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4:49 AM
@SamyakMarathe An isolated electron is stable and will never decay so the half life is infinite.
@cOnnectOrTR12 It depends on whether you want to use a potential energy associated with the external force. If you define a PE for the external force then you'll find the total energy PE+KE is constant.
@user157860 I'm free now if you're around.
 
 
3 hours later…
7:39 AM
Good noon to @JohnRennie
 
@cOnnectOrTR12 Hi :-)
 
7:56 AM
@JohnRennie when the spring mass system is falling then gravity is external. But when we throw a mass up then applied force is external ?
 
I'm busy for a few minutes. I'll ping you when I'm free.
 
8:13 AM
@cOnnectOrTR12 Hi
I think you're worrying unnecessarily about the exact definition of "external".
It's usually obvious if we should regard a force as external to the system.
 
@JohnRennie ok. But how is external force gravity is conservative? I didn’t get that part.
 
Suppose you have mass on a frictionless table, and you exert an external force by pushing on the mass.
If you just consider the mass than energy isn't conserved.
So you could argue that the external force doesn't conserve energy. But obviously it does really as that energy came from me i.e. my energy decreased by the same amount that the energy of the mass increased. Yes?
 
So it isn't useful to discuss whether or not the external force is conservative.
 
What do you mean it isn’t useful? Shouldn’t the concept be applicable from all angles?
 
8:28 AM
Whether or not energy is conserved just depends on how you look at the system. If you just look at the mass and ignore the person pushing the mass you're going to say it isn't conserved. But if you look at the person as well you're going to say it is conserved.
 
@JohnRennie, Hi, :) ready when you are!
 
@user157860 As I recall, where we'd got to is that you said you'd been reading the Encyclopaedia Brittanica article. Yes?
 
yes, if you have access I'll give you page and volume
it says that two co-moving charges exert Fm0 Fe x v^2/C^2
 
I don't have access, but the point is that Einstein couldn't have read that article because the article relies on Einstein's discoveries for its equations.
 
makes no difference
 
8:36 AM
We understand what is going on now because that equation is derived from special relativity, but Einstein didn't know about special relativity until he discovered it. That's why he was puzzled.
 
If albert is co-moving he'll use a charce and measure Fm even if he sees static charges
 
@user157860 He didn't know that until he discovered SR!
 
sure , but there's no need for it, there is no problem
Albert measure a Fm even he sees no moving charges
he was wrong to worry about it
it is not weird
 
Right, but how do you explain that when you don't know that the equation relating the magnetic and electric forces exists?
 
I do not know if he knew, I an saying he was wrong to see a problem
there is no need fot SR since there is no problem
he surely knew that his compass measured a Fm even if there are no moving charges in sight!!!!
 
8:40 AM
You've lost me now. Before we discovered SR there was a problem because have a situation where the field looks like an electric field to one observer and a magnetic field to another observer.
@user157860 Hang on, there is NO magnetic field if no charges are moving.
 
but there is one if you do not SEE them but there are!!
you said it was weird that two observers measured different things
 
A magnetic field is what is measured by a magnetometer. If I give you a magnetometer and I place a stationary charge Q at a distance r from you your magnetometer will read zero. Yes?
 
but if you are comoving yyou see stationary charges yet your magnetometer will record a Fm
 
Wrong.
If you are comoving with the charge your magnetometer will measure zero.
 
from the beginning again:
two charges moving parallel to the ground at 1 cm/sec feel attraction Fm, right?
a stationary observer sees then moving
OK?
 
8:46 AM
@JohnRennie so let’s take two magnets 🧲. Now if somehow there are made apart then a force will cause them to come again to each other. That is internal force. If we somehow fix the magnet and apply a force on another to make them apart then here the force is conservative and external like gravity. And this applied force is external. Did I got it?
 
@user157860 Wrong.
Two charges moving at the same speed feel only an electrostatic force.
 
No! if you do not trust Encyclopedia Britannica, think that in a synchrothron electrons are sent at an angle so they spiral damping the attraction by Fm
 
We're at an impass aren't we. You don't believe what I'm telling you.
 
