Static and Moving Electric Fields
When an electron is static, it has an
electrostatic field. When it is put in motion, there is a
current making an electromagnetic field.
How soon does a electrostatic field change to an electromagnetic
field and back?
Does it change automatically as soon as it moves OR proportionally
to the acceleration of the electron (as the electric field
diminishes, the magnetic field grows in respect to the
acceleration of the electron)?
It does not really change from one to the other, because the field observed
depends on the relative motion of the charged particle and an observer.
Think of a stationary charged particle viewed by two observers, one moving
and one stationary. The moving observer will see an electromagnetic field;
the stationary observer will see an electrostatic field.
Here is an analogy: If you look at a piece of paper edge on, it looks like
a line segment. But if you change the angle of the paper, or if you move
around it, you will notice that the paper has a width as well as a length.
The same kind of thing is going on with charged particles, but it is the
relative speed, rather than the relative angle, that makes the difference.
Technically speaking, there is no such thing as a static
electron--it will always be traveling at (or very close to) the
speed of light. So in a situation where a circuit is turned on and
off, there will be cohesive movement in one direction of the
electrons. It is the movement of the electrons all in one direction
that generates the magnetic field. So how quickly does it take the
electrons to start moving in the same direction. I do not know if
there have been experiments that have tried to measure this time,
but my guess would be something very close to the speed of light. As
far as automatically or proportionally, you have to ask yourself to
what degree of certainty do you need this answer. The speed of
light is increadibly fast, so whether it is proportional or
instantaniously, it would be very difficult to tell. Figure it this
way, even if the switch is proportional, it would be proportional to
such a very large number that by assuming it is proportional or
instantaniously and being wrong, you error would be very very small.
You have asked a great question! Seriously!
There are several subtleties that occur here, but I will attempt to give
you a concise answer first.
The short answer: The change radiates out at the speed of light.
Now to explain that...
The first thing to keep in mind (and this is not nessecarilly an easy
thing to consider!) is that you have got to talk about where you are
relative to the electron and what motions are involved.
To keep things simple, let us assume that you are near the electron, and
both of you are sitting still. And, suppose you have got a device that
measures both electric fields and one that measures magnetic fields too.
When you are both sitting still, all you measure is a reading of the
electric field of the electron. Once the electron begins moving
relative to you, then things can start changing.
The first question is: How fast does it change? Is it instantaneous,
does it lag, what is going on?
The answer is that it depends upon how far you are from the electron(and
let us assume things are not moving super fast). The change in the
electric field(and the manifestation of the magnetic field) will radiate
out from the electron at the speed of light. Therefore, if you are close
it will appear possibly very quickly. However, if you are far away, then
even after the electron has begun moving(as measured by someone close to
the electron), you will not see any change in the electric field or in the
position of the electron until after enough time has passed for the
variation in the electric field to reach you.
Now, from the language above, you may have figured out that my answer is
leaving out much. And it is true!
Electric fields and magnetic fields are related in a very intimate way.
Depending upon where you are and how the electron is moving relative to
you, electric fields and magnetic fields can be interchanged. That is
another way of saying that they manifest themselves from the same thing
and that your observation will depend upon your perspective relative to
So, now let us assume that your electron is moving past you with a
constant velocity in a straight line. You will now measure both an
electric field and a magnetic field and both of them will change with time.
Now contrast that with the same situation in which you AND the electron
are moving parrallel to each other with the same constant velocity.
Now, even though I might say that there is an electric and magnetic
field and that they are both changing, you would only measure a constant
electric field. In other words, so long as your constant motion is the
same as that of the electron, it appears to be standing still and so
you will only measure an electric field.
There are huge jumping off points here. I have used the word "relative"
quite a bit while attempting to avoid "special relativity" However, the
honest truth is that you cannot escape it! It was a very similar line of
questions that led Einstein to discover special relativity . in fact,
Einstein's paper was not called "Special Relativity" or anything like
that, but instead "A paper on the electrodynamics of moving bodies."
I would suggest looking up some material on special relativity if you feel
Michael S. Pierce
Materials Science Division
Argonne National Laboratory
When talking about individual electrons, we must actually use quantum
physics. The photons randomly emitted due to the temperature of the
electron are electromagnetic at all times. An electron is always vibrating.
When vibrating but not going anywhere, these photons combine into a wave.
When we see this wave, it produces an AVERAGE magnetic field of zero but a
noticeable electric field.
When the electron is moving, the AVERAGE magnetic field is just as
noticeable. The electron absorbs energy from the warm air around it,
perhaps even from the equipment used to measure the electron. This heat
energy is in turn emitted as electromagnetic waves. Exactly when cannot be
measured. There is no exact time of emission. Multiply the energy in the
wave. Rest assured that, usually, it is emitted in a time that is on the
order of billionths of a second or shorter.
When you get into quantum mechanics, often a college course, you will learn
how individual particles work very different from groups of particles. In a
physics class that has not reached quantum theory, individual electrons are
treated the same as larger charges to make the work easier. You can spend
more time understanding electric and magnetic fields now. When you have had
the preparation everyone needs (calculus, electromagnetic theory,
light,...), a professor can tell you the wonders of how individual particles
seem to behave.
God bless you,
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Update: June 2012