Inductor opposing change in current.

  1. Hello,

    I'm wondering, if you have an inductor and resistor that has the switch open, so current is zero. Then you turn on the switch, the inductor is going to resist this change, it wants to be zero, right? Therefore, why doesn't the current indefinitely stay as zero, since the inductor would like to be at that value that it was at initially.
     
  2. jcsd
  3. berkeman

    Staff: Mentor

    Can you show us the equation that relates current and voltage for an inductor?
     
  4. ε = -L di/dt

    So if it opposes a change (di/dt = 0) then ε is 0, then no current
     
  5. berkeman

    Staff: Mentor

    There is no negative sign, first of all. And the equation should show the current as a function of time for an applied voltage:

    [tex]V(t) = L \frac{dI(t)}{dt}[/tex]
     
  6. Yeah, so my conceptual understanding of an inductor is that it would like to maintain the same current through it. If that is the case, why does it ever change? I guess unless something can overcome the resistance that the inductor puts forth to resist that change. What is it?
     
  7. Nugatory

    Staff: Mentor

    Look at the equation berkeman posted again. L is non-zero, V is non-zero, therefore dI/dt must be non-zero. And if dI/dt is non-zero at some time t, then even if I is zero at time t it won't be zero a moment later; the voltage is pushing current through the inductor.

    An analogy which is really quite good if you compare the equation with Newton's [tex]f=ma=m\frac{dv}{dt}[/tex]

    Because of inertia, a mass hanging from a rope resists changes in speed unless you apply a force to it. If it's not moving and you push on it, it resists. But it will start swinging, and once it does it will resist any attempt to stop it.
     
  8. Woopydalan,

    Your equation is correct. It holds true for self-inductance.

    You are right about the inductor resisting current. It holds true because of Lenz's Law.

    As for your actual question on why the current doesn't remain zero: it is because, in steady state, flux through the inductor is zero. Hence, as stated by Faraday's Law, induced emf which opposed the current in the first instance eventually disappears, that is, self-inductance (L) becomes zero.

    Thus, current flows. This is the simplest and only explanation.
     
  9. My text books show induced emf = -d∅/dt. = -L.di/dt
    The minus sign is taken to be an indication of Lenz's law.
    Where is my text book (or my reading of the book!) going wrong
     
  10. It resists changes in current.

    If the current through the inductor is the only source of flux linking its turns, then the flux through the inductor is only zero when the current through it is zero.

    If by steady state you mean the rate of change of flux will tend to, or become, zero as t->∞, then you're assuming something which isn't true in general, e.g. it's not true for a constant voltage applied across an ideal inductor.

    If the inductor induces an emf, then the rate of change of current through it must be nonzero, i.e. the magnitude of the current changes with time. You have assumed that which makes your conclusion true.
     
  11. Redbelly98

    Redbelly98 12,039
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    The negative sign is usually included in intro physics textbooks, and omitted in electrical engineering books. But the two use a different convention for the direction of the current in an inductor, relative to the potential difference. They are equivalent descriptions.
     
  12. Redbelly98

    Redbelly98 12,039
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    If that were true, we would not describe an inductor with that equation. We would simply say ε=0, i=0, and be done with it.

    You can think about it this way:

    If there is no current in the inductor, then there's no current in the resistor either.
    In which case, there is no voltage difference (V=iR) across the resistor.
    So then, the inductor must get the full battery voltage ε.
    Therefore, di/dt = ε/L is not zero.
     
  13. I guess my question is that if an inductor resists changes to current, if the inductor had 0 current in it, why would it have current in it if the switch is closed? Wouldn't it resist having a nonzero value?
     
  14. When the switch is closed, the current and the resulting magnetic field increases from zero to the maximum value in a short time. This results in a increase in magnetic flux through the inductor and, hence, again induces an electric current in the inductor.

    The above forms the basis of Faraday's Law which was observed during experiments conducted around 1830.
     
  15. You have the wrong idea about this, there are degrees of "resisting change", it's not absolute.

    If the inductor didn't oppose changes in current, you would be able to make the current through it "jump" instantaneously from one value to another, like you could for an ideal resistor. At the other end of the spectrum, it would oppose any and all changes to current and the rate of change of current through it would always be zero.

