Thread: intervening meadium in gravity

what does it actually mean that gravity is not affected by intervening meadium

Originally Posted by saurab dulal

what does it actually mean that gravity is not affected by intervening meadium

It means that if you place an object in back of another object, say a satellite on the opposite side of the Earth from the sun, it feels the attraction of both bodies, in this case both the Earth and the Sun.

That is contrasted with, for instance, an electrostatic force, which is shielded by conductive materials. A body inside of a condutive box, does not feel an electrostatic force from charges outside of the box. (see "Faraday cage").

There is no Faraday cage for gravity.

Post removed as off-topic

simply means it's unaffected by a Blockage


A stream of water moving from point A to point B is affected if you drop a wall in it, but not if you shoot Light at it, so a Wall is an affecting medium on water, while light isn't


Gravity appears to be unaffected by any medium. Mass, light, energy, Nothing blocks the suns gravitational attraction to us....that we know of

If such a medium existed, you could build a room out of it and objects would be weightless inside. I may be wrong, but you could also build a rocket out of it and lift the payload and fuel for free, since they'd be weightless too. I'm pretty sure that if you thought about it for long enough, you could find even crazier uses for the stuff, probably crazy enough to prove it can't exist. (Sorry if that's all a bit off topic.)

thanks a lot to all the guys who help me to understand it,but i still have a lot of question regarding it,i think i need to analyse it first.

Originally Posted by DrRocket

It means that if you place an object in back of another object, say a satellite on the opposite side of the Earth from the sun, it feels the attraction of both bodies, in this case both the Earth and the Sun.

That is contrasted with, for instance, an electrostatic force, which is shielded by conductive materials. A body inside of a condutive box, does not feel an electrostatic force from charges outside of the box. (see "Faraday cage").

There is no Faraday cage for gravity.

A tiny exception is taken to the stated rule: The default condition within a Faraday cage holds that all internal contents are devoid of electrical charge and are therefore not subject to feeling of any electrostatic force from outside of the box. Therefore, the rule seems to imply an impenetrability that really goes untested as a rule.

The reason behind the phenomena manifested by a Faraday cage is the fact that a negative charge, for instance, brings all excess electrons out to the outermost surface in their flight due to mutual repulsion. They remain at the outer surface due to sustained global electrostatic force. An ionizing event could split an electron far enough away from an atom within a Faraday cage, so that the sustained negative electrostatic force pushing electrons against or toward the outer surface could exceed the attraction felt between the dislodged electron and its erstwhile atomic nucleus. As a result, the electron would respond by joining the excess electrons, and the positive ion remaining would migrate or propagate into the opposite direction until it reached the electrical center of the Faraday cage. Thus, a Faraday cage can be expected to maintain itself against dynamic conditions that bring transient disruption to the normal exclusion of ions from its interior.

That very slight oversimplification hereby declared for the rule that implies electrostatic impenetrability for a Faraday cage becomes a catastrophic misnomer when recited by creditable scientists as an invalid rebuttal against truth that is vital to many useful scientific clarifications.

Originally Posted by dalemiller

Originally Posted by DrRocket

It means that if you place an object in back of another object, say a satellite on the opposite side of the Earth from the sun, it feels the attraction of both bodies, in this case both the Earth and the Sun.

That is contrasted with, for instance, an electrostatic force, which is shielded by conductive materials. A body inside of a condutive box, does not feel an electrostatic force from charges outside of the box. (see "Faraday cage").

There is no Faraday cage for gravity.

A tiny exception is taken to the stated rule: The default condition within a Faraday cage holds that all internal contents are devoid of electrical charge and are therefore not subject to feeling of any electrostatic force from outside of the box. Therefore, the rule seems to imply an impenetrability that really goes untested as a rule.

The reason behind the phenomena manifested by a Faraday cage is the fact that a negative charge, for instance, brings all excess electrons out to the outermost surface in their flight due to mutual repulsion. They remain at the outer surface due to sustained global electrostatic force. An ionizing event could split an electron far enough away from an atom within a Faraday cage, so that the sustained negative electrostatic force pushing electrons against or toward the outer surface could exceed the attraction felt between the dislodged electron and its erstwhile atomic nucleus. As a result, the electron would respond by joining the excess electrons, and the positive ion remaining would migrate or propagate into the opposite direction until it reached the electrical center of the Faraday cage. Thus, a Faraday cage can be expected to maintain itself against dynamic conditions that bring transient disruption to the normal exclusion of ions from its interior.

