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Coulomb's Law

Hi this is regarding fusion power, for when binding the two nuclei so that it overcomes the electromagnetic repulsion so that the nuclear bindin energy takes over. I need to do some calculations involving coulombs law in my coursework, but i dont really know. can someone give me a header? please?
If so that would be very helpful!
Also im trying to work out temperature needed for fusion, my teacher said using coulombs law and then using E=3/2Kt and so on, but how can i do this? please help its quite urgent.
Thanks
Reply 1
Avatar for Drummy
Drummy
10
electrical potential energy = K q^2 / r^2 where K = 9 x 10^9 and q is the charge on a proton

assume that to join they must effectively "touch"
at which point r = 2 x 10^-15m (the sum of 2 proton radii)

if KE --> EPE then put this energy equal to kT to get the temperature required ( where k = Boltzmann's const)

crudely simplified but probably what your teacher wants
Reply 2
Avatar for Itsmoe07
Itsmoe07
OP
Thats great thanks, theres some confusion though with coulumbs law, the one im looking at is a bit different its written:
F= q^2/ 4(pie)Er^2 , whats E in this and the 4 (pie) i dont see energy from this one? whats the difference?
Reply 3
Avatar for CubeDude
CubeDude
Drummy
electrical potential energy = K q^2 / r^2 where K = 9 x 10^9 and q is the charge on a proton

assume that to join they must effectively "touch"
at which point r = 2 x 10^-15m (the sum of 2 proton radii)

if KE --> EPE then put this energy equal to kT to get the temperature required ( where k = Boltzmann's const)

crudely simplified but probably what your teacher wants


Isn't that meant to be r :p: ?

Drummy is right to say that this is an awfully simplified model. When you calculate your value for T it might be fun to check it with the temperature of the sun's core...
Reply 4
Avatar for CubeDude
CubeDude
Itsmoe07
Thats great thanks, theres some confusion though with coulumbs law, the one im looking at is a bit different its written:
F= q^2/ 4(pie)Er^2 , whats E in this and the 4 (pie) i dont see energy from this one? whats the difference?


Yes this is Coulomb's law. Drummy has replaced the 14ΠE0 \frac{1}{4\Pi E_0} term with K to simplify things a bit.

Coulomb's law is a formula expressing Force. What you need is energy.

Do you know a formula that links force and energy?
Reply 5
Avatar for Itsmoe07
Itsmoe07
OP
Um im not to sure could you help me out? Also could i show how to calculate coulumbs constant from that equation, but what is the E thing? and is there a value for it?
Reply 6
Avatar for teachercol
teachercol
9
http://www.thestudentroom.co.uk/showthread.php?t=756292

Are we doing all your coursework?

the E thingy is epsilon zero = 8.85E-12
Reply 7
Avatar for CubeDude
CubeDude
Itsmoe07
Um im not