OK let's change subject, re my question on PSE light needs no medium, how come?
 
Light is an oscillating electric field. Yes?
(It also has a magnetic field but we'll ignore that and concentrate on the electric field)
 
8:53 AM
Before you go on pleas clarify this:
 
@JohnRennie am I right?
 
I read that a m oscillating E-field creates a M-field, fair enough
 
@cOnnectOrTR12 Can we leave this for now. I feel like we have talked about it for hours without getting very far.
 
but then an F-field creates an E- field , how that?
 
@JohnRennie can you answer another question?
 
8:55 AM
"F-field"? What is an "F-field"?
@cOnnectOrTR12 give user157860 a chance now.
 
sorry M-field magnetic
 
Ok I’ll wait:)
 
We usually use B for magnetic fields so I'd call it a B-field.
 
an oscillating B-field cannot create an E-fiel
it can move a charge, but light has no charge, right?
 
Anyhow, it isn't really true that the E-field creates the B-field or that the B-field creates the E-field. If that was true you'd be going round in a loop trying to decide which field created the other. It's more correct to say the two fields cannot exist without each other.
 
8:59 AM
ok, go on with propagation now
 
OK now let's do a thought experiment. Imagine we have an electric field in space, and at any position (x,y,z) in space the field has some value E(x,y,z).
We'll start with the field zero everywhere except at the origin (0,0,0) where we make the field have the value E within some tiny volume dV surrounding the origin.
OK so far?
 
if we have an e-field in space, that implies that space has properties?
 
No space doesn't need to have properties. We can have an electric field without worrying about space.
An electric field is just an electric field.
 
This is the most voted reply PSE: " I would say the electromagnetic field is the medium.

For like the medium water oscillates when a water wave is observable after throwing a stone, so the electromagnetic field oscillates when excited by an antenna, say. If nothing oscillates there are no waves, neither in water nor in the electromagnetic field.

The medium disappears only when one thinks of an electromagnetic field as being nothing, only a vacuum
To me it is nonsense, am I just dumb?
 
I read that answer, but I'm not sure that regarding the electric field as the medium is helpful. It isn't the way I would try and explain it.
 
9:07 AM
the big question: isn't it simpler just to assume that space is not a vacuum and is sort of medium in which light propagates?
that solves all problems
and we need not specify what it really is
only is has to be elastic 100%
 
That might seem simpler to you, but it causes a problem because then there is a reference frame in which the medium is stationary, and this picks out that reference frame as special i.e. it's the only frame in which the medium is stationary.
 
the medium is not IN the space it IS space
no need for a reference frame
 
If we have a medium that medium must have properties, and one of those properties is its speed. Like in a sound wave in air the air has a speed, or a water wave in water the water has a speed.
That means the "space medium" must have a speed.
 
sorry, air has speed?
 
It's called "wind" :-)
 
9:13 AM
"" Like in a sound wave in air the air has a speed,""...? propagation has speed, air is still omly oscillates, right?
 
Suppose you're sitting in still air and you measure the speed of sound then you get the result 340 m/s. Yes?
 
ok?
 
Now suppose there is a wind coming towards you at 100 m/s. Now when you measure the speed of sound you'll get 440 m/s i.e. the sound moves at 340 m/s relative to the air, and air moves at 100 m/s relative to you, so the speed of the sound relative to you is 340 + 100 = 440 m/s.
Yes?
 
yes, but why do we need to consider the case where there is wind? why can 'tspace be stationary?
 
What I'm saying is that any time we have a wave moving in a medium we get this effect. If you were standing in a river flowing at some speed v and you measured the speed of a water wave your result would be the wave speed plus v. Yes?
 
9:20 AM
yes
 
But now suppose the air or water is stationary, and this time you are moving at some speed v. Again the measured speed of the wave is the wave speed plus v.
 
This time it's because both you and the wave are moving relative to the medium.
Now, you ask "why can't space be stationary?". But if space is a medium then you can be moving relative to that medium just like you can move relative to air or water. Yes?
 
btw, if water or air were 199% elastic, would propagation go on indefinitely?
 
