Well no. It is convenient. Easy to calculate. Especially when particles get relativistic. Plus it is very common with scientist and engineers working in the field. I think I learned it in high school physics 47 years ago.icarus wrote:TallDave:Indeed you should. If a 250kV well accelerated an ion it would be an ion volt .... a 250kV electric potential well accelerates an electron to 250keV ... electron volts.Does a 250KV well accelerate an ion to something like 250KeV? I should find the correlation.
http://en.wikipedia.org/wiki/Electron_volt
"By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt."
Myself, I think the electron-Volt is a vulgar unit that stems from the inability to reduce electromagnetic phenomena to more fundamental geometric relationships whereby they would involve only units of length and time ... with appropriate constants .... it does more to advance confusion than understanding .... imho.
Maintaining Quasi neutrallity - a question.
Engineering is the art of making what you want from what you can get at a profit.
Definitely:MSimon wrote:Well no. It is convenient. Easy to calculate. Especially when particles get relativistic. Plus it is very common with scientist and engineers working in the field. I think I learned it in high school physics 47 years ago.icarus wrote:TallDave:Indeed you should. If a 250kV well accelerated an ion it would be an ion volt .... a 250kV electric potential well accelerates an electron to 250keV ... electron volts.Does a 250KV well accelerate an ion to something like 250KeV? I should find the correlation.
http://en.wikipedia.org/wiki/Electron_volt
"By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt."
Myself, I think the electron-Volt is a vulgar unit that stems from the inability to reduce electromagnetic phenomena to more fundamental geometric relationships whereby they would involve only units of length and time ... with appropriate constants .... it does more to advance confusion than understanding .... imho.
v does NOT equal SQRT(2E/m) [E = energy]
v DOES equal SQRT(c^2-[c^3/({E/m}+c^2)]^2)
So at keV energies for electrons, Newtonian mechanics don't follow.
I don't argue that it is common. Probably it is convenient because that is the way you learned how to do it .... 47 years ago.It is convenient. Easy to calculate. Especially when particles get relativistic. Plus it is very common with scientist and engineers working in the field. I think I learned it in high school physics 47 years ago.
Converting everything back to electrons accelerating in electric fields, even temperatures is pretty byzantine. If Newtonian mechanics had an analogue, we'd be converting all kinetic energies to that gained by a 1 gram mass accelerating across a 1 [m^2/s^2] gravitational potential difference or some such madness.
It's a weakness, not a strength, imho.
It serves as a sanity check and for BOE calculations. Very handy. And reaction rates are given not in terms of relative velocity but in terms of eV. Thus you can design a machine (roughly) without resort to a calculator. Very handy.icarus wrote:I don't argue that it is common. Probably it is convenient because that is the way you learned how to do it .... 47 years ago.It is convenient. Easy to calculate. Especially when particles get relativistic. Plus it is very common with scientist and engineers working in the field. I think I learned it in high school physics 47 years ago.
Converting everything back to electrons accelerating in electric fields, even temperatures is pretty byzantine. If Newtonian mechanics had an analogue, we'd be converting all kinetic energies to that gained by a 1 gram mass accelerating across a 1 [m^2/s^2] gravitational potential difference or some such madness.
It's a weakness, not a strength, imho.
Besides. We have lots of different energy units. BTUs, joules, temperature, calories, etc. each useful in a different domain for simplifying calculations.
You can always convert to the desired units at the end of calculating.
Don't forget all this came about because for most of engineering time even slide rules and log tables were not available let alone calculators or computers. Anything that simplified the intermediate steps was very handy. And engineers and scientists STILL love that simplification.
Engineering is the art of making what you want from what you can get at a profit.
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Art Carlson
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I don't use eV as a unit of energy, I use it as a unit of temperature. It fits into the other units better than the completely arbitrary kelvins do. With temperature measured in eV, Boltzmann's constant becomes 1.6e-19 J/eV. This is a number you have to carry in your baggage anyway: It is numerically equal to the fraction of a Coulomb in an elementary charge.icarus wrote:Converting everything back to electrons accelerating in electric fields, even temperatures is pretty byzantine. If Newtonian mechanics had an analogue, we'd be converting all kinetic energies to that gained by a 1 gram mass accelerating across a 1 [m^2/s^2] gravitational potential difference or some such madness.
