Current, Power and Energy in an Inductance

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Discussion Overview

The discussion revolves around a homework problem involving the analysis of an inductor's voltage over time, specifically focusing on finding the current, power, and energy stored in the inductor at a given time. The scope includes mathematical reasoning and conceptual clarification related to inductance and electrical circuits.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant attempts to calculate the current by summing the areas of rectangles under the voltage-time graph but finds this method yields incorrect results.
  • Another participant suggests rearranging the voltage equation for inductors and integrating to find the current, hinting at the importance of considering areas beneath the time axis.
  • A participant expresses uncertainty about how to derive power from voltage.
  • There is a question raised about the implications if the area under the graph were zero.
  • One participant clarifies that the problem asks for instantaneous power at a specific time, indicating that the voltage can be directly read from the graph at that moment.
  • A participant confirms they found the answer with assistance from others.

Areas of Agreement / Disagreement

Participants generally agree on the need to derive current and power from the voltage graph, but there are differing approaches and some uncertainty regarding the calculations and interpretations of the graph.

Contextual Notes

Participants have not fully resolved the method for calculating current and power, and there are indications of missing assumptions regarding the integration process and the handling of areas beneath the time axis.

dudforreal
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Homework Statement
The graph below shows the voltage across a 820 mH inductor vs. time.
Find the current (Ix), the power delivered (Px) and the energy stored (Wx) at time tx = 9 μs.


The attempt at a solution

I tried adding up the areas of the rectangles up to tx and multiplying it by 820m but this gives the wrong answer for Ix.
 

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You are very close! Keep plodding along.

The voltage across an inductor is given by:

V = L\dfrac{dI}{dt}

Now, after rearranging to isolate \dfrac{dI}{dt}, and then integrating, what do you get? Hint: you shouldn't be multiplying the sum of the areas of the rectangles by 820mH.

Also, are you ensuring that you subtract contributions from areas beneath the t axis?
 
cool...not sure on how to get voltage in order to get power
 
Last edited by a moderator:
...and what if the area was 0 altogether?
 
Last edited by a moderator:
The question asks for the power at tx = 9 μs. I.e. it's asking for an instantaneous power, and so you only need the voltage at that point in time. This you can easily read from the graph to be 700 V.

If the voltage V = 0, then the power (at that point in time) P = VI = 0.
 
cool got the answer thanks for the help :smile:
 
No worries!
 

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