| Topic: |
Science > Physics |
| User: |
"Emwizsoon" |
| Date: |
29 Dec 2004 06:35:29 PM |
| Object: |
Why does current create magnetic field? etc. |
Hi,
I'm a student. I have 4 questions.
1. What's the different between a power line and an
antenna? Both has current, but antenna emits em wave
while power line emits magnetic field in small frequency.
Can the power line elf emission be consider
electromagnetic field too?
2. What's the difference between a magnetic field
and electromagnetic field? Both current can produce
them as in antenna and power lines. Is it in the
frequency?
3. Why does current create magnetic field or in
case of antanne electromagnetic field. I mean, Where
does the "field" come from? How can it propagate in
empty space.
4. Why is the speed of light 186,282 miles per
second and not 500,000 miles per second? What's so
special about 186,282?
Thank you.
Emwizsoon
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| User: "Oliver Keating" |
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| Title: Re: Why does current create magnetic field? etc. |
29 Dec 2004 07:05:40 PM |
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"Emwizsoon" <emwizsoon@yahoo.com> wrote in message
news:1104366929.497354.188060@f14g2000cwb.googlegroups.com...
Hi,
I'm a student. I have 4 questions.
1. What's the different between a power line and an
antenna? Both has current, but antenna emits em wave
while power line emits magnetic field in small frequency.
Can the power line elf emission be consider
electromagnetic field too?
Really there is no difference between a power line and an antenna, except
the job they are supposed to do. This can be a problem for example when
there is a solar event that increases the solar wind, and changes the
magnetic field around the earth - long distance power lines can "pick up" on
this, and it can cause blackouts if systems are overloaded with current.
2. What's the difference between a magnetic field
and electromagnetic field? Both current can produce
them as in antenna and power lines. Is it in the
frequency?
Really there is no difference between a magnetic field and an
electromagnetic field. Or more specifically, there is only one phenonenan,
the electromagnetic force, of which the electric force and the magnetic
force are two components of the same thing.
3. Why does current create magnetic field or in
case of antanne electromagnetic field. I mean, Where
does the "field" come from? How can it propagate in
empty space.
A changing electric field gives rise to a magnetic field, and a changing
magnetic field gives rise to an electric field. If an electric field starts
changing, as will happen if a current starts to flow as there are moving
charges, then this will create a magnetic field. But because a new magnetic
field is being created - that too is changing and this in itself starts to
make its own electric field, but this makes its own magnetic field and so
on- it propogates as an "electromagnetic wave" and this is what gives you
radio and indeed light.
Now, how can it propogate in empty space is an interesting question, a
hundred years ago it was thought that space was not empty but there was a
material called "the ether" that exists all through the universe. However,
various experiments showed this to be false, so it would appear it needs no
medium.
4. Why is the speed of light 186,282 miles per
second and not 500,000 miles per second? What's so
special about 186,282?
It is just one of natures constants, there is no reason for it, although it
is related to the ease with which electric and magnetic fields can permeate
through empty space,
Thank you.
Emwizsoon
Interesting questions!
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| User: "Emwizsoon" |
|
| Title: Re: Why does current create magnetic field? etc. |
29 Dec 2004 07:40:03 PM |
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Oliver Keating wrote:
Really there is no difference between a power line and an antenna,
except
the job they are supposed to do. This can be a problem for example
when
there is a solar event that increases the solar wind, and changes the
magnetic field around the earth - long distance power lines can "pick
up" on
this, and it can cause blackouts if systems are overloaded with
current.
I have a magnetic field meter used to measure power lines.
How come I can't pick up the magnetic field of let's say a
power line miles away. I mean. If they increase the current a
hundred fold, should I be able to pick them up. How can I
measure the magnetic field of radio. If I increase the
frequency sensitivity (I don't know how) of my magnetic
field meter from 60 hz to that used by radio wave. Can I
pick its magnetic field. Or not. If not. Why not?
Now, how can it propogate in empty space is an interesting question,
a
hundred years ago it was thought that space was not empty but there
was a
material called "the ether" that exists all through the universe.
However,
various experiments showed this to be false, so it would appear it
needs no
medium.
When we jump from airplane, we fell down to earth.
Science can't detect anything between let's say the
ground and our body. Gravitons have never been detected.
Einstein says it is the curvature of space/time. What
do you guys really believe? Gravitons or Einstein?
Also couldn't it be possible that what enable the
electromagnetic field to travel is because there is
similar distortion in vacuum that enable em wave to
exist in the first place?
4. Why is the speed of light 186,282 miles per
second and not 500,000 miles per second? What's so
special about 186,282?
It is just one of natures constants, there is no reason for it,
although it
is related to the ease with which electric and magnetic fields can
permeate
through empty space,
Why does nature produce that constant? There must be
a reason for it. Everything has a reason. Anyone got
any idea?
Also since they say photons are wave and particles.
How do I turn the let's say electromagnetic wave used
by radio into particles? Do I put a phosphor material
at the antenna.
Thanks.
Emwizsoon
.