That very slight oversimplification hereby declared for the rule that implies electrostatic impenetrability for a Faraday cage becomes a catastrophic misnomer when recited by creditable scientists as an invalid rebuttal against truth that is vital to many useful scientific clarifications.

You have it wrong.

The effect of a Faraday cage is not to make the contents devoid of charge.

The basic idea is that in a conductor charges are mobile. A charge introduced only a closed conductive surface causes the charges to arrange themselfes so that the field internal to that surface is zero. This is a result of an idealization and of Maxwell's equations.

It has little to do with actual materials that are composed of atoms, and nothing whatever to do with atomic physics or ionization.

As an engineering tool, real Faraday cages work pretty well in providing shielding to internal components, even in the case of time-varying fields, so long as the frequencies are low relative to relaxation times for the conductive material and so long as penetrations of the box are appropriately engineered to prevent excess leakage.

There is a gravitational analogy. Given a spherical shell of uniform density, the internal gravitational field is zero. Thus the gravity internal to a radially symmetric ball depends only on the material at a radius less than that of the point in question. Note that this has nothing whatever to do with any sort of gravitational shelding, and is simply the result of the nature of fields that obey an inverse square law.

You can block the movement of charge with a Faraday cage but not the electromagnetic potential, the field if you will. Electromagnetic fields, like gravitational fields, cannot be blocked.

Originally Posted by MigL

You can block the movement of charge with a Faraday cage but not the electromagnetic potential, the field if you will. Electromagnetic fields, like gravitational fields, cannot be blocked.

That is just plain wrong.

Electromagnetic sheilding is designed to do just that.

That is why shielded coaxial cable is used in many applications. That is why twisted shielded pairs are used in others. That is why devices are enclosed in conductive boxes to limit emissions. That si why .......

Originally Posted by DrRocket

]

You have it wrong.

The effect of a Faraday cage is not to make the contents devoid of charge.

.

You have it wrong.
Since all of the charge is confined to the outside surface of a Faraday cage, no charge remains to be distributed within it. The truth is so very simple that it is saddening to see esoteric ramifications introduced that posture as mechanisms of higher authority. For some reason, a truth that is such common sense to blue-collar evaluation becomes incomprehensible to elite scholars.

I strongly disagree. Fields cannot be blocked. Shielding around wires and circuits attenuates some frequencies, because of absorption and re-radiation at particular frequencies. A capacitor is a simple shield and often used for filtering because it blocks current but not fields (voltage).

Originally Posted by MigL

I strongly disagree. Fields cannot be blocked. Shielding around wires and circuits attenuates some frequencies, because of absorption and re-radiation at particular frequencies. A capacitor is a simple shield and often used for filtering because it blocks current but not fields (voltage).

A capacitor is not a shield, never was and never will be. A field is not voltage and voltage is not a field. That comment is nonsensical.

A capacitor is a very very simple filter, which is an entirely different thing.

You apparently don't know what a shield is or how it works.

Shields do most assuredly attentuate electromagnetic fields (nothing is 100% so a very pure interpretatin of "blocked" obviates use of the literal meaning of that word).

An ideal conductor will block an electrostatic field, however.

You need to learn something about Maxwell's equations and classical electrodynamics.

An ideal conductor will block an electrostatic field, however.

I.e. a superconductor?

Originally Posted by KALSTER

An ideal conductor will block an electrostatic field, however.

I.e. a superconductor?

The idealization in the case of static fields is not so much a superconductor as a conductor with evenly distributed charge, i.e. ignoring the discrete nature of electrons. The effect is classical -- Maxwell's equations -- not quantum mechanical.

A static field is also an idealization. In reality any time varying field is not static, and there are no fields that are not time varying at some level (the universe is not of infinite age for instance).

