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Electromagnetism

02/24/2009 4:15 AM

Assume you have an infinitely long conductor and constant current is flowing at one direction. You will be able to measure the magnetic field around it. Suppose you start moving towards the directing of current flow at the same speed. Now for you there is no current flow. Will you experience a magnetic filed around the conductor in this condition?

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#1

Re: Electromagnetism as an entity

02/24/2009 4:36 AM

you can measure magnetic field

you cannot move at the speed at which electricity(current) flows as it is speed of light

so no point in going in it

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#2
In reply to #1

Re: Electromagnetism as an entity

02/24/2009 5:05 AM

The actual drift speed of electrons is much much smaller than that of light and in theory we can travel at this speed. So the question is still valid.

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#3
In reply to #2

Re: Electromagnetism as an entity

02/24/2009 8:58 AM

haha, you are really artist, instead of physicist.

electron can be slower movement, but current is another thing, which run according to field, so we can say it runs as fast as the light.

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#4

Re: Electromagnetism

02/24/2009 11:00 AM

The matter is too complex and is in the realm of theoritical physics and not engineering.

The current carrying conductor is creating a circular magnetic field around the conductor.

The velocity of the field (assuming the current is stabilised) is at the speed of light.

The velocity of current too is speed of light.

Now the observer moves at the speed of light.

But as per the special theory of relativity (read long back and not as a part of text book)

c-c=c as c+c=c

Hence in the end the magnetic field will radiate. And though the observer is moving at the speed of light, Electric field will exist and current will flow.

For explanation , contact any theoretical physician (preferably with a nobel in theory of relaitvity).

Too lazy to again pick up that book and read it.

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#6
In reply to #4

Re: Electromagnetism

02/25/2009 2:02 AM

Thanks for your answers.

The core point of my question was, irrespective of the observer if the magnetism will exists as a phisical entity or not. Your answer suggests yes.

Electric current has much slower speed than light in vacuum by the way!

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#16
In reply to #6

Re: Electromagnetism

02/25/2009 11:26 AM

Excuse me. Reread your original post -- the "core" of your question WAS concerning the observance by an observer. There are no questions without an "observer" to ask them!

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#8
In reply to #4

Re: Electromagnetism

02/25/2009 5:11 AM

Hi SB. You said:".....Now the observer moves at the speed of light....." No... Noone and nothing can move at exactly c (only massless particles like photons and gravitons can do that.)

So I have to make it more right: Because an observer can travel only at v<c (to be honest, at v<<c) he, always, "see" the velocity of the electic field (on the wire) to be exactly c (due to SR).

So I agree with the others who said that you, always, "observe" a magnetic field.

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#9
In reply to #8

Re: Electromagnetism

02/25/2009 5:39 AM

I have already referred this is a topic of special theory of relativity. In this you do have observer moving at the speed of light.

It also have been noted that it is a theoritical physics subject.

For this I have to explain in detail what is meant by the observer, but that much will be out of context and also got to study back the definitions from the books i read , as said long back, just for the fun of reading (knowing).

And the flow of current creates the magnetic field (or as can be called EM field) which will exist and move/ spread at the speed of light,

The observer moving at the speed of light will sense this field since in this area the classical physics (with red shift, doppler etc fails). If get the time and the book (in my home lib) will try to locate the equation.

Why didn't you object to c-c=c ?

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#23
In reply to #9

Re: Electromagnetism

02/26/2009 5:19 AM

Sorry I'm intervening, but I think there is a fundamental error committed here. Current is just and plain the sum of moving charges. These charges do not move at the speed of light. The electric field along the wire it does indeed, but not the electrons themselves.

For a moment, think of the situation that there is no wire at all. Just an electron moving at the ridiculously slow speed of 1 cm/s. Doesn't it create a magnetic field around it? Well, sure it does!

I wonder whether relativity has anything to do with the matter.

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#24
In reply to #23

Re: Electromagnetism

02/26/2009 9:12 AM

The current is not the movement of one electron. As I remember the drift speed of electron was of the order of a few mm /sec my memory may be wrong.

If it is so, then say when I switch on the light in my room, assume it is incandescent one (it is not ) then the llengh of the wire at farthest corner is about 6 or 7 mt then after my switchin on it should take about 1/2 hr to start heating up of the filament.