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| User: "Don Kelly" |
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| Title: Re: Why does current create magnetic field? etc. |
01 Jan 2005 10:11:37 PM |
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"Emwizsoon" <emwizsoon@yahoo.com> wrote in message
news:1104370803.604040.317880@z14g2000cwz.googlegroups.com...
Oliver Keating wrote:
Really there is no difference between a power line and an antenna,
except
the job they are supposed to do. This can be a problem for example
when
there is a solar event that increases the solar wind, and changes the
magnetic field around the earth - long distance power lines can "pick
up" on
this, and it can cause blackouts if systems are overloaded with
current.
I have a magnetic field meter used to measure power lines.
How come I can't pick up the magnetic field of let's say a
power line miles away. I mean. If they increase the current a
hundred fold, should I be able to pick them up. How can I
measure the magnetic field of radio. If I increase the
frequency sensitivity (I don't know how) of my magnetic
field meter from 60 hz to that used by radio wave. Can I
pick its magnetic field. Or not. If not. Why not?
-------------
The magnetic field of a single current carrying conductor decays at 1/r
where r is the distance. The effect of another nearby conductor carrying the
same current in the opposite direction has a cancellation effect. Power
lines have multiple phases or a return conductor so that at distances
appreciably larger than the distances between conductors, the net magnetic
and electric fields are negligable due to this cancellation effect.
Note also that 1/4 wavelength (or multiple) is the magic length for
antennae - a 1/4 wavelength at 60 Hz is about 780 miles. A bit awkward for
an antenna.
--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer
Now, how can it propogate in empty space is an interesting question,
a
hundred years ago it was thought that space was not empty but there
was a
material called "the ether" that exists all through the universe.
However,
various experiments showed this to be false, so it would appear it
needs no
medium.
When we jump from airplane, we fell down to earth.
Science can't detect anything between let's say the
ground and our body. Gravitons have never been detected.
Einstein says it is the curvature of space/time. What
do you guys really believe? Gravitons or Einstein?
Also couldn't it be possible that what enable the
electromagnetic field to travel is because there is
similar distortion in vacuum that enable em wave to
exist in the first place?
4. Why is the speed of light 186,282 miles per
second and not 500,000 miles per second? What's so
special about 186,282?
It is just one of natures constants, there is no reason for it,
although it
is related to the ease with which electric and magnetic fields can
permeate
through empty space,
Why does nature produce that constant? There must be
a reason for it. Everything has a reason. Anyone got
any idea?
Also since they say photons are wave and particles.
How do I turn the let's say electromagnetic wave used
by radio into particles? Do I put a phosphor material
at the antenna.
Thanks.
Emwizsoon
.
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| User: "Ron G" |
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| Title: Re: Why does current create magnetic field? etc. |
11 Jan 2005 05:28:25 PM |
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Don Kelly <dhky@peeshaw.ca> wrote in message
news:ZvKBd.662891$nl.73409@pd7tw3no...
The magnetic field of a single current carrying conductor decays at 1/r
where r is the distance. > Don Kelly
dhky@peeshaw.ca
Doesn't the magnetic field reduce by the inverse cube root of the distance
r?
That's what I've always learned/heard.
Electrical fields reduce by the inverse square root of the distance r.
Best----
Ron
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.822 / Virus Database: 560 - Release Date: 12/22/04
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| User: "redbelly" |
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| Title: Re: Why does current create magnetic field? etc. |
11 Jan 2005 06:30:40 PM |
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Ron G wrote:
Doesn't the magnetic field reduce by the inverse cube root of the
distance r?
That's what I've always learned/heard.
Electrical fields reduce by the inverse square root of the distance
r.
Ron, there are two misconceptions at work here:
1. The reduction of fields with distance depends on the geometry of
the objects producing those fields (more on this below).
2. You're memory of what you heard is somewhat faulty; in certain
cases taught in physics courses, the magnetic field decays as the
inverse cube (not cube root) of the distance, and the electric field
decays as the inverse square (not square root) of the distance.
Now, back to #1, and the geometry issues.
For a long, straight wire carrying a current, the magnetic field decays
as 1/r, as Don Kelley said.
For a small circular loop carrying a current, the magnetic field decays
as the inverse cube or 1/r^3. This is probably what you are
remembering.
(But this is true only at distances that are substantially larger than
the diameter of the loop.) Also, the value changes if you move from
directly above the loop to one side of the loop, even if you remain at
a fixed distance from it while you move around.
For a point-like electric charge, the electric field decays as 1/r^2,
which is probably what you are remembering.
For an electric dipole (two equal-but-opposite charges close to each
other), the electric field decays as 1/r^3, as in the case of the
current loop's magnetic field. (But you must be at a distance
significantly greater than the charge separation.)
Final note: I have been talking about either constant currents or
stationary charges. If the currents or charges oscillate, that will
change things.
Mark
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| User: "Franz Heymann" |
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| Title: Re: Why does current create magnetic field? etc. |
19 Jan 2005 11:46:18 AM |
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Ron G wrote:
Doesn't the magnetic field reduce by the inverse cube root of the
distance r?
