
#1
Jul2707, 11:06 AM

P: 1

Hello
I am Fusina, I wish to ask you Three questions which bother my mind. Its all about Newton's third Law. Is newtons third law always obeyed? Is there an example of two moving particles, who's velocities are such that thier mutual magnetic force doesn't obey Newton's 3rd law? I tried answering this by thinking of two particles moving along the x and y axes respectively, but i cant explain. Also, i dont know the reason why in practice, the mutual magnetic force of two moving electons is not the same (i heard it has something to do with the momentum lost to the medium of interaction) 



#2
Jul2707, 04:44 PM

P: 1,983

It is not true always. Magnetic forces are the classical example of a situation, where Newton's third law gets violated.
A conservation of momentum is a kind of law that doesn't get violated ever. The Newton's third law is true if you can assume that the objects are interacting so directly, that their combined momentum is conserved. When you have charged particles interacting, they are in fact interacting with the electromagnetic field. Both the particles and electromagnetic field contain energy and momentum. So in this case the particles' total momentum is not conserved, but instead it is the total momentum of both particles and the field, that is conserved. This is why Newton's third law doesn't apply with magnetic forces anymore. That's the idea. If you want it more precise, you need to get into mathematics of it. 



#3
Jul2807, 05:35 PM

P: 167

jostpuur wrote:
Thanks. 



#4
Jul2907, 03:26 AM

P: 1,743

Is Newton's third law disprovedE. Breitenberger, "Magnetic interactions between charged particles", Am. J. Phys. 36 (1968), 505 There are experiments which can be interpreted as an evidence that magnetic interactions between charges are not described by the BiotSavart law: P. Graneau, N. Graneau, "Electrodynamic force law controversy" Phys. Rev. E 63, 058601 (2001) Eugene. 



#5
Jul2907, 03:47 PM

P: 167

jostpuur:
meopemuk: 



#6
Jul2907, 03:56 PM

P: 1,743

Eugene. 



#7
Jul2907, 07:02 PM

P: 1,983

alvaros, I don't know any site at the moment. I could try to search web, but I probably won't find anything that you couldn't find yourself. The Lorentz force and BiotSavart law got confirmed by experiment more than 100 years ago (I don't know in fact very accurately when, but something like that anyway). If you know these laws, you can verify yourself by calculating some examples, that Newton's third law can get violated.
One way to go around the problem of infinite energies in classical fashion is to not use point charges, but instead replace them with small balls of finite but large charge density. When you solve the electric field for them, and its energy, you can see that it approaches infinity when the radius of the ball is taken to zero. But the problem is solved to some extent simply by not letting the radius go zero, and instead giving it some small but nonzero value. 



#8
Jul2907, 08:25 PM

P: 1,743

There are experimental groups, which continue to investigate these questions. These are not "mainstream" activities, but they are not "crackpot" either. One recent example is: N. Graneau, T. Phipps Jr., D. Roscoe, "An experimental confirmation of longitudinal electrodynamic forces" Eur. Phys. J. D 15 (2001), 87. Eugene. 



#9
Jul3007, 04:33 PM

P: 167

Right now Im confused, Ive never thougth about Newton 3rdlaw and magnetic forces. I must study the issue.
Anyway Thanks to all. 



#10
Jul3007, 04:37 PM

P: 1,743

Eugene. 


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