Work Done on Charges by a Capacitor and an Inductor

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Homework Statement


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The Attempt at a Solution


I believe that the inductor does negative work on the charges via the electric field (by way of induced EMF).
I am unsure about how the capacitor would be doing positive work, though. Can anyone help steer me in the right direction? The only thing I see as of now is the E field lines pointing to the left, and so the electrons are being pushed clockwise, which is opposite of the current direction.
 

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  • #2
Can you give a picture of good quality ?
 
  • #4
Merlin3189
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I believe that the inductor does negative work on the charges via the electric field (by way of induced EMF).
I am unsure about how the capacitor would be doing positive work, though. ... The only thing I see as of now is the E field lines pointing to the left, and so the electrons are being pushed clockwise, which is opposite of the current direction.
I agree the inductor does negative work on the charges.

For the capacitor, remember that conventional current is from +ve to -ve, so your worry, "electrons are being pushed clockwise, which is opposite of the current direction" is unfounded. The positive current is in the opposite direction to that in which the negative electrons move .

For the capacitor to do work on the charges, it has to use its stored energy (in the dielectric) to give energy to the charges, which they then use to cause heating, create magnetic field, to radiate em or to charge another capacitor..
 
  • #5
After the switch is closed and the capacitor begins to discharge,the energy stored in the electric field decreases.The capacitor's discharge represents a current in the circuit ,and some energy is now stored in the magnetic field of the inductor.Therefore, energy is transferred from the electric field of the capacitor to the magnetic field of the inductor.Hence,capacitor does positive work.
 
  • #6
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I agree the inductor does negative work on the charges.

For the capacitor, remember that conventional current is from +ve to -ve, so your worry, "electrons are being pushed clockwise, which is opposite of the current direction" is unfounded. The positive current is in the opposite direction to that in which the negative electrons move .

For the capacitor to do work on the charges, it has to use its stored energy (in the dielectric) to give energy to the charges, which they then use to cause heating, create magnetic field, to radiate em or to charge another capacitor..
How do you know that it would be doing positive work, other than looking at the fact that the capacitor is losing energy?
I’m trying to look at it in terms of the actual force that does positive work on the charges.
I’m aware current is opposite to actual electron motion, but I just noted this because when I’m thinking about the actual force lines, they only seem to be doing positive work on the electrons.
 
  • #7
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After the switch is closed and the capacitor begins to discharge,the energy stored in the electric field decreases.The capacitor's discharge represents a current in the circuit ,and some energy is now stored in the magnetic field of the inductor.Therefore, energy is transferred from the electric field of the capacitor to the magnetic field of the inductor.Hence,capacitor does positive work.
Thanks for the reply.
Please see the reply I just made and feel free to answer this as well.
 
  • #8
Merlin3189
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Ok, so the (conventional) current is flowing from the positive plate to the negative. So the negative electrons are moving in the opposite direction, from the negative plate which is repelling them towards the positive plate which is attracting them.
So the charged capacitor plates are exerting a force on the electrons in the same direction in which they are moving. Therefore it is doing positive work.

Later when the emf from the inductor is pushing the electrons towards the negative plate (b to c) then the electrons are doing work on the capacitor. Now the capacitor is doing negative work on the electrons, because the force from the negative plate is repelling the electrons, but they are still moving towards the negative plate (aided by the inductor emf.) Force and movement are in opposite direction, so work is negative.
Edit:
electric_work.png
 

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  • #9
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Ok, so the (conventional) current is flowing from the positive plate to the negative. So the negative electrons are moving in the opposite direction, from the negative plate which is repelling them towards the positive plate which is attracting them.
So the charged capacitor plates are exerting a force on the electrons in the same direction in which they are moving. Therefore it is doing positive work.

Later when the emf from the inductor is pushing the electrons towards the negative plate (b to c) then the electrons are doing work on the capacitor. Now the capacitor is doing negative work on the electrons, because the force from the negative plate is repelling the electrons, but they are still moving towards the negative plate (aided by the inductor emf.) Force and movement are in opposite direction, so work is negative.
Edit:View attachment 235363
Great reply. Thanks for the time and effort.
This is what I suspected. That the work is positive because the electrons are moving in same direction as the force from the electric field.
The second scenario also makes sense.

Thank you again.
 

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