Discharging a high voltage capacitor using a low voltage source?

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Discussion Overview

The discussion revolves around the energy dynamics involved when discharging a high voltage capacitor using a low voltage source, specifically examining the energy consumption from the voltage source and the total energy loss during the process. The context includes theoretical considerations and mathematical reasoning related to capacitors and voltage sources.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related
  • Debate/contested

Main Points Raised

  • One participant poses a scenario involving a capacitor charged to 2V being discharged to 1V using a 1V DC voltage source, asking about energy consumption and total energy loss.
  • Some participants suggest that the analysis of energy dynamics is homework-related, while others clarify that they are exploring the implications of current flowing into the voltage source.
  • A participant provides a formula for the energy gained by the voltage source and the reduction in stored energy in the capacitor, concluding that the energy loss is independent of the resistor value used in the circuit.
  • Another participant questions whether a non-rechargeable battery can gain energy from the capacitor, indicating a potential misunderstanding of energy dynamics in non-ideal systems.
  • A later reply emphasizes that while a battery can be modeled as an ideal voltage source under certain conditions, it is not ideal in practice, suggesting limitations in the initial analysis.
  • One participant mentions that using an inductor can minimize energy loss in the circuit, proposing an alternative approach to the problem.

Areas of Agreement / Disagreement

Participants express varying views on the nature of energy transfer between the capacitor and the voltage source, with some agreeing on the theoretical framework while others question the applicability to real-world scenarios, particularly regarding non-rechargeable batteries. The discussion remains unresolved regarding the implications of these dynamics.

Contextual Notes

There are assumptions regarding the ideal behavior of voltage sources and capacitors, and the discussion does not resolve the complexities introduced by non-ideal components like batteries. The impact of the resistor value on energy loss is also noted but not fully explored.

seahs
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Suppose a capacitor is initially charged up to 2V, and then a 1V DC voltage source is used to discharge the capacitor to 1V, what is the energy consumption from the voltage source and what is the total energy loss?

Assume the capacitor is connected to the voltage source using a resistor and the capacitor is 1F.
 
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Homework ?
 
Carl Pugh said:
Homework ?

nope, just wondering what happens when the current flows into the voltage source
 
If Vb is the battery (voltage source) voltage and Vc is the initial capacitor voltage then :

The energy gained by the voltage source is Vb*Q = Vb C ( Vc - Vb )

The reduction in stored energy in the capacitor is 1/2 * C ( Vc^2 - Vb^2 )

The energy lost is the difference of the above, which equals 1/2 * C (Vc - Vb)^2.

Note that this energy loss is independent of the value of the resistor used to connect the two.
 
uart said:
If Vb is the battery (voltage source) voltage and Vc is the initial capacitor voltage then :

The energy gained by the voltage source is Vb*Q = Vb C ( Vc - Vb )

The reduction in stored energy in the capacitor is 1/2 * C ( Vc^2 - Vb^2 )

The energy lost is the difference of the above, which equals 1/2 * C (Vc - Vb)^2.

Note that this energy loss is independent of the value of the resistor used to connect the two.


Can the voltage source gain energy? what if the voltage source is like a non-rechargeable battery?
 
A battery is not an ideal voltage source, but under some circumstances is a good approximation. The above analysis is based on an ideal voltage source. It is applicable to the case of a battery only to the extent that the battery can be modeled as an ideal voltage source.
 
seahs said:
Can the voltage source gain energy? what if the voltage source is like a non-rechargeable battery?

Yes - the 'voltage source' receives charge from the Capacitor. But, as stated above, half the energy is lost in a 'simple' connecting circuit. However, using a suitable inductor can ensure (virtually) no energy loss.
 

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