Hi All,
I've been struggling for a very long time (years, if truth be known) to reconcile my understandings of various aspects of the relationship between AC current and AC voltage across an inductance. I've read innumerable writeups on the topic, some more understandable, and some less, but all to no avail when it comes to completing my understanding. So, in my first post here (after much lurking), I'm turning to you all for help.
Obviously, I'm thinking about part of the puzzle wrong. I'm hoping someone can point out to me where I'm going wrong, and can also tell me the right way to think about it.
First, let me recite the aspects that do make sense to me:
What I see in such a plot is, at all points in the 2π radians of the cycle, the applied voltage exactly opposed and balanced by the cemf. Since the net voltage is always exactly 0 (i.e. the push of the applied voltage is always exactly countered by the reverse push of the cemf, whose magnitude is the same but whose sign is different) my intuition is telling me there shouldn't be any current flowing, at any point in the cycle.
But clearly this is contradicted by experience.
What the heck am I missing here? Where's the extra hole in my head that I need to plug?
Thanks,
M
I've been struggling for a very long time (years, if truth be known) to reconcile my understandings of various aspects of the relationship between AC current and AC voltage across an inductance. I've read innumerable writeups on the topic, some more understandable, and some less, but all to no avail when it comes to completing my understanding. So, in my first post here (after much lurking), I'm turning to you all for help.
Obviously, I'm thinking about part of the puzzle wrong. I'm hoping someone can point out to me where I'm going wrong, and can also tell me the right way to think about it.
First, let me recite the aspects that do make sense to me:
- I believe I understand why the (counter) emf developed by an inductor is proportional to di/dt . Basically, the more flux lines cut per unit time by a conductor (regardless of whether the conductor is moving past the flux lines, or the flux lines of an expanding or collapsing field are moving past the conductor), the greater the induced voltage.
- I believe I understand why, by Lenz's law and the right hand rule, the emf developed by the inductor is a counter emf; if it weren't, the magnitude of the current would tend toward infinity.
- If I only consider the fundamental e = L di/dt formula, and then look at graphical plots of current and voltage, it's fairly obvious that the current wave lags the voltage wave by 90 degrees, and why.
All of this seems reasonable, too -- at least, until I look at a plot of the applied voltage, the cemf, and the current. Then everything falls apart for me.[Suppose that] a sine wave of voltage is applied to a pure inductance... By Kirchoff's [voltage] law it is known that the algebraic sum of the voltage drops about any closed circuit is equal to 0. Thus:
e (applied) + cemf = 0;
e = -cemf .
Since: cemf = -L di/dt
Therefore: e = +L di/dt .
... [T]he cemf is, by definition, a voltage opposite in phase to the applied voltage, or 180 degrees out of phase with the applied voltage.
What I see in such a plot is, at all points in the 2π radians of the cycle, the applied voltage exactly opposed and balanced by the cemf. Since the net voltage is always exactly 0 (i.e. the push of the applied voltage is always exactly countered by the reverse push of the cemf, whose magnitude is the same but whose sign is different) my intuition is telling me there shouldn't be any current flowing, at any point in the cycle.
But clearly this is contradicted by experience.
What the heck am I missing here? Where's the extra hole in my head that I need to plug?
Thanks,
M