Delay of free fall with electromagnets

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Discussion Overview

The discussion revolves around the phenomenon observed when a metallic ball suspended by an electromagnet is released after disconnecting the power. Participants explore the reasons behind a perceived delay in the ball's fall, considering various physical principles and circuit characteristics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question whether the delay is real or an optical illusion, suggesting it may appear as if the ball is hanging briefly.
  • There is a proposal that residual magnetism in the electromagnet's material could contribute to the delay, although this is characterized as a guess.
  • One participant mentions the need to account for voltage changes in their measurements, indicating a potential correction in their calculations.
  • Another participant raises questions about the circuit design, including the presence of a snubber circuit and the inductance of the electromagnet, suggesting these factors could influence the delay.
  • Lenz's Law is introduced, with a participant explaining that the collapsing magnetic field induces a back emf that opposes the change, affecting the timing of the ball's fall. The decay of the magnetic field is described as exponential, influenced by the self-inductance and resistance of the coil.

Areas of Agreement / Disagreement

Participants express differing views on whether the delay is a real phenomenon or an illusion. Multiple hypotheses regarding the causes of the delay are presented, and no consensus is reached on the definitive explanation.

Contextual Notes

Participants mention various factors that could influence the delay, including circuit design, material properties, and electromagnetic principles, but do not resolve the implications of these factors.

tecnica
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When you have a metallic ball hanging from a electromagnet and you disconnect the electromagnet, there is a delay between the moment you unplug the machine, and the ball starts falling. Why does this happen?
 
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Are you sure there is a delay, or does it just appear to hang there briefly?
If there really is a delay I would think it has something to do with some brief residual magnetism somehow stored within the material of the electromagnet, but it is quickly lost.
 
rootone said:
Are you sure there is a delay, or does it just appear to hang there briefly?
If there really is a delay I would think it has something to do with some brief residual magnetism somehow stored within the material of the electromagnet, but it is quickly lost.
There is a delay because, when I change the voltage, the time that I measure changes as well. I need to add a correction into my calculations so that I can get a better result, but I don't know how.
 
I can't really offer any more than my first idea, that somehow the material of the electromagnet, a lot of which will be iron, temporarily retains some magnetism as if it were a permanent magnet.
Purely a guess, I'm sure somebody will be able to state categorically what is happening.
I'd be interested to know about it too.
 
tecnica said:
When you have a metallic ball hanging from a electromagnet and you disconnect the electromagnet, there is a delay between the moment you unplug the machine, and the ball starts falling. Why does this happen?

What is the switch circuit like? Does it have a snubber circuit to keep the switch-opening arc to a minimum? What is the inductance of the electromagnet? Can you post the snubber circuit and any other components that are part of the circuit?

Do you have access to an oscilloscope? If so, and if there is a snubber circuit on the switch, you can measure the time constant of the collapsing field...
 
Lenz's Law is applicable here. When you open the circuit the magnetic field will begin to collapse and this changing magnetic field will induce a back emf such as to oppose this change. The self inductance L determines how long this back emf will persist. Thus after opening the circuit the magnetic field will decay exponentially with a decay constant of L/R where R is the resistance of the coil. Also depending on the core there may also be a residual magnetization left in the core too. So depending on the self inductance, resistance, the core material and weight of the object there will be a delay in dropping.
 
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