# Time For Magnetic Field To Be Created

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### #1 EinsteinIsUseless199

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Posted 05 May 2018 - 02:47 PM

Well lately I was revising Faraday's-Lenz's law of induction (V= -N*dΦ/dt).

What doesnt this law say is in how much time a magnetic field is created:

Any proposals?

### #2 exchemist

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Posted 06 May 2018 - 03:29 AM

Well lately I was revising Faraday's-Lenz's law of induction (V= -N*dΦ/dt).

What doesnt this law say is in how much time a magnetic field is created:

Any proposals?

I think you will need to explain what scenario you are considering. The equation you quote simply relates induced e.m.f. to rate of change (or cutting) of magnetic flux. It says nothing about what creates the field in the first place, which could be a number of things, such as a current flowing through a wire or coil or the presence of a ferromagnetic material.

If what you mean is at what rate does the associated magnetic fleld appear, in response to a current starting to flow through a wire, that is something which this equation (which is about e.m.f.) does not deal with. In that situation, the rate at which the current starts to flow is slowed down by the simultaneous creation of the associated magnetic field - a phenomenon known as inductance. More here: https://en.wikipedia...wiki/Inductance

But I'm having to guess what it is you are after. If you can explain a bit more, I may be able to help further.

### #3 Farsight

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Posted 10 May 2018 - 04:10 PM

Well lately I was revising Faraday's-Lenz's law of induction (V= -N*dΦ/dt).

What doesnt this law say is in how much time a magnetic field is created:

Any proposals?

There is no time in the expression because a magnetic field doesn't actually get created. That's because the field concerned is the electromagnetic field, which is "the greater whole". The thing called a magnetic field is just one aspect of it, which is revealed by induction. As to how it works, if you’ve ever read Maxwell’s On Physical Lines of Force, you may have noticed this: “a motion of translation along an axis cannot produce a rotation about that axis unless it meets with some special mechanism, like that of a screw”. This is why we have the right-hand rule which applies not just to electromagnetism, but to screw threads

Inducing a magnetic field is akin to pushing down on a pump action screwdriver. There's no appreciable time lag between you pushing down and the bit starting to turn. So if you wrote an expression relating your downward push to the rotation, you wouldn't include a time lag.term for the bit to start turning. In similar vein there's no time lag term for the magnetic field to get created. Because like I said it doesn't actually get created.

### #4 JMJones0424

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Posted 11 May 2018 - 09:06 PM

Of course there is time in the expression, it would be silly for there not to be.  There is a significant difference between rotating a rotor in a stator once in a second and once in a year.  FFS, what do you think the "t" is in the formula?

Einstein- You might find this explanation helpful: http://hyperphysics....ric/farlaw.html

### #5 OceanBreeze

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Posted 12 May 2018 - 01:47 AM

Of course there is time in the expression, it would be silly for there not to be.  There is a significant difference between rotating a rotor in a stator once in a second and once in a year.  FFS, what do you think the "t" is in the formula?

Einstein- You might find this explanation helpful: http://hyperphysics....ric/farlaw.html

Yes, of course time is involved as nothing happens instantly.

Even light has a finite speed and light consists of electromagnetic fields. There is probably a clue in there about the time it takes to set up a magnetic field.

### #6 JMJones0424

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Posted 13 May 2018 - 03:34 PM

Maybe I'm missing something.  There's a good chance I don't understand the question posed in the OP.  However, Faraday's law is pretty straight forward, and needn't require Sherlock Holmes to determine what it is describing.  It is a fact that if you move a magnet across a metallic object, then you will induce a voltage.  It is a fact that the faster you move the magnet, the greater the voltage induced.  This seems obvious to me, but maybe I'm missing something.

What doesnt this law say is in how much time a magnetic field is created

I don't understand what you are asking, as the law precisely says in how much time a magnetic field is created.  Movement is change in distance over change in time.  The greek letter delta that looks like a triangle is a stand in for "change" in physics.  Also, in mathematics, d is a stand in for derivation, which is functionally the same as the change of a variable over time in a given function, and is therefore equivalent to delta in physics.  The circle with the line through it is phi, a greek letter that is assigned to the magnetic flux as described by the hyperphysics frame that i linked to previously (equivalent to BA).  "t" is always time.  "V" is voltage. "N" is the number of turns in the coil, or what I would normally describe as the stator.

So, this law describes the voltage induced by moving a magnet of phi magnetic flux through a number of N coils in a t amount of time.  As the description of movement necessarily depends upon a time component, then the answer of why time is in this law should be obvious.

However, if your question is, instead, "does a magnetic field act instantaneously rather than at some fixed rate?", then I don't know the answer to your question.  I am inclined to answer that magnetic fields, like gravitational fields, change at the speed of light.  One could imagine a test of such a case as a highly rotating neutron star.  I think that magnetic fields propagate at the speed of light, because if they propagated instantaneously, then they would defy special relativity.  I don't know that this is accurate though, and I'm not sure if this is the question you are asking.

Edited by JMJones0424, 13 May 2018 - 03:38 PM.

### #7 exchemist

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Posted 16 May 2018 - 07:49 AM

Maybe I'm missing something.  There's a good chance I don't understand the question posed in the OP.  However, Faraday's law is pretty straight forward, and needn't require Sherlock Holmes to determine what it is describing.  It is a fact that if you move a magnet across a metallic object, then you will induce a voltage.  It is a fact that the faster you move the magnet, the greater the voltage induced.  This seems obvious to me, but maybe I'm missing something.

I don't understand what you are asking, as the law precisely says in how much time a magnetic field is created.  Movement is change in distance over change in time.  The greek letter delta that looks like a triangle is a stand in for "change" in physics.  Also, in mathematics, d is a stand in for derivation, which is functionally the same as the change of a variable over time in a given function, and is therefore equivalent to delta in physics.  The circle with the line through it is phi, a greek letter that is assigned to the magnetic flux as described by the hyperphysics frame that i linked to previously (equivalent to BA).  "t" is always time.  "V" is voltage. "N" is the number of turns in the coil, or what I would normally describe as the stator.

So, this law describes the voltage induced by moving a magnet of phi magnetic flux through a number of N coils in a t amount of time.  As the description of movement necessarily depends upon a time component, then the answer of why time is in this law should be obvious.

However, if your question is, instead, "does a magnetic field act instantaneously rather than at some fixed rate?", then I don't know the answer to your question.  I am inclined to answer that magnetic fields, like gravitational fields, change at the speed of light.  One could imagine a test of such a case as a highly rotating neutron star.  I think that magnetic fields propagate at the speed of light, because if they propagated instantaneously, then they would defy special relativity.  I don't know that this is accurate though, and I'm not sure if this is the question you are asking.

I think the question has to either one about inductance or else about the rate of propagation of a change in a magnetic field. The latter, as you say, must take place at c, while the former is governed by the formulae for deriving inductance. But our poster seems to have vanished. Perhaps it was a homework question and we got suckered into answering it.