And from power station to my home, once the generator is synchronised, it may take a few years (if not centuries)

The current is moving at a speed of light since it is the effect of the Electric field (moving at speed of light)

In your argument, even light does not move at speed of light ( if I forget about photon, and consider the other part of the dual nature of EM Energy, ie the wave form) then the waves move by the energy transfer - a la relay race- and then the individual particles move at slower speed, but the effect is the C

We have been taught the examples as a row of billiards balls in a table , the first ball moves out as a new ball is moved in the back. The balls are added in the queue at a much slower speed but the effective speed of moveing in aball vs moving out of the que is much faster. In this case we don't even know the queue is full of them. We add one and think this is the one that has come out.

So the speed of current and speed of electron are not the same. Though we have learnt the current is the movement of electron, but is it with this modification.

It is the nett movement (otherwise the movement of electron in the atomic orbit is current too ?) well may be - got to think about it. Then again why not really ?

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#27
In reply to #24

Re: Electromagnetism

02/27/2009 3:26 AM

So the speed of current and speed of electron are not the same. Though we have learnt the current is the movement of electron, but is it with this modification.

Exactly! Current appears at the other end of the wire almost at the speed of light. It's the "perturbation" that you caused by switching the light on which travels at that speed. What you won't see in this other end, or at any point in the wire for that matter, is charges traveling at the speed of light. Instead, you can easily follow the "average" electron in its motion without needing any space rocket for that.

Now, the question is whether the magnetic field is caused by the perturbation as explained before (which anyway, it is instanteneous and happens only at the onset), or by the motion of the charges themselves. For making it easier to see it, consider for a moment the case of a cathode tube, which starts emitting electrons. These electrons will start their travel at 0 speed, and accelerate during their course, but certainly they don't reach the speed of light. There is nothing traveling at the speed of light here, except of course the very moment of the setup of the electric field when you apply a voltage at the two ends. Now, do these electric charges create a magnetic field? Well, of course they do: Magnetic force is caused by the motion of charges.

It would be interesting to consider the case of charges moving in open space, like inside a cathode tube. I would be very interested to know what two electrons experience when they run side by side in parallel routes . It is expected to be an attractive magnetic force between those two (beyond the electric repelling force), and the issue is how they would explain the phenomenon if you asked them? Mind you, for each electron, there seems to be no apparent motion of the other one.

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#5

Re: Electromagnetism

02/25/2009 1:22 AM

My answer is: YES!

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#7

Re: Electromagnetism

02/25/2009 2:19 AM

The current intensity is in my way to see an absolute value after the relative speed between charges and conductor wire.-

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#10

Re: Electromagnetism

02/25/2009 6:24 AM

It is not correct that if you move at the same speed as the electrons that there is no current flow. The conductor has no net charge - i.e. for each electron there is a positive charge in the nucleus of a metal atom. If you move along the wire at the same speed as the electrons, then these metal atoms are moving relative to you in the other direction. No matter how you move along the wire, the sum of the speed of the electrons and metal atoms remains the same. Therefore the magnetic field remains the same.

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#18
In reply to #10

Re: Electromagnetism

02/25/2009 1:14 PM

I don't understand because i think i tried to say the same than you even not being sure:Let's imagine two wires close each other paralel, etc. same charge densities d1=-d2, different sign,speeds v1 an v2 to observer and lets say a law in case i'm correct,B1=a*d1*v1,B2=a*d2*v2 so B=B1+B2=a*d1*(v1-v2), so since i'm trying to represent relative speed between electrons and wire by v1-v2, i conclude magnetic field remains the same so i must insist in what i said before even if i'm wrong.-

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#11

Re: Electromagnetism

02/25/2009 6:37 AM

First of all, the electron flow is not fast but the vibe between them is, whether you're going in their direction or against them, it doesn't matter.

You will still experience the magnetic force because it is perpendicular to the direction of current (electron) flow i.e. it is outward bound in concentric circles around the conductor.

The direction of these concentric circles is unidirectional for DC current and alternating for AC current - end of story.

You can read about this here!

The actual arrows can be confusing if you're in different parts of the world because the Anglos use the right-hand rule whereas the Europeans use the left-hand rule (it's a crazy world) but the argument still stands as shown.

Further you can also see how these individual concentric circles add up to form a magnetic field when you start to coil-up this conductor.

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#12

Re: Electromagnetism

02/25/2009 8:27 AM

Tongue in cheek.... would there be a current flow in an infinitely long wire???? If so, the current flow would be relative to the static wire, therefore a field would be created.

