Question

E Field, B field and relativity
http://www.youtube.com/watch?v=1TKSfAkWWN0&list=PL0WOi2rip9KGv_2qXouxQF-23LTp5PsnV


as this video points out, magnetic field is basically electric field viewed from some moving reference..

but.. if that was true, then that positive charge should have experienced a attractive force when looked from a stationary frame of view, cause the negative charges are moving and hence undergoing length contraction and thereby increasing the density!

Answer 1

I think you're right - the video doesn't explore that though, right? The cation is only treated as a moving charge in his thought experiment.

Answer 2

i checked other sites.. they do the same thing as well.. since i dunno much of relativity, i can't comment much.. but that was my common sense talking :P

but then again, relativity completely destroys common sense.. so there must be some explanation.. we need an expert!

Answer 3

other sites just use the analogy of the moving charge in relation to the current through the wire and not vice versa?

Answer 4

http://galileo.phys.virginia.edu/classes/252/rel_el_mag.html

this site.. proves it mathematically

perhaps if u know the math of relativity, u can do the case that i am talking about.. and maybe you ll be able to come up with some answer?

Answer 5

ya.. check that site.. it only considers taht case.. and proves it mathematically!

Answer 6

http://www.google.com/url?q=http://chemiitm.files.wordpress.com/2012/12/david-j-griffiths-introduction-to-electrodynamics-3rd-edition.pdf&sa=U&ei=u2CCUtO0E8SyyAG5kYHIAg&ved=0CC8QFjAG&usg=AFQjCNG0JBC16CLmdM1LyrA_BlV9CCXPyQ

This was my electrodynamics book, and talks about this scenario starting on page 538... looking through it...

Answer 7

he only talk about the moving charge as well, though....

Answer 8

dang.. so why don't u try doing the math of our case? :P the stationary case?!

Answer 9

trying to figure out how to set it up :P electrodynamics wasn't my best class in the world..... :/

Answer 10

wait.. why m i downloading that book? i have that book :P

Answer 11

hahaha ^_^

Answer 12

now you have an infinite number of copies!

Answer 13

no.. i would just have 2 :O

Answer 14

copy paste

Answer 15

digital magic

Answer 16

i think you need to reboot :P

Answer 17

i have a philosophical question.. that questions the free will scientifically..

Answer 18

?? (also, brb, gotta run out real quick. I'll look at it when I get back ^_^)

Answer 19

lemme post it!

Answer 20

Also, I dunno why I thought there should be a force in the first place - the wire is neutral, so there's no force on the stationary charge in the laboratory frame, so there **can't be any force on the charge in any other frame (there must be agreement between inertial frames).

So, to argue that to fruition: in the laboratory frame, both the outside charge and the positive charges in the wire are stationary, so the negative charges are the only things moving - **they're already contracted at the measured linear charge density.

In the frame of the moving negative charges, there's a *decrease in negative charge density and an increase in positive charge density; however, the stationary charge is now moving in relation to the negative charges, so experiences a magnetic force towards them
\[\textbf F=q \textbf v \times \textbf B\]
that balances out the increase in positive charge density.