It's a weakness, not a strength, imho.
In addition, eV's allow you to get your head around a problem quickly. Chemistry, ionization, and visible light occurs in the eV range. Tens of keV means fusion reactions and x-rays. MeV's are for nuclear reactions and gamma rays. Do that trick with either Joules or kelvins!
I guess it should actually be measured in pV (proton-Volts) so that a drive of 250kV gives a 5B11 ion 1250kpV, but the charge of a proton is equal in magnitude to the electron, so energy content-wise, the values would be the same.TallDave wrote:Does a 250KV well accelerate an ion to something like 250KeV? I should find the correlation.
I'm not sure how the e got in there, but I remember it being relevant in some context. I should probably go look it up, but exam on Monday...
It's a bizarre debate. If you work with 'eV' then it'll be second nature to you to use it. If you don't, then don't let it worry you!!..
If you want to convert everything into velocities so you get a 'feel' for the dynamics of a fusion device, then it's simple enough to say that most of these devices have ions running around at around a million metres per second to get fusion (that'd be a 10keV deuteron) and fusion products are in the 10 million m/s range (that'd be a 2MeV alpha). Energy scales as velocity squared in this range, so up or down a little will cover the range of energies involved in fusion.
(Electrons won't be going anywhere very quickly, if there's a magnetic field around. Instead they'll be mostly spinning around on the spot, emitting at a fequency of 2.8E10Hz x B [in teslas].)
Taking these approximations as read, you'll then be comfortably less than an order of magnitude out, in pretty much any 'intuitive' thinking you care to do, regarding ion velocities in a fusion device.
Hope that helps!....
If you want to convert everything into velocities so you get a 'feel' for the dynamics of a fusion device, then it's simple enough to say that most of these devices have ions running around at around a million metres per second to get fusion (that'd be a 10keV deuteron) and fusion products are in the 10 million m/s range (that'd be a 2MeV alpha). Energy scales as velocity squared in this range, so up or down a little will cover the range of energies involved in fusion.
(Electrons won't be going anywhere very quickly, if there's a magnetic field around. Instead they'll be mostly spinning around on the spot, emitting at a fequency of 2.8E10Hz x B [in teslas].)
Taking these approximations as read, you'll then be comfortably less than an order of magnitude out, in pretty much any 'intuitive' thinking you care to do, regarding ion velocities in a fusion device.
Hope that helps!....
Electrons won't be going anywhere? Possibly true (in a radial vector), but only in the case where they are traped on a magnetic field line. The same could be said of an ion. In the Polywell the electron is injected at a cusp where due to the opposing magnetic fields there is a thin corridor where there is no magnetic field (or a field that is so week that the inertia of the electron completly dominates the electrons motion). The electron would not be traped on a field line (Wiffleball border) untill it's speed had droped enough that it's gyroradius was smaller than the magnetic flux gradient (which is presumably very tiny at the compressed Wiffleball border), or it was scattered so that it's vector was almost parellel to the field line (in which case the entrapment may only be temperary?). When the machine starts up and the Wiffleball is still forming a substantial percentage of the electrons may be traped but this would be a temperary condition.chrismb wrote:It's a bizarre debate. If you work with 'eV' then it'll be second nature to you to use it. If you don't, then don't let it worry you!!..
If you want to convert everything into velocities so you get a 'feel' for the dynamics of a fusion device, then it's simple enough to say that most of these devices have ions running around at around a million metres per second to get fusion (that'd be a 10keV deuteron) and fusion products are in the 10 million m/s range (that'd be a 2MeV alpha). Energy scales as velocity squared in this range, so up or down a little will cover the range of energies involved in fusion.
(Electrons won't be going anywhere very quickly, if there's a magnetic field around. Instead they'll be mostly spinning around on the spot, emitting at a fequency of 2.8E10Hz x B [in teslas].)
Taking these approximations as read, you'll then be comfortably less than an order of magnitude out, in pretty much any 'intuitive' thinking you care to do, regarding ion velocities in a fusion device.
Hope that helps!....
In that situation, what is the initial b-field gradient in the central region of the Polywell (due to the natural inverse square law, and repulsion from opposing magnets) before charged particles are introduced to push these borders back and compress them further?
Dan Tibbets
To error is human... and I'm very human.