A superconductor, again in the classical setting of uniform distribution of charge, etc, , would have zero relaxation time and threfore ought to shield against dynamic as well as static fields.

dr rocket got owned in this topic lol

nice attribution to the thread, and just as pointless and wrong as the virtual entirety of the topic.

Originally Posted by DrRocket

You have it wrong.

The basic idea is that in a conductor charges are mobile. A charge introduced only a closed conductive surface causes the charges to arrange themselfes so that the field internal to that surface is zero. This is a result of an idealization and of Maxwell's equations.

It has little to do with actual materials that are composed of atoms, and nothing whatever to do with atomic physics or ionization.

No matter: whatever idea you speak of, because since there are no perfect electrical insulators, then everything is a conductor. Electrons are more mobile than the other things, but so what? Charged particles can be moved, but their mobility is not so much of a feature with static electricity to be called its basic feature. It seems better from here to avoid the wooziness of fields when all we need to say is that upon arrival of equilibrium, no particles of net charge fall between the center and the outside layer of charged particles of a Faraday cage.

By all means, unless actual materials are involved, there would be no host for any electricity. Atomic structure is a difficult thing to deny actual materials that are made of atoms or some of their parts. No mention has been made here anywhere of nuclear energy. Splitting an electron away from a nucleus is not what is meant by splitting of atoms. Nothing that I have ever learned would be germain without the phenomena of ionization. Charges that arrange themselves have been doing so long before Maxwell drew up any equations and before any Shakespearian sonnets were written.

Book-learning is wonderful, but must take the rap for so many folks reciting rules they do not comprehend. The charged particles that seek outside surfaces are not external to the Faraday cage, they simply become the outermost portion of the cage. That is why, when one momentarily disrupts the equilibrium of a Faraday cage by inserting a charged particle into it, if he or she can detect the direction of migration taken by that particle, then the polarity of that Faraday cage can be known.

Originally Posted by dalemiller

Originally Posted by DrRocket

You have it wrong.

The basic idea is that in a conductor charges are mobile. A charge introduced only a closed conductive surface causes the charges to arrange themselfes so that the field internal to that surface is zero. This is a result of an idealization and of Maxwell's equations.

It has little to do with actual materials that are composed of atoms, and nothing whatever to do with atomic physics or ionization.

No matter: whatever idea you speak of, because since there are no perfect electrical insulators, then everything is a conductor. Electrons are more mobile than the other things, but so what? Charged particles can be moved, but their mobility is not so much of a feature with static electricity to be called its basic feature. It seems better from here to avoid the wooziness of fields when all we need to say is that upon arrival of equilibrium, no particles of net charge fall between the center and the outside layer of charged particles of a Faraday cage.

By all means, unless actual materials are involved, there would be no host for any electricity. Atomic structure is a difficult thing to deny actual materials that are made of atoms or some of their parts. No mention has been made here anywhere of nuclear energy. Splitting an electron away from a nucleus is not what is meant by splitting of atoms. Nothing that I have ever learned would be germane without the phenomena of ionization. Charges that arrange themselves have been doing so long before Maxwell drew up any equations and before any Shakespearian sonnets were written.

Book-learning is wonderful, but must take the rap for so many folks reciting rules they do not comprehend. The charged particles that seek outside surfaces are not external to the Faraday cage. That is why, when one momentarily disrupts the equilibrium of a Faraday cage by inserting a charged particle into it, if he or she can detect the direction of migration taken by that particle, then the polarity of that Faraday cage can be known.

The electrons, including the electrons in the conduction band, in a metal are generally contained in the metal, unless the work function has been exceeded, but that is an entirely different topic. The action of a Faraday cage pertains to the exclusion of externally applied electric fields from penetrating the surface that forms the cage. It is not a result of any sort of "charge containment" as you describe it.

If you replace the charged surface with an insulating surface there will still be no charged particles internal to the surface. But an externally applied electric field will penetrate the surface as though it were not there.

On the other hand a charge introduced in the interior of a Faraday cage will create an internal field.

Sonny, I comprehend these "rules" rather well. It it you who do not know what in the hell you are talking about.

There is no such thing as the "polarity of a Faraday cage".

Originally Posted by DrRocket

Sonny, I comprehend these "rules" rather well. It it you who do not know what in the hell you are talking about.