    If you apply a voltage V across an ideal inductor with inductance L, you have from Faraday's law:

    [tex]
    V = L \frac{\mathrm{d}i}{\mathrm{d}t} \Leftrightarrow \frac{\mathrm{d}i}{\mathrm{d}t} = \frac{V}{L}
    [/tex]
    Clearly then, the rate of change of current through the inductor can be nonzero. If you imagine plotting the current through the inductor as a function of time, you'll notice that V/L gives you the slope of this line. Decreasing the inductance L would give you a larger and larger slope, i.e. the change of current through the inductor would occur more rapidly.

    You might say the inductance is a measure of "how much" the inductor resists changes in current.
     
  16. Redbelly98

    Redbelly98 12,039
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    I'll give a short and simple answer.

    Resisting change means that the current does not (nearly) instantaneously jump to a nonzero value, as it can in a resistor. Think about what happens in your circuit if there is no inductor.
     
  17. vanhees71

    vanhees71 3,815
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    2014 Award

    Perhaps it's easiest to consider the most simple problem of this kind analytically. Take the usual assumptions of quasistationary circuit theory, i.e., the extensions of the whole circuit is much smaller than the wavelength of typical em. waves of the problem, so that the displacement current and retardation can be neglected. Then everything is described by linear ordinary differential equations and the circuit is described by resistance, capacitance and self-inductance.

    We consider a coil of self-inductance L in series with a resistor, attached to a DC battery. We switch on the circuit at [itex]t=0[/itex]. From Faraday's Law we find
    [tex]L \dot{i}+R i=U(t),[/tex]
    where
    [tex]U(t)=U_0 \Theta(t).[/tex]
    First we solve the homogeneous equation
    [tex]\dot{i}=-\frac{R}{L} i.[/tex]
    Obviously the general solution is
    [tex]i(t)=A \exp \left (-\frac{R}{L} t \right).[/tex]
    Then we need one special solution of the inhomogeneous equation. Obviously this is given by the ansatz [itex]i_0=B =\text{const}[/itex]. Plugging this in the equation, we get for [itex]t>0[/itex] that [itex]B=U_0/R.[/itex] The general solution of the full equation thus is for [itex]t>0[/itex]
    [tex]i(t)= A \exp \left (-\frac{R}{L} t \right) + \frac{U_0}{R}[/tex]
    Since [itex]i(0)=0[/itex] we finally can determine [itex]A[/itex] and then get
    [tex]i(t)=\frac{U_0}{R} \left [1-\exp \left (-\frac{R}{L} t \right ) \right ].[/tex]
    As you see the current won't flow immediately at full strength but only approaches exponentially the static value [itex]i_{\infty}=U_0/R[/itex].
     
  18. I would like to thank you all for contributing to this thread. I think I am now fully convinced that an inductor can resist change, which doesn't mean that it completely prevents the flow of changing current, much like a resistor doesn't prevent the flow of current entirely.
     
  19. There is something very strange here !!! usually introduced in intro physics textbooks !!!!
    Why do some introduce it and others do not, which ones do and which ones don't. The hyperphysics site, often quoted here....what do they do??
    I did not know that electrical engineers had a different convention for electric current...how do they cope with kirchoffs laws when communicating with physicists?
    An emf is generated across an inductor....not a pd....is there some confusion here about which way current flows.
    Your statement does not help anyone wanting to learn physics....especially those using text books.
    The minus sign is a recognition of Lenz's law. It is as important in electrical studies as the minus sign is in the analysis of SHM in mechanics. I doubt if anyone would claim that a minus sign is not necessary in SHM and that some intro physics text books decide not to use it.
    It is PHYSICS.
     
  20. Redbelly98

    Redbelly98 12,039
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    Maybe getting off topic here, but I am compelled to defend myself against some criticism of what I posted earlier.
    SHM is different. You define displacement to be positive in some direction, and it's understood that's the positive direction for the first and second derivatives and restoring force as well. But when defining positive current to go with a positive voltage difference, it can -- and in practice does -- go either way.

    People can -- and do -- define positive current flow to be in either direction: either entering the positive terminal of a device, or entering the negative terminal. For most devices (resistors, batteries, diodes), there is pretty universal agreement. Not so for inductors:

    If you're not familiar with the terms "active sign convention" and "passive sign convention", try a google search and learn about them. To be very brief

    [​IMG]
    Active sign convention, V = -L di/dt


    [​IMG]
    Passive sign convention, V = +L di/dt
     
  21. berkeman

    Staff: Mentor

    Thanks RB, that's interesting.
     
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