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#13
In reply to #12

Re: Electromagnetism

02/25/2009 9:26 AM

Current flows between points of different electrical potential, so currents in that infinitely long wire would be in segments defined by points of differing potential. Since the wire has no "ends" there would be no uniform flow across the entire "length" (also defined by ends). In any given segment the magnetic field will exist around the wire and can be measured by an observer, no matter how fast he travels relative to the wire. If it were not so, Spock's instruments would not work at warp speed.

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#14

Re: Electromagnetism

02/25/2009 10:14 AM

Say Slick? Just exactly are you going to get current flow in an infinitely long wire? An infinite wire will have an infinite resistance so there can be no current flow...

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#15
In reply to #14

Re: Electromagnetism

02/25/2009 11:19 AM

super conductors ?

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#17

Re: Electromagnetism

02/25/2009 11:48 AM

My understanding is that the speed of light is constant- not effected by any other energy, power or environment & thus the other two factors- time & space are effected. Time is said to slow down for the person that is traveling at or near the speed of light. Mass (a significantly large one) can redirect, bend light & consequently our perception of space can be effected & any movement of our own would have to compensate for this distortion. Though in our common current 'reality' we are only assuming that everything is 'as it should be'. So just like now... we are under the influence of gravitational pull- however we will not feel its effects unless we try to move against it. Likewise I think you would not feel the effects of the magnetic field you are describing- until you try & counter it's effect on you & your surroundings. Carlos

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#19
In reply to #17

Re: Electromagnetism

02/25/2009 3:52 PM

Hang on a minute. While the speed of light may be a constant.....it is not the same in all environments. For instance, the speed of light in water is much slower. Because of this you get refraction. What is really interesting is that once the light exits the water, it resumes its previous speed. But this is really a different subject.

I have already forgotten if the current in this "very long" wire was to be static or dynamic (DC or AC). If it is DC your ability to measure it will depend more on the Hall effect detection equipment than you will need. The magnetic field will radiate outward from the wire and it will be static (i.e. not moving...like rings in a tree). If the current is AC then your relative motion will increase the measured frequency (as you increase the rate at which you cross the existing lines of flux) which will most likely adversely affect the accuracy of your reading. To the best of my knowledge there is no (current or magnetic field) instrument in existence that operates from DC to Light. So the question is mute.

Incidentally, research has shown that at absolute zero, you can bring the speed of light to nearly zero and it regains its properties with a few hundredth of a °K.

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#20

Re: Electromagnetism

02/25/2009 3:52 PM

Let's get a few things straightened out. The electric field is what travels down our infamous wire at the speed of light. The electrons experience an electric force due to the field which causes them to move. Consequently, the electrons in the wire between the switch on the wall and the light bulb on the ceiling begin to move at practically the same time since the wire is much shorter than the infamous infinite wire and since the electric field travels down that wire at the speed of light. The electrons on the other hand, which are our moving charges, travel at a much slower speed since they push eachother away and bump into atoms surrounded by electrons, often bouncing backwards. As a result of all of this, we average out the motion of all the electrons moving down our wire and get an average electon speed down the wire which we call the drift velocity since their motion is very much like bumper boats drifting lazily down a stream with lots of obstacles in the way to make it exciting. This drift velocity is affected by the magnitude of the electric potential (how hard we are pushing the electrons), the material the wire is made of, and the temperature of the wire (the higher the temperature the more the atoms bounce around and get in the way of the electrons).

Current is the amount of charge that moves past some measurement point in some specific period of time. I = dQ/dt Current does not travel at the speed of light. It travels, if you can call it that, at the electron drift velocity since that is the rate at which the charges (electrons) are moving.

Let us start by taking a look at a much simpler example than the infinite wire (which could just as well have been a mile long or even shorter). We have a single electron moving through space at velocity V with respect to you and your laboratory magnetic field measuring device. If that electron is moving such that it passes through your machine in your lab, your machine will measure a temporary magnetic field due to the passing charge. If your lab happens to be traveling at exactly the same velocity (speed and direction) as the electron such that the electron spends all of its time motionless (as far as you can tell) inside your magnetic field measurement device, you will measure no magnetic field since you have no charges moving with respect to you and your machine. You will be able to measure an electric field however, verifying that there is a charge present.

Now imagine an accelerator firing a continuous string of closely but evenly spaced electrons at velocity V. The same arguement holds for the line of charge as it did for a single point charge except the device will measure a reasonably constant magnetic field as electrons move through it rather than just a pulse. When moving with the electrons, you will measure no magnetic field.

