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Physics question, is a true DC or constant magnetic field truly possible?

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Nominal Animal:

--- Quote from: BrianHG on March 28, 2024, 12:38:39 am ---Can we or the universe ever achieve
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This is not physics.

Physics is a science that tries to describe observable phenomena (using mathematical models).  It cannot answers questions like "is X possible"; you'd have to reformulate the question into "in what kind of situation or system could X happen".

Because physics only involves observable phenomena, there is no such thing as "perfect" or "exact"; it is always limited by our ability to observe, to measure the phenomena.

The stated question is like asking a mathematician, "If you had to specify a real number for \$\lim_{x \to \pm0} \frac{1}{x}\$, what would it be?".  The expression evaluates to positive or negative infinity depending on the direction you approach zero from, so assigning any real number would be an error.

As mentioned by several others above, we aren't even certain if the electron charge is a constant (the same everywhere and unchanging as time progresses).  Thus, "absolutely constant DC current" (even if considered only in some limited time interval) needs to be specified with respect to something, for example as the rate of one-electron charges passing through a surface.

In case you're tempted to insist and play along that definition: even if we found some phenomena where a steady beam of electrons at a specific energy (velocity) is emitted from some source, changing your velocity with respect to the source will change the observed current in the electron beam.  Any change in the gravitational potential will change the observed current in the electron beam, too.  If you examine the system at shorter and shorter timescales or currents, you'll notice the current (and the related electromagnetic fields) being quantized.

Thus, the only non-nitpicking/language-lawyer answer one can give to the stated question is "According to our current understanding, no: perfectly direct current is impossible, for example due to quantization of current and electromagnetic fields alone.  At sufficiently short time intervals, there is always at least some ripple or noise due to quantum mechanics."

Siwastaja:

--- Quote from: ejeffrey on March 29, 2024, 03:41:59 am ---It's not "DC" if it didn't already exist since the  beginning of the universe.

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Why would beginning of the universe be enough? If we follow this logic, I think it should extend to minus infinite time.

coppercone2:

--- Quote from: Kleinstein on March 28, 2024, 09:53:47 am ---One can get a pretty stable magnetic field with superconductors.  Superconductors can work as essentially perfect shield to external fields (type 1 superconsuctors have exactly 0 internal flux). The flux captured in a superconducting ring stays constant as long as the ring is superconducting.
There is still a rather minor uncertainty in how much the parts move relative to each other. So the problem is not electrical noise but more mechanical and vibrations.
For the "DC" ideal, it really depends how one defines DC - there are way than looking at the fourier tranformer over all time. It is more about stable over the measurement time with settling effects completed.

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I think the ring is radiating on a quantum level because the current is still fluctuating, the ring just has a very high radiation resistance . Won't it depend on how stable the power supply used to charge it is?

I think the most stable current would be if electrons are single file like a ball chain that way there is no interaction between them trying to pass each other like cars on a high way. And then it depends on how well they hold to the lane lines I think. So you reduce noise in a dimension?

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