It's not the elastic properties that matter, it's the damping properties.
The damping measures how much energy is lost as the wave travels.
 
9:23 AM
which depend on what?
 
If the damping is zero then no energy is lost, and in that case the wave would indeed travel forever.
@user157860 it depends on the medium. In water the damping is due to water molecules colliding with each other and converting elastic energy to heat.
 
@JohnRennie is the same valid if space is not a medium but itself THE medium?
 
That doesn't seem like a real distinction. What do you mean by "the medium"?
 
ok, thanks for the detours, please go on with your view od propagation of light,
 
Why is that different from "a medium"?
 
9:26 AM
water and air are the medium of sound in space, spase as The medium is in itself
 
> spase as The medium is in itself
 
has no other frame of reference. I suppose that makes a lot of didfference
 
That doesn't make sense. A medium is just somethign with elastic properties in which a wave can travel.
 
we supposed that space is elastic 100%
 
OK, so ... ?
 
9:28 AM
ok, thanks for the detours, please go on with your view of propagation of light!
 
We start with our electric field zero everywhere except in a small volume dV centred at the origin where the field has the value E. OK so far?
 
This means the energy stored in the field is ¹⁄₂ε₀E² dV. Yes?
 
sorry, we are starting from scratch ,how do you justify permeabilty and permittivity?
 
This discussion keeps spreading out in all directions :-)
I need to go I'm afraid. We'll need to pick this up later.
 
9:32 AM
I regret that but you know it is THE most complex issue on earth
 
I'm guessing you're still at school, so you haven't learned about Maxwell's equations yet. Yes?
 
I am 80+, there's a lot to say about them equations , too!
 
Ah OK, my apologies, I assumed you were a budding physics student.
The problem is that when we first start teaching students about stuff we have to dumb it down because the students don't have the background in physics they need for a detailed description.
 
no apologies, i studied physics in the '70s , but from an epistemiologic point of view, I try to get to the root of issues dsregarding all prejudices
 
The result is we end up saying "you just have to accept this" and this does tend to annoy the students.
As you learn more about physics you'll find things become simpler, and the things you were told you just have to accept suddenly make sense because you can see how they fit in with everything else.
 
9:38 AM
the point is that I accpt only what is really justified an not eecause "it works"
 
That's an excellent attitude to have, but if you want things to be "really justified" your only option is to start learning physics in earnest so you get the relevant background.
Then and only then you'll see how it all fits together.
 
I am really grateful to you for putting up with me, i suppose I sound like am 'old codger'.
 
I can't get remotely close to covering everything in the detail you'd need ina few minutes of Internet chat. There is no alternative to knuckling down to learn the material.
I'm 61, so I'm dangerously close to being an old codger too :-)
As far as the students are concerned we're both "infinitely old"! :-)
 
if you are intrigued by the previous issue I can copy the relevant part from the Encyclopedia
 
I would be interested to read it. Can you post the link and I'll see if I can get at it. Sometimes these sites have free trials.
But I need to go. If you post the link I'll have a look later today.
 
9:44 AM
I'd copy it from the book, if you want to try the article on line it is Magnetism, I suppos you'll have to accept EB's word
 
What about my question @JohnRennie?😑
 
10:14 AM
@JohnRennie 61 and 81. Two Codgers. That is a very disrespectful way as google says😂
I am in my twenties 😂
 
@cOnnectOrTR12, then you are a young codger!!
 
10:30 AM
@user157860 haha
Sounds like cougar!
 
10:52 AM
k
@JohnRennie, EB 1974, Magnetism Vol 11 page 318:" two electric charges moving parallel to one another experience a magnetic force of attraction, like two parallel currents, but the magnetic force is usually s,all compared to the electrostatic force of repulsion....
...for two separated electric charges moving at velocity v...Fm= Fe x v^2/c^2...For currents flowing in wires,,, currents may be large whereas electrostatic forces are absent...""
therefore, as stated, an observer moving parallel to the charges, will use a test charge and measure a magnetic attraction even though he sees no moving charges
As I said, this is not strange since your compass measures a B-field yet you see no moving charges. Einstein was wrongly worried
 

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