Now back to our wire. When the observer is stationary watching electrons flow past, he/she will observe a magnetic field. When walking along the wire at precisely the electron drift speed, the observer will see no net flow of electrons going behind them or in front of them. One would then guess that the observer would then measure no magnetic field. This would be true if all we had in the wire were electrons. However, we also have metal atoms with a net positive charge (since the free electrons are moving at the drift speed down the wire) which look like they are going the opposite direction. Consequently we observe not negative charges going forward as we did when we were stationary, but positive charges going backward. This results in an apparent current (as far as we can tell with our instruments) and a magnetic field will be measured. This magnetic field will have the same directionality.

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#21

Re: Electromagnetism

02/25/2009 8:51 PM

suppose it would be a zero speed relatively. its really a static electric field. run your brain, do an electric charge has a megnetic field around it?

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#22

Re: Electromagnetism

02/25/2009 9:10 PM

Dear cnpower, ...even not running our brains, as long as the author is saying that the "constant current is flowing at one direction" is not a static charge, do you agree? You are just an observer, even sleeping on the electron, you observe what's happening around him

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#26
In reply to #22

Re: Electromagnetism

02/26/2009 10:24 PM

nh, you make thing complex. set asid of current. just relate to sitting on the electron, Im afraid I cannt tell you exactly now. needless to say I forget neerly all of the atomic knowledge.

I search my mind as possible as I could, I remember around the atomic core is layers electron cloud, every electron has its own rotation movement, this cause a pair of megnetic poles.( this is why we difficult to find a single megnetic pole.) now I may say he might be absorb into electron track, or some pole of megatic field or impulse with electron to produce a wonderful anti mass.

continue...

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#25

Re: Electromagnetism

02/26/2009 5:25 PM

Lets not forget that the wire is infinitely long, so no matter how fast you are going relative to it, you may as well be save your hypothetical energy for something else.

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#28

Re: Electromagnetism

02/27/2009 3:53 AM

I'm afraid that we haven't reached a confident conclusion yet.

At first I thought that we could consider current "moving" at c (as it is related with the movement of the electric field... ??? ...)

But then I read the analysis (in the post #20) by Physics Prof and I found it really logical. If it's o.k. we should consider the "speed" of current being the same as the speed (statistical average) of the electrons.

Hence, these are two contrary considerations. And I can't tell for sure which of them is the correct one... ...

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#29
In reply to #28

Re: Electromagnetism

02/27/2009 7:45 AM

may I consider AC current, in this the electrons oscillate and do not move in one direction

so no current flow ?

Drift speed of electron is not the speed of current.

Let us understand the context here, The magnetism is created by the sum total of the electrons and not by only the first electron entering the conductor from source.

As the first enters, the last leaves and inbetween everyone moves in the direction of the field.

This movement of the mass creates the magnetic field (not any specific one moving) since anyway the electrons are moving whether or not you have a field. On the field only the movement is getting regulated.

The problem here is the basic misunderstanding between the current and electrons.

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#30
In reply to #28

Re: Electromagnetism

02/27/2009 9:55 AM

Current is proportional to the sum of charges that crosses a given section of the wire. It is a flux quantity. The term "the speed of current" has no meaning at all. Now, if you are asking: "how fast will current appear at the other end of the wire" then the answer is "almost as much as the light takes to travel the same distance". This holds for any perturbation that is caused to the current (e.g. in the case of AC, the wave will be somehow delayed at the other end). But whether we have perturbations or not, the magnetic field is there and it has to do with the motion of the electrons alone.

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#31
In reply to #30

Re: Electromagnetism

02/27/2009 11:34 AM

Hi Tkot. You said:"The term "the speed of current" has no meaning at all." I don't like the term "speed" for the current either. That's why I putted " " on the word speed and some ? afterwards... You are right that the current is "the sum of charges that crosses a given section of the wire" in a given time duration (i.e. I=Q/t).

The problem is what happens if we are, also, moving along at the speed of electrons. We observe no current flow. But, certainly, the magnetic field is present: in the case of the two parallel wires flown by currents of the same direction (see the latest CR4 challenge) the two wires attract each other (due to their magnetic fields). And we observe this attraction even when we are moving along at the same speed of electrons. I.e. we observe no movement of the electrons but we still observe the attraction between these wires. The magnetic field is still present.

