Energy Stored in an Inductor

  • #1
AlphaMaleMatt
3
0

Homework Statement



Not really relevant here.

Homework Equations



U = LI^2 -- maybe?

The Attempt at a Solution



http://i.imgur.com/Pq4dOex.png

The picture is there, as well as the answer. Why is that the answer? How do inductors work when completely disconnected, and not in a circuit? Thanks.

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
36,103
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The inductor is in a circuit.

1/2 LI^2 (note the prefactor) is indeed the energy stored in an inductor.
 
  • #3
AlphaMaleMatt
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I'm an idiot. The question asks what happens when the switch is switched over to position B, after being in position A for "a very long time"
 
  • #4
rude man
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I'm an idiot. The question asks what happens when the switch is switched over to position B, after being in position A for "a very long time"

That sine curve in the illustration tells you what happens to UL.

Hint: the instantaneous sum of stored energies in C and L is a constant.
 
  • #5
AlphaMaleMatt
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That sine curve in the illustration tells you what happens to UL.

Hint: the instantaneous sum of stored energies in C and L is a constant.

I guess I'm just confused as to why the energy in the inductor decreases, but then increases again?
 
  • #6
rude man
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I can't give you a good verbal explanation. A publication like the ARRL Handbook can.

Mathematically, the integro-differential equation 1/C∫0t i(t') dt' = -L di/dt is solved with initial condition i(0+) = E/R

where i is the current flowing out of the inductor and into the capacitor. Each term is the voltage at the capacitor and inductor.
 
  • #7
NascentOxygen
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I guess I'm just confused as to why the energy in the inductor decreases, but then increases again?
Because the L-C elements represent a resonant circuit (having no resistive losses). Just as a child's swing oscillates when you release it from some height, so does the energy in the analogous L-C circuit.
 

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