Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 19/09/2007 01:36:25
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if you had a wire, and it was stretchy, would running a current through it cause it to contract?
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if you had a wire, and it was stretchy, would running a current through it cause it to contract?
If you mean that the diametre contracts, the answer is yes.
Of course we are talking about stationary currents.
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If the magnetic field generated causes a contraction in diameter ( I am reminded of the ZETA experiments of the fifties) is there a corresponding increase in length or does the density increase.
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i was refering to the length of the wire contracting
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i was refering to the length of the wire contracting
Lenght doesn't contract in that case.
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If the magnetic field generated causes a contraction in diameter ( I am reminded of the ZETA experiments of the fifties) is there a corresponding increase in length or does the density increase.
It's a good question. I would say it depends on the rapidity of the action and on the compressibility of the material. For a conductive liquid under "normal" conditions I would expect a corresponding increase in lenght; for a rarefied gas or for very rapid actions (contraction magnetic force) I would expect a significant increase in density, at the beginning and then the increase in lenght.
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I'm picturing the electromagnetism experiment where you have two wires running parallel to one another. if both have a current traveling through it at the same time and in the same direction then attractive magnetism arises.
My understanding as to why that happens is because when both wires have negative current flowing through them, the negative charges view a contracted positive wire, and the positive wire views a contracted negative current due to relative motion. the contracted charges then have a higher amount of electrical force per area relative to the opposite charge due to the fact that there are more opposite charged particles in that area.
so I don't see how the experiment i described above wouldn't cause a magnetic force from the front of the wire to the back, thus causing the stretchy wire to contract. Because just like in the case of the moving currents in the two parallel wires, in the stretchy wire, the negative electric charge has motion relative to the positive charge in the wire and vis a vis. the negative and positive charges are also stationary relative to each other, causing each charge to view more opposite charges nearbye, hence causing attraction.
So how come magnetism arises in the parallel wires experiment but not in the experiment I just created?
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First of all, are we sure that the longitudinal magnetism does in fact not arise? One would have to devise an experiment to test that hypothesis. With lateral magnetism, it is easy to coil wire many times, concentrating the effect; and also, the displacement of wire laterally is easy to observe due to bendability. When you try to do this longitudinally, there are some obvious geometrical difficulties. Still, it is an interesting question. One envisions setting this up using, perhaps, mercury in a trough. The effect is likely to be weak, requiring some exacting tests. I really don't know what to expect. One has to be careful about additional effects that may also be present, that could complicate matters, such as the deflection of current within the conductor as it passes through the magnetic field created by the other current in the conductor, which is a traverse force. Another complication is that the theory of contracting charge densities that you cite, is of course a relativistic phenomenon. Now, here it is useful to have a more complete understanding of the effect of relativity on electromagnetic fields. Relativity says that not only do the particle distances contract, but also the field itself distorts. According to relativity, the field of a moving charged particle is not spherically symmetric, but is stronger at right angles to the motion and weaker along the motion. Any analysis of the effect of moving charge upon forces parallel to the motion must take this into account.
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Large currents flowing along parallel cables can cause substantial forces between the wires and you have to take this into account when designing powerful electromagnets. They recently had a problem loke this with the new LHC (large hadron collider at CERN) when the internal forces on one of the steering magnets damaged the magnet. I have also watched a large electric furnace starting up by melting cold scrap iron. There were three large carbon electrodes with incredible voltages and current available to them being pushed slowly down into the heap of solid metal bits. The electrodes were connected to the power source by a catenery of dozens of seperate cables and as the contact was being made large arcs werte happening as bits of metal melted in the vast current surges. The magnetic effects of the surge currents were causubf the flexible cables to thrash around like they were demented.