In order to get rid of the "positive current" (see #20) -which can be considered equivalent to the "negative current" of the electrons- we could consider the flow of electrons inside an electronic vacuum tube (as you already said). The magnetic field is still there even when we are moving along at the same speed of electrons.

Let me put it another way: We have two parallel plastic, thin rods with some static charges (e.g. electrons) on them. Of course, no attraction force will take place between these two rods (no movement of electrons → no magnetic field). Now we start to move relative to these rods (in a parallel direction). Hence we observe an apparent movement of the electrons relative to us. This is considered as a "current" by us. But we don't observe any attraction between the two rods (no magnetic field is produced).

These are in contradiction with the opinion presented by the PhysicsProf in post #20 (who said that we observe no magnetic field when we are moving in accordance with the electrons.)

So, what's really going on???

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#32

Re: Electromagnetism

02/27/2009 1:26 PM

Are we entering the area of theoritical physics ?

In this area we have only one ineraction out of four (EM, Gravity, Strong and Weak interactions)

Let us make it more complicated (it is complicated enoght as it is)

A negatively charged sphere - does it have a magnetic field ?

Now you move away, as the observer moves, with reference to him the charge moves. So though the charge is stationery, on observer it is exerting an EM field

Now any Field EM or otherwise is it depending on the frame of reference ?

In the end it boils down to Maxwells equations.

Based on the frame of reference (observer) the two fields vary and the observer feels a field or the other.

Thus a charge in space (whether moving or not) is fgoing to have its EM field and is going to apply a force on other charge at the speed c.

The force that is going to be applied (or the other charge is going to experience E, M or EM field) depends on how the second charge is moving with reference to other.

The second charge may as well be the observer.

But what is stationary with respect to a charge ? One electron, or the flow of electrons ?

As per maxwells law, it is the current density factor and not an individual charge that is necessary.

And the current, being effect of EM field is established at the speed of light.

For the magnetic field not to be established

x B must be zero

or εoδE/δt +μJ= 0

So if the observer moves in a way that the above is satisfied, then only the B will become zero.

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#37
In reply to #32

Re: Electromagnetism

03/03/2009 9:42 PM

x B must be zero

I know you are familiar with the formula, I think this might be yhour written wrong, people often like to write x sign instead of .(point). but here, there will be different meaning.

x B =rot B =0 may be not found everwhere, but

. B =div B =0 can be.

howeve, at your assumption . its really equal to aero.

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#38
In reply to #37

Re: Electromagnetism

03/03/2009 11:24 PM

XB (or for that matter any vector) gives an idea of time variation of that vector in the three co-ordinates. So for the Magnetic field to be zero, this equation also is to be satisfied.

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#33

Re: Electromagnetism

03/02/2009 5:15 PM

Assume you have an infinitely long conductor and constant current is flowing at one direction.

So there will be magnetic field exist before i start moving the speed of current. So during trvelling i will observe magnetic field.

You will be able to measure the magnetic field around it. Suppose you start moving towards the directing of current flow at the same speed. Now for you there is no current flow.

Ofcourse. If i didn't travel towards the current flow and hanging in the conductor still there will be no current flow through me. Since current will choose the low resistance path.

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Guru

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#34
In reply to #33

Re: Electromagnetism

03/03/2009 12:56 AM

AKA the Faradic Cage?

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Guru
Engineering Fields - Control Engineering - New Member China - Member - New Member

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#35
In reply to #34

Re: Electromagnetism

03/03/2009 4:20 AM

a brave man with tasla coil.

3D?

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Guru
Hobbies - DIY Welding - New Member

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#39
In reply to #34

Re: Electromagnetism

03/04/2009 4:38 AM

Is that David Copperfield?

Every time I see a man with a stick in his hand playing and throwing sparks like that it makes me more & more convinced that Tesla Coil = Magician's coil and whether it works or not you can always poke it.

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Canada - Member - uWaterloo/PI represent! lol

Join Date: Feb 2009
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#40
In reply to #39

Re: Electromagnetism

03/04/2009 9:02 AM

I saw footage on Discovery channel of one Australian inventor who took one for a nice little swim with his tin foil hat and grounds in the pool.

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#36

Re: Electromagnetism

03/03/2009 9:07 PM

You could have so complex thinkings, any kind of ideas, but at the end some equation and numeric values can be verified or not.Above i wrote couple equations. Are those right or wrong after your point of view? please a single answer...

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