28, 2017 #10
    No. That only happens in the special case of a purely resistive circuit (as anorlunda noted). But as you are only now learning about inductors and capacitors all your previous experience has been with resistive circuits and that special case. The general case is that you have circuits with R, L and C and the voltage and current zero-crossing points don't coincide.

    In a steady state, sustained case there can't be (well, unless you're playing with superconductors). But here we are not considering a sustained zero voltage, we are looking at sinusoidal waveforms where the instantaneous values are always changing. We get a non-zero current only because the voltage is changing as it passes through the zero crossing point. If we froze the voltage as it reached zero and kept it at zero the current wouldn't keep flowing. This only works because the voltage is always changing in a sine wave.

    The differential equations for capacitors posted by Jim Hardy and anorlunda describe how the current flow in or out of a capacitor is proportional to how fast the voltage across the capacitor is changing (with time). A feature of a sine wave is that its value is changing at its fastest as the value crosses zero. So the current flow in or out of a capacitor reaches a maximum as the sinusoidal voltage across it crosses zero. (A positive or negative maximum, depending on which direction the voltage is crossing zero.) Another feature of a sine wave is that its rate of change falls to zero at the peak positive and peak negative values. So the current in a capacitor will pass through zero as the sine voltage across it reaches peak positive and negative values.

    The corresponding equation for an inductor describes how the voltage across an inductor is proportional to how fast the current through the inductor is changing. Hence the voltage across an inductor will be at a (pos or neg) maximum as the current through it crosses zero. Similarly the voltage will cross through a zero as the current reaches peak +ve and -ve values.

    The voltage across a resistor is just the resistance times the current through it, from Ohm's Law as you are presumably already aware. It doesn't care how fast the current or voltage are changing. When the current is zero the voltage is zero. Hence for a resistive circuit the voltage and current zero-crossing points occur at the same time.
     
  12. Jun 29, 2017 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It would be a good thing to dump that approach. No one uses Electrons when doing this sort of analysis. They don't help at all.
    If you associate movement of electrons (I guess you mean current) with Power then you have the problem of deciding which way the Power is actually flowing. It can be either way, depending on the Potential Difference.
    It's not clear, from what you write, where you level of knowledge sits for this topic. The theory is very well established and, if you can bring yourself to accept that the Mathematical models of circuits are valid then your questions are answered if you just read a text book. Trying to apply intuition against Maths usually leads nowhere. If you haven't done the Maths yet then just wait and it will all be clear when you have. The elementary rules for DC circuits do not apply in a straightforward way for time varying circuits with reactive components which Store Energy for a finite time.
     
  13. Jun 29, 2017 #12

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Because current through a capacitor is not proportion to voltage. It's in proportion to rate of change of voltage.

    It only does that for sine waves. They are a special mathematical case. We get so wrapped up in our studies we forget about that.
    https://www.mathsisfun.com/algebra/trig-sin-cos-tan-graphs.html
    upload_2017-6-29_8-18-47.png
    Differentiating any other waveform will change its shape.

    Take it to DC. Put 100 VDC across a capacitor.
    It charges quickly and current drops to zero.
    Right there's proof that Zero Current and Zero Voltage needn't coincide.
     
  14. Jun 29, 2017 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I'm afraid that I have to blame all you Power Engineers for this guy's problems. All the rules about sinusoidal waveforms work so well for you and you are so familiar with it that you forget about
    1. DC and
    2. The exponential decay function that's always involved with Capacitors and Inductors.
    You get on with your Phases, Reactive power and Power Factors etc etc and solve very hard probs. KVL and KCL appear to go out of the window for the beginner.
    No need to get defensive guys (:smile::smile::smile:) but the cognitive dissonance no longer gets to you because you are so familiar with it all.
    This problem, in its own way, is a bit like the Particle Wave Duality that gave people such problems a hundred years ago. Wear the right hat and the problem just goes away.
     
  15. Jun 29, 2017 #14
    I think a very good way to think of this is a pendulum. Where gravity is the Voltage and the movement of the weight is the current.

    At max height, the force of gravity (in the direction of movement) is at it's maximum, but you have zero movement ( current) at the point of maximum force. At the bottom there is no force in the direction of movement and gravity is now contributing nothing to the movement. ( so maximum movement with no force) .

    This is a general condition of all undampened oscillating systems.

    A weight hanging or "bouncing" from a vertical spring may be even clearer, since it is a 1 dimensional model.
     
  16. Jun 29, 2017 #15
    Wow thanks! It's nice to have backup. Also, I was thinking the same thing regarding the particle/wave duality. I know it's the best explanation we have at the moment, but it's a bit lazy to say it's sometimes this and sometimes that.

    So I don't have a lot of time to post, but I'm trying to keep up with all your replies. I just have the following to say.

    In the case of DC being applied to a capacitor, yes, there's no current once the voltage in the capacitor is equal to the supply. But that's voltage without current, not current without voltage.

    Furthermore, when I said that the zeroes must align, I meant just the zeroes.
    I understand that current might not be always proportional to voltage, but surely there must always be voltage and the current and voltage must always be the same polarity.

    I also don't subscribe to the notion that thinking of electricity in terms of the electrons in the wire is not useful. How can ignoring what is actually happening in the system and replacing it with what mathematicians reckon be any more useful? Especially when the maths gives us answers where the voltage is zero and the current is not, and we know that can't happen.

    That being said, I haven't looked to see if anyone's observed this on an oscilloscope, so I'll look into that when I have some spare time.
     
  17. Jun 29, 2017 #16

    nsaspook

    User Avatar
    Science Advisor

    You're not the first or last one with this notion. When people say "It would be a good thing to dump that approach" it comes from decades of practical experience of teaching others about electricity because most of the time the person with that notion does not understand what's actually happening.
     
    Last edited: Jun 30, 2017
  18. Jun 29, 2017 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Read what I wrote. it is not backup.
    It is not 'what is actually happening'. The electrons are not 'carrying the power' there is no way that such low mass particles, travelling at 1mm/s could be carrying any appreciable power. It's about Potential Difference and Current, acting together. (Which is why it's called Electromagnetic theory)
    The mathematical description is a way of describing what is happening and it gives good, quantitative predictions. Waving arms about and saying that the particles describe it all is demonstrating a very limited knowledge of the evidence. If you don't like the Maths then you just have to stay out of the discussion because there is no better model than the mathematical one.
    If you re-read my words, you will see that I was referring to the situation 100 years ago and that 'duality' is not a current view.
    A good idea. When you have some time, do some serious reading and improve the basis for any arguments you use.
     
  19. Jun 29, 2017 #18

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Picky, picky. Point was zeroes don't have to align.

    You have much to unlearn.
     
  20. Jun 29, 2017 #19

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    Viewing it on an oscilloscope is one of the first things I did with a scope in school. Trust me, it's there. Why is it you just know that can't happen concerning the voltage being zero and the current not being zero? How do you "just know"? There is no other way to shift phase between current and voltage without having times throughout the cycle where one is positive and the other negative.
     
  21. Jun 29, 2017 #20

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    phase_shift.jpg
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Voltage and current phase shifting/Current without voltage?
  1. Voltage and Current (Replies: 5)

Loading...