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Simple Inductor Question

  1. Jun 24, 2009 #1
    Ok, this was not a homework problem, but a question I had on a test for electrical engineering. The question was as follows;

    "What is the inductor voltage at 9uSeconds."

    Ok, the way I approached the problem, and was marked as being wrong. I said that I know the equation for an inductor is

    V = L( delta current/delta time)

    BUT, if you are only choosing the point 9uSeconds, which is ZERO amps on the chart, and not over a range, then the voltage must technically be zero also then...

    I mean, if the question asked what is the voltage at 9uSeconds, as it passed through that point, then I would have answered differently.

    Anyhow, I'm new to this whole thing, and I'm just trying to understand if I'm technically correct, or just way off... does an inductor STILL produce a voltage even if it has zero current?

    Thanks for any response :)
  2. jcsd
  3. Jun 24, 2009 #2


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    Staff: Mentor

    Welcome to the PF. It may not be a currently-due assignment, but it is still academic coursework, and as such needs to go in the Homework Help forums, where I've moved it for you.

    On your question, you have the correct equation, so it is the change in current that is related to the voltage, not the current itself. Can you plot the V(t) for the I(t) graph that you were given?

    Consider also a parallel LC circuit. The energy oscillates back and forth between the voltage on the capacitor and the current in the inductor. What is the capacitor voltage when the inductor current is max? What is the inductor current when the capacitor voltage is zero?
  4. Jun 24, 2009 #3
    Thanks for the response :) I'm attaching the orinal problem to this post.

    I guess the point that really threw me with the question, is that it only referenced a single point in time, where you would normally obtain the slope of the line, based upon a series of point. I just regarded the question, that a single point in time- a snapshot per say, can have no current flow, if no time passes. Hence, 0 current divided by 0 change in time, times the inductance leaves an error since, you cant divide 0 by 0....

    But, I guess, thinking about it, you can have voltage in a line with no current. Also, with the same respect, current lags voltage in an inductor.

    The way I understood it, was that the inductor acts as a sort of flywheel to maintain current flow.

    Am I working this out correctly in my head now?

    Attached Files:

  5. Jun 24, 2009 #4


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    Staff: Mentor

    The best way to think about it is with differential calculus. Have you studied derivatives yet? The correct way to write the relationship for an inductor is with differentials, not with deltas:

    [tex]v(t) = L \frac{di(t)}{dt}[/tex]

    For functions like you have in the problem graph, the derivative di/dt exists for all values of i(t) shown in the graph. di/dt at any time t there on the graph is defined as the delta(i)/delta(t) as the time interval shrinks infinitely small. So it's basically the slope.

    Hope that helps. You're on the right track!
    Last edited: Jun 24, 2009
  6. Jun 24, 2009 #5
    I have studied calculus. I was originally a math and physics major some years ago- never finished though. Left college, and I'm back now to finish my degree in engineering. I really love college/engineering- glad I'm back :)

    I just sort of regret signing up for this summer class. It really leaves no room for error. I mean, the professor spent maybe 30 mins talking about inductors. The book was helpful a bit. I also read a bit at allaboutcircuits. A course like this just needs to sink in, to allow the basics to really be well understood- i think.

    I absolutely understand your statement, about always having a value. But if, in my thinking, would an inductor have any voltage if there is zero time? like a snapshot? I'm really sorry to be a pain, its been 15 years since I had calculus.

    I GREATLY appreciate your help :) You've been terrific!
  7. Jun 24, 2009 #6


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    Staff: Mentor

    Yes, zero time is just a snapshot on a graph of something versus time. The vertical scale of the graph gives the value at a point in time. So the plot of whatever versus time, is just a series of snapshots of the values at those times.

    Draw the full plot of v(t) for your i(t) plot. Put a 2nd vertical axis next to the first, and label it v(t) with appropriate numbers on it. Then use the i(t) plot and the equation above, to draw the plot of v(t). That should help you see how it works overall.

    Hang in there, and have fun with your EE studies!
  8. Jun 24, 2009 #7


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    Science Advisor

    Yes, this one is best looked at as a changing current causing the voltage.
    You can work out the current's rate of change by the graph given and, knowing the inductance, work out the voltage.
    The current passing through zero at that time is not important as it is only the rate of change that matters.
  9. Jun 24, 2009 #8
    I agree with your asesement :) I think I just thought the teacher was asking a trick question per say. Like, if there is no change in time, and only considering 9uSeconds, and not the change in slope- what would the voltage equal....

    Thanks for the responses :)
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