EM Phenomenon: Magnetism from a Cathode Ray?

[Charles Link]

I always get twitchy when I read a phrase like that. That way of putting things is very dodgy. Just perhaps, I / we know what you mean but why not use the Mathematical "one twenty ninth'? This is a relatively new phenomenon all goes back to the use of percentage reductions of prices in the Bank Holiday Sales etc., but that always refers to a Subtraction. You would never get 'a reduction of 101%'
I am not just being picky for the sake of it. It just adds confusion and I just wish that on PF, at least, we could avoid using the phraseology of the uninformed press and commercial market when discussing matters in which accuracy is vital.

I'm simply following "nasu's" terminology in the previous post. I would normally say something like " lower by a factor of 29"

 

[sophiecentaur]

I realize that but I am not 'attacking' the messenger - just the message.
I'm sure you both know better. But are you saying you disagree with me?

 

[Charles Link]

I realize that but I am not 'attacking' the messenger - just the message.
I'm sure you both know better. But are you saying you disagree with me?

No, it is important to use good grammar. The post I was responding to about the "free electrons" was perhaps a little picky, because I computed exactly what I said I was computing. It is important to keep the factor of 1/29 in mind if one attempts to estimate the electron velocities, so it was still a very good comment. In the way the "29 times less" was used, I do think we all knew what it meant.

 

[sophiecentaur]

Yes. What you wrote was clear - in that context- but it could be the thin end of the wedge. We have enough problems with text speak from people who can be bothered to post correctly. I wonder what the opinion is amongst the Mods - or amongst people for whom English is a second or third language.
I feel a bit like the last bastion in this respect. (And my spelling is correct, in that instance. lol) OWCH, I used "lol"
I'm now waiting for NASA to give me a kicking.

 

[nasu]

I always get twitchy when I read a phrase like that. That way of putting things is very dodgy. Just perhaps, I / we know what you mean but why not use the Mathematical "one twenty ninth'? This is a relatively new phenomenon all goes back to the use of percentage reductions of prices in the Bank Holiday Sales etc., but that always refers to a Subtraction. You would never get 'a reduction of 101%'
I am not just being picky for the sake of it. It just adds confusion and I just wish that on PF, at least, we could avoid using the phraseology of the uninformed press and commercial market when discussing matters in which accuracy is vital.

Yes, I agree that it is an usage frown upon (yet).
Maybe confusing too, you are right, I did not realize it.

 

[sophiecentaur]

Yes, I agree that it is an usage frown upon (yet).
Maybe confusing too, you are right, I did not realize it.

Well - at least no fisticuffs.

 

[nasu]

I computed the electrical charge "sitting in the wire". Yes, the charge involved in any current flow is 29 times less.

Well, you called it "free" and this term is usually (in solid state physics) used for the de-localized electrons.
This is why I wrote this comment. Not that it makes a great difference. I don't think that this number (with or without 29 in it) is very relevant in understanding electrical current.
There is some surface charge density which I suppose is many order of magnitude less than these numbers, which has a role in producing the electric field which drives the current through the wire. I don't actually know any number about typical magnitudes of this surface charge.

 

[nasu]

Well - at least no fisticuffs.

I knew that you are an Englishman. But it seems it's not enough for you to use this expression.

http://www.urbandictionary.com/define.php?term=fistycuffs

 

[sophiecentaur]

I knew that you are an Englishman. But it seems it's not enough for you to use this expression.

http://www.urbandictionary.com/define.php?term=fistycuffs

I was regressing about a century.

 

[Chris Frisella]

Did you try the link?
I can give it a go - but if you are hoping for explaining relativity phenomena in terms you find familiar already I don't think I can. The reason we need relativity is because there are things the more familiar does not explain.

The principle assumption of relativity is that nothing travels faster than light in a vacuum (formally: the speed of light is the same for all observers).
Consequences of that are consequences of special relativity.

naively you'd think that the electric field of a charge moving with velocity v in the z direction would be: $\vec E = (kq/|\vec r - vt\hat k|^3)(\vec r - vt\hat k)$ ... basically just the regular point charge field lines whose center moves along the z axis. But this violates the principle assumption above: do you see why?

The forward field would be propagating faster than c?

Thanks. That and the simulator helped.

 

[Simon Bridge]

Not just that, but the field through all space changes at the same time... this would allow ftl communication, so does not happen.

 

[Chris Frisella]

Not just that, but the field through all space changes at the same time... this would allow ftl communication, so does not happen.

Gotcha :)

 

[jartsa]

I think I know what would happen if the beams were parallel. Now I am just wondering why an incoming beam perpendicular to the wire would receive a force which is parallel to the wire. I made a little graphic to illustrate what I think would happen. My question is why does that force exist?

View attachment 101382

See this page, particularly the chapter "A Charge Moving Perpendicular to a Wire"

http://physics.weber.edu/schroeder/mrr/MRRtalk.html

 

[Chris Frisella]

See this page, particularly the chapter "A Charge Moving Perpendicular to a Wire"

http://physics.weber.edu/schroeder/mrr/MRRtalk.html

Great reference! Thank you!