No.
The field of a magnetic dipole decreases as the inverse cube of
distance
(note inverse cube, not inverse cube root)
All magnetic fields are not produced as dipole fields. The field from
a magnetic pole decreases as the inverse square law. Magnetic poles
can be produced in the lab to any desired degree of approximation.
That's what I've always learned/heard.
Electrical fields reduce by the inverse square root of the distance
r.
No, they don't. They decrease as the inverse square of the distance
from a point charge
Franz
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| User: "John C. Polasek" |
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| Title: Re: Why does current create magnetic field? etc. |
30 Dec 2004 10:15:45 AM |
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On 29 Dec 2004 17:40:03 -0800, "Emwizsoon" <emwizsoon@yahoo.com>
wrote:
Oliver Keating wrote:
Really there is no difference between a power line and an antenna,
except
the job they are supposed to do. This can be a problem for example
when
there is a solar event that increases the solar wind, and changes the
magnetic field around the earth - long distance power lines can "pick
up" on
this, and it can cause blackouts if systems are overloaded with
current.
I have a magnetic field meter used to measure power lines.
How come I can't pick up the magnetic field of let's say a
power line miles away. I mean. If they increase the current a
hundred fold, should I be able to pick them up. How can I
measure the magnetic field of radio. If I increase the
frequency sensitivity (I don't know how) of my magnetic
field meter from 60 hz to that used by radio wave. Can I
pick its magnetic field. Or not. If not. Why not?
Now, how can it propogate in empty space is an interesting question,
a
hundred years ago it was thought that space was not empty but there
was a
material called "the ether" that exists all through the universe.
However,
various experiments showed this to be false, so it would appear it
needs no
medium.
When we jump from airplane, we fell down to earth.
Science can't detect anything between let's say the
ground and our body. Gravitons have never been detected.
Einstein says it is the curvature of space/time. What
do you guys really believe? Gravitons or Einstein?
Also couldn't it be possible that what enable the
electromagnetic field to travel is because there is
similar distortion in vacuum that enable em wave to
exist in the first place?
4. Why is the speed of light 186,282 miles per
second and not 500,000 miles per second? What's so
special about 186,282?
It is just one of natures constants, there is no reason for it,
although it
is related to the ease with which electric and magnetic fields can
permeate
through empty space,
Why does nature produce that constant? There must be
a reason for it. Everything has a reason. Anyone got
any idea?
Also since they say photons are wave and particles.
How do I turn the let's say electromagnetic wave used
by radio into particles? Do I put a phosphor material
at the antenna.
Thanks.
Emwizsoon
Emwi, I like the way you won't take no for an answer, you keep
chugging away.
A broadcast antenna and the transmitter are especially designed to
couple efficiently to the impedance of space which is 377 ohms. In
that way a large part of each cycle is stolen from the antenna and
dispersed in space. If the broadcaster is not so tuned, then the
energy stays in the tuned "tank" circuit.
The power line at 60hz has simply no impact on space where
capacitivity is 8.8e-12 farad/meter, and likewise with its
permeability 4pie-7 hy/meter. Impedance is sqrt(mu/eps) = 377ohms.
As for your other question you posted, don't worry about the professor
asking you questions to which even he does not have an answer. Just be
sure you can quote his notes back to him in the final. Of course there
must be extra credit if you can tell him something that he can tell
his students next year!
Mr. Dualspace, sometime lecturer
If you have something to say, write an equation.
If you have nothing to say, write an essay
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| User: "Guy Gordon" |
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| Title: Re: Why does current create magnetic field? etc. |
31 Dec 2004 02:50:35 AM |
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"Emwizsoon" <emwizsoon@yahoo.com> wrote:
2. What's the difference between a magnetic field
and electromagnetic field?
Just your frame of reference relative to the field.
3. Why does current create magnetic field or in
case of antanne electromagnetic field. I mean, Where
does the "field" come from? How can it propagate in
empty space.
A field doesn't propogate. By definition a field exists everywhere in
space, and every point has some value (which may be zero). If the
values are fixed, you have a static field. If they vary sinusoidally
with time, you have a wave.
4. Why is the speed of light 186,282 miles per
second and not 500,000 miles per second? What's so
special about 186,282?
Wish I knew the answer to that one. :-)
Posted Via Usenet.com Premium Usenet Newsgroup Services
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| User: "Rene Tschaggelar" |
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| Title: Re: Why does current create magnetic field? etc. |
31 Dec 2004 04:35:24 AM |
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Emwizsoon wrote:
Hi,
I'm a student. I have 4 questions.
4. Why is the speed of light 186,282 miles per
second and not 500,000 miles per second? What's so
special about 186,282?
Well, light travels a certain distance within a certain time.
We've choosen the second to be 1 second, and we've choosen the
mile to be 1 mile long. Others have choosen a km to be 1km long.
Now the speed of the light was measured with these value.
Yes, there is a wave equation outputting the speed of
light in terms permissivity and permeability, but these
are also just some measured values in a certain measuring
context.
Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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