There is no such thing as the "polarity of a Faraday cage".

Sir: I congratulate you on your self esteem and good manners. I apologize for trying to help you and promise to never try that again.

Originally Posted by dalemiller

Originally Posted by DrRocket

Sonny, I comprehend these "rules" rather well. It it you who do not know what in the hell you are talking about.

There is no such thing as the "polarity of a Faraday cage".

Sir: I congratulate you on your self esteem and good manners. I apologize for trying to help you and promise to never try that again.

Please don't.

Just as you can set up conditions where it seems gravity is shielded, your own example DrRocket, so you can set up conditions where it 'seems' the electromagnetic field is shielded, the Faraday cage.
I stand by my opinion that a charge, by its very existance, implies a field, a gradient, a potential, a voltage which cannot be shielded by another charge or a sequence of moveablr charges ( a conductor).
Similarily, mass/energy implies a field, a gradient, a potential which we call space/time and it also cannot be shielded.

Originally Posted by MigL

Just as you can set up conditions where it seems gravity is shielded, your own example DrRocket, so you can set up conditions where it 'seems' the electromagnetic field is shielded, the Faraday cage.
I stand by my opinion that a charge, by its very existance, implies a field, a gradient, a potential, a voltage which cannot be shielded by another charge or a sequence of moveablr charges ( a conductor).
Similarily, mass/energy implies a field, a gradient, a potential which we call space/time and it also cannot be shielded.

That is just wrong.

Fields add vectorially, and it is quite possible to arrange charges so that the resultant field, in a given volume or area, is zero -- which is what shieldiing does.

A Faraday cage does just that to a static electric field.

But since there is no version of a "conductor" with respect to gravity, and since gravity is alway attractive, there is no analog of a Faraday cage.

Spacetime is not a potential. Spacetime is a Lorentzian manifold. Neither is it a field or a gradient. In fact that assertion is meaningless.

Don't be so sure of yourself DrR.
Lets consider the simplest case. A positive charge charge separated by a distance, say x, from a negative charge exerts an attractive force on the negative charge due to Coulomb potential. If we now introduce a second negative charge behind the first, at a distance of say 2x, you claim this second negative charge is screened by the first and does not feel an attractive force to the positive charge. My claim is that it does, but this attractive force is overwhelmed by the repulsion of the negative charge in front of it which is four times stronger. Should you wish you can also consider strictly repulsive potential. Do the math and convince yourself.

The fact that space/time may have a large energy density could imply that it is a potential.
I only meant that the space/time curvature around mass/energy is very similar and analogous to the 3-dimensional potential well which surrounds a charge. The mathematical symbolisim used to describe it is a 'grad' or gradient from which I drew the other terms I used. This is after all a forum of words, not equations, so excuse the flowery language.

Originally Posted by MigL

Don't be so sure of yourself DrR.
Lets consider the simplest case. A positive charge charge separated by a distance, say x, from a negative charge exerts an attractive force on the negative charge due to Coulomb potential. If we now introduce a second negative charge behind the first, at a distance of say 2x, you claim this second negative charge is screened by the first and does not feel an attractive force to the positive charge. My claim is that it does, but this attractive force is overwhelmed by the repulsion of the negative charge in front of it which is four times stronger. Should you wish you can also consider strictly repulsive potential. Do the math and convince yourself.

I said no such thing.

If you have two negative charges and a positive (or negative) halfway in between them, the net force on it is zero.

I have said absolutely notihng that is not contained in Maxwell's equations.

Very well, I cannot speak for you , only for myself. I am saying the field(s) is still there but the conditions negate it effect as opposed to the field(s) disappearing.

The point being that that is exactly what you can't do with gravity.

Originally Posted by MigL

This is after all a forum of words, not equations, so excuse the flowery language.

Not really. That is why the forum supports Tex.

I,m sorry but a charge without a field ? What is that ? A charge is a meassurement in the first place. So what meassures (observer or observing device, not "the observer of a device" off course) has a charge difference to the charge it meassures. These differences come quantumized but as it is always measured relative (to the measurment device as observer) it is never really a property. Charge maybe considered to be a property but that,s all there is to it.

A coulomb has a field implied from the first moment it is defined (and vice versa). It,s in how a coulomb is defined. A field doesn,t result from a charge nor does a charge result from a field you can,t seperate them.
That you can shield what you first meassure as a charge charge doesn,t change this. If you want to speak of a charge then it is a charge relative to what shields it (so outside the shielding it is no longer measured as a charge. To the charge the shielding is like a black hole and as with black holes the question is if it can ever be a complete shielding.

I think it is not possible. Suppose you have a faradaycage. You meassure its potential.
and determine it at zero. Then put a charged objekt in it. Allthough it is shielded, it will influence the charge of the cage material. Electrons of the metal will tend towards the charged objekts. Hence the material turned to the outside changes its charge also. So the electric field runs through the material. Same as when you hold a charge near a steelsphere there will be a charge difference over the spheres circumference. So the charge is not hundred procent shielded.

Originally Posted by Ghrasp

I,m sorry

yep

As I said I know charge is considered a property but an atom is not negative or positive but both. An objekt with a negative charge of 0,000.001 C can have a negative charge of 1 coulomb if the positive charge is 0,000001 microC less then the negative the resulting - meassured - charge to the outside would be 1 microC.

That shows that the charge is not a property but a resultant of negative and positive difference. The total of charges can be completely different with the same resulting charge. That on itself allready implies electric fields running through material. Nothing shields completely.
But also a field perpendicular perpendicular to it (magnetic) always comes with it. (the two fields always come together..the total of the two : spacetime). It would not be the case if it is static but it is never completely static only sometimes it is considered to be. Considering something doesn,t make it realistic.

What makes a farady cage work is based on both fields and their perpendicularity. If you have a negatively charged objekt and a positively charged objekt at a certain distance they are connected with an electric field that implies em radiation. With electricity the electrical component is in the direction of the wire and the perpendicular (cyclic) component or the wavecharacter can be seen in the induction.

If a shielding objekt would be a single ring it divides the space between the objekts in an area running through the ring and an area running around it.

Based on Lorentz the induction component of the field outside and inside is opposite direktion for the material the ring consists of.

If the field as far as it runs around the ring would give an induction leftround the field that runs through it combines with a ring induction rightround. These direktions oppose and that gives a shielding effect due to internal resistance of the material both directions oppose each other everywhere in the ring, the ring works as an obstacle more or less, dependant on it,s size and meassurements.

Change the ring for a plate with two holes in it every hole has a ring of material around it ; two rings are - topology - connected.
Put a field to it perpendicular to the plate and draw a circle around both holes left round the direction for the material in between the two holes is opposed. The field where it runs through one ring it runs outside the other. The shielding is an effect of that topology and only to understand with the notion of the perpendicular field to the electric field. It can,t be understood if only the electric component is considered.

Sorry for complex - lengthy - way of explaining but with some drawing it is easy to see (and would be easier to explain).

Originally Posted by DrRocket

You have it wrong.

Here is how it works: Suppose we had a golden sphere (globe 1) isolated in an electrically neutral environment in space. Suppose that it was coated with 2000 extra electrons. We could say that it was electrically charged to some negative value. We need not care whether or not such a globe is hollow. But if any care, let them call it hollow.

A perfectly spherical shape might present an equidistant spacing of adjascent surplus electrons upon our sphere. Hence. There would be 1000 extra electrons on either hemisphere no matter how you cut it so to speak.

Let us then presume that an identical sphere (globe 2) with an identical charge moseyed up toward globe 1 from the left of an observer. (Of course this would include an endothermic process.) With increased proximity for the globes, a reduced number of electrons would remain upon their facing hemispheres because those particles would be retreating from the advancing array of like charges, shifting to the outlying hemispheres. At any point to the observer’s right where globe 1 intervenes before globe 2, an increased repulsion to the observer’s right will have been produced against any negatively charged particle or body. This is because the outlying hemisphere of globe 1 then contains between but not including 1000 and 2000 surplus electrons. The increased electron or ion density implicit for that hemisphere represents an increased amount of stored energy on the surface of that globe. The same number of surplus electrons would be crammed into a smaller area. Hence, an intruding Faraday cage does not obstruct electrostatic influence from one side to its other.