Something Vexing me about Magnetism and Conservation of Energy

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Discussion Overview

The discussion revolves around the relationship between magnetism, superconducting magnets, and the conservation of energy. Participants explore the implications of using a superconducting magnet to lift weights and the potential misconceptions regarding energy conservation in this context.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions whether using a superconducting magnet to lift a weight and then capturing the energy from its fall violates the conservation of energy principle.
  • Another participant suggests that the superconducting magnet behaves like a transformer, indicating that the coupling limits the current despite the absence of resistive losses.
  • It is pointed out that superconductors still obey inductance laws, which may impose restrictions on current flow.
  • A participant emphasizes the significant energy expenditure required to maintain the superconducting state, suggesting that this must be considered in any energy balance.
  • Another participant argues that energy conservation still holds, noting that as the potential energy of the weight increases, the magnetic energy decreases, implying a dynamic balance between the two forms of energy.
  • Concerns are raised about losses due to induced currents in the weight, which should also be factored into the energy considerations.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using superconducting magnets in energy systems. There is no consensus on whether the described system violates conservation of energy, as multiple perspectives on the mechanics and energy dynamics are presented.

Contextual Notes

Participants highlight various assumptions, such as the need for cooling in superconductors and the effects of inductance, which may influence the overall energy dynamics in the system. The discussion remains open-ended regarding the implications of these factors.

Matt Luckham
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Something Vexing me about Magnetism and Conservation of Energy

I have been thinking about an issue at work and it has been bugging me so I thought I would post something on here to see if anyone can explain where my thought process is going wrong.

Say you have a super conducting magnet. It is a big one. The current is switched on and so the magnet becomes active. As it is super conductive the current is not impeded and continues to flow. The magnet is used to lift a large weight off the ground. At somepoint the current is channeled off somewhere else, the magnet stops working and the weight falls. The energy of the weight falling under gravity is captured somehow. The current is channelled back to the magnet, weight is lifted again and so on...

So, my question is, does this system not break the conservation of energy principle? Are we not getting something for nothing here? I am sure I am missing something in my understanding of magnetism, but I thought it was just a field and so no actual mass is lost when it is active?
 
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Since the superconducting magnet is lifting a load, it is coupled to it like a transformer. That coupling will limit the current even though you are pushing electrons into a superconducting material. Once the weight stops moving, the superconductor will reach full current flow.
That's my understanding.
 
Welcome to PF.

You seem to be under the misconception that a superconducting magnet will not create any restriction to the flow of current. This is not correct: a superconductor has no resistive loss, but it does still obey the laws of inductance.
 
I would also offer to keep in mind the large energy EXPENDITURE necessary to keep a superconducting magnet cool enough.
Thus, no matter what one does, the output will never meet or exceed the energy required to keep the system running.
What about hypothetical room temp. superconductors? Ok, we removed the cooling factor but Russ addressed that other factors come into play.
 
In other words, the energy conservation still applies.
As the weight potential energy will increase, the magnetic energy will decrease, and so should the magnetic field and the current.
Pulling the weight back will restore the magnetic field.

Losses due to currents induced in the weight should be added to this picture.
 

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