Art:
The division into degrees however is not quite arbitrary but it is anthropic, since it is based on water freezing and boiling points, something humans can get an intuitive feel for.
I guess the question is, how many electron-Volts does the weather feel like to you outside today?
If you can't answer that off the top of your head, you probably are not using them as intuitively as you imagine but only ever in calculations, that are necessarily abstract. Once your are into abstractions any unit is as good as any other, imho .... it's just what you first learnt that works best I find.
At the risk of going completely off topic (apologies to Aero here), I have to take issue with this statement. Absolute zero is zero Kelvin. This is anything but arbitrary.completely arbitrary kelvins
The division into degrees however is not quite arbitrary but it is anthropic, since it is based on water freezing and boiling points, something humans can get an intuitive feel for.
I guess the question is, how many electron-Volts does the weather feel like to you outside today?
If you can't answer that off the top of your head, you probably are not using them as intuitively as you imagine but only ever in calculations, that are necessarily abstract. Once your are into abstractions any unit is as good as any other, imho .... it's just what you first learnt that works best I find.
About .025 eV. And I didn't look it up.icarus wrote:Art:At the risk of going completely off topic (apologies to Aero here), I have to take issue with this statement. Absolute zero is zero Kelvin. This is anything but arbitrary.completely arbitrary kelvins
The division into degrees however is not quite arbitrary but it is anthropic, since it is based on water freezing and boiling points, something humans can get an intuitive feel for.
I guess the question is, how many electron-Volts does the weather feel like to you outside today?
If you can't answer that off the top of your head, you probably are not using them as intuitively as you imagine but only ever in calculations, that are necessarily abstract. Once your are into abstractions any unit is as good as any other, imho .... it's just what you first learnt that works best I find.
But that is a great idea. I'm going to give the daily temp in eV occasionally just to confuse the uninitiated.
==
I just used the (aprox) conversion factor of 11,605 and came up with .0258 eV for 300K.
The current local temp is .02512 eV
Last edited by MSimon on Sat Jul 18, 2009 10:30 pm, edited 1 time in total.
Engineering is the art of making what you want from what you can get at a profit.
MSimon ... but you just did two conversions to get there .. albeit in your brain.
If I said what does the temperature feel like you'd have said ... 65 or 550 (F) if you.re an American and 12 or 25 or something (Celcius) if you're from most other places. ... and you'd be accurate to within a few degrees.
If you were using 25.8 milli-eV in daily life then that would be suprising.... but kinda cool I suppose.
EDIT: Room temperature, 20 C, 293.15 K is 25.26 milli-eV for anyone who's considering switching.... freezing (0 C) is 23.5 milli-eV, boiling (100 C) is 32.2 milli-eV
If I said what does the temperature feel like you'd have said ... 65 or 550 (F) if you.re an American and 12 or 25 or something (Celcius) if you're from most other places. ... and you'd be accurate to within a few degrees.
If you were using 25.8 milli-eV in daily life then that would be suprising.... but kinda cool I suppose.
EDIT: Room temperature, 20 C, 293.15 K is 25.26 milli-eV for anyone who's considering switching.... freezing (0 C) is 23.5 milli-eV, boiling (100 C) is 32.2 milli-eV
Last edited by icarus on Sat Jul 18, 2009 10:36 pm, edited 1 time in total.
But Simon, that will get complicated. What if it hails, hailstones are reported in penny sized, quarter sized, golf ball, baseball and softball sized. I'm sure their speed of impact can be approximated by terminal velocity, but what is the keV of a quarter sized hailstone?
You'll need this so you can report the temperature, .025 eV, and the hailstones in keV, that is, consistent units.
You'll need this so you can report the temperature, .025 eV, and the hailstones in keV, that is, consistent units.
Aero
I don't use liters for board-feet usually either.icarus wrote:MSimon ... but you just did two conversions to get there .. albeit in your brain.
If I said what does the temperature feel like you'd have said ... 65 or 550 (F) if you.re an American and 12 or 25 or something (Celcius) if you're from most other places. ... and you'd be accurate to within a few degrees.
If you were using 25.8 milli-eV's in daily life then that would be suprising.... but kinda cool I suppose.
And I don't report air temps in deg. Rankine either. Deg. F is usual where I live.
Engineering is the art of making what you want from what you can get at a profit.