Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 05/09/2014 07:38:16
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An insulator can stop an electric current flowing but what about the electric and magnetic fields. Do they still penetrate insulators. If we had a ball bearing in the hollow centre of a transparent sphere which is made of a material insulator would a magnet attract it? This is an important consideration when studying the penetration of the gravitational field.
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No.
Some ferromagnetic materials, which can be used for magnetic shielding, are poor electrical conductors, but insulators do not in general exhibit strong magnetic properties of any kind.
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Electric fields will penetrate an insulating ball, and will attract or repel the ball bearing (depending on whether the ball has the same or an opposite chargeto the external electric charge).
Magnetic fields will penetrate an insulating ball, and will attract an iron ball bearing, or make a magnetized ball bearing spin around to line up South Pole with North pole.
Gravitational fields will penetrate an insulating ball, and will attract a ball bearing inside.
However, if the ball (and ball bearing) are freely orbiting a mass like the Earth, then the ball bearing will float freely inside the insulating ball - the attraction of the Earth bends its path into an ellipse.
PS: I don't understand how electric or magnetic shielding might achieve gravitational shielding - but it would be great if you could accomplish it!
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I agree with the above posts, but would like to add on minor point. In electrodynamics, the electric field and magnetic field are linked, so if you block the electric field, you also block the magnetic field that is linked to it. In this case, an insulator can block the magnetic field indirectly.
In detail, an electric field that is changing in time induces a magnetic field. So if you block the electric field, it is zero for all time (not changing) and the induced magnetic field also vanishes.
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That is as I expected but I wanted to make sure I hadn't overlooked something. Sometimes effects are discovered that were not expected. I am looking into the penetration of a mass by the gravitational field. JP I will be coming back with some questions over the next couple of weeks. At the moment I am looking at Gauss's Law so it is early days.
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Worth doing a simple experiment. Measure the gravitational attraction between a test mass M and two identical moveable masses m and m at distances r and R respectively. Now if r is less than R you can interpose one of the moveable masses so that it eclipses the other as seen from M
Classical dogma says that F = GMm(1/r^2 + 1/R^2), i.e the gravitational force on M is additive in the eclipse, and if the two m's are in opposition, F = GMm(1/r^2 - 1/R^2), i.e. subtractive. So you can measure the effect, if any , of gravitational shielding.
It's quite a difficult experiment to do but the direct measurement of G has been done many times and I'm sure most national laboratories still have the equipment in a museum somewhere.
Good luck.
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I'm not interested in gravitational shielding as I don't believe it to be possible. That was not the purpose of the original question. Field penetration is one of the mysteries of physics. What is a field composed of? We know it's there and it has a range and it can cause seemingly remote effects. This field penetration when considered for gravity has to explain the equivalence principle and the fact that gravity has been described as a fictitious force.
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An insulator can stop an electric current flowing but what about the electric and magnetic fields. Do they still penetrate insulators. If we had a ball bearing in the hollow centre of a transparent sphere which is made of a material insulator would a magnet attract it? This is an important consideration when studying the penetration of the gravitational field.
Do you really not understand that air is an insulator?
Have you never seen a compass with a case round it?
Have you just not been paying attention?
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An insulator can stop an electric current flowing but what about the electric and magnetic fields. Do they still penetrate insulators. If we had a ball bearing in the hollow centre of a transparent sphere which is made of a material insulator would a magnet attract it? This is an important consideration when studying the penetration of the gravitational field.
Do you really not understand that air is an insulator?
Have you never seen a compass with a case round it?
Have you just not been paying attention?
Air is also a gas and as such does not have a lattice of molecules bound together. If you stood in a lightening storm in a Faraday cage you would assume yourself safe. A Faraday cage is not one continuous mass but is as stated a cage with holes in it. There are effects that surprise us all the time. I already knew what the answer would be but there may have been something that came up in the discussion that described one of these effects.
Of interest was the comment by JP "I agree with the above posts, but would like to add on minor point. In electrodynamics, the electric field and magnetic field are linked, so if you block the electric field, you also block the magnetic field that is linked to it. In this case, an insulator can block the magnetic field indirectly.
In detail, an electric field that is changing in time induces a magnetic field. So if you block the electric field, it is zero for all time (not changing) and the induced magnetic field also vanishes."
There are some subtle interactions that become important in some circumstances.
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One such example of these counter-intuitive effects can be found here.
http://news.bbc.co.uk/dna/place-lancashire/plain/A37235784
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You seem to have not noted a few things.
Firstly, you said nothing about a solid insulator.
Secondly that there are magnets which are, themselves insulators and any ordinary magnet will be covered by a thin layer of oxide which may or may not be magnetic.
Thirdly, the analogy to a Faraday cage is to use a magnetic "conductor", rather than an insulator.
Something like this
http://en.wikipedia.org/wiki/Mu-metal (http://en.wikipedia.org/wiki/Mu-metal)
Also, you may find the physics of dropped springs "counter intuitive, but I don't.
And you seem to have missed the point about a compass.
Don't you realise that things like this
http://en.wikipedia.org/wiki/Compass#mediaviewer/File:Walkers_compass_arp.jpg
work.
Before you concern yourself with " subtle interactions that become important in some circumstances." perhaps you should find out the basics of what happens in ordinary circumstances.
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I agree with the above posts, but would like to add on minor point. In electrodynamics, the electric field and magnetic field are linked, so if you block the electric field, you also block the magnetic field that is linked to it. In this case, an insulator can block the magnetic field indirectly.
In detail, an electric field that is changing in time induces a magnetic field. So if you block the electric field, it is zero for all time (not changing) and the induced magnetic field also vanishes.
Except that insulators do not block electromagnetic radiation. Your radio and mobile phone work perfectly well inside wooden buildings, and light travels pretty well through glass and many plastics. You can however exclude low frequency electromagnetic radiation by (as jeffrey points out late) a Faraday cage or a solid conductor, and yet you can never competely absorb gamma radiation. Fact is, it isn't that simple.
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You're right--I missed the "insulator" point in the OP's question.
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I don't think insulators would be very good at blocking magnetic fields.
But superconductors are great at it. The magnetic field inside a superconductive object is zero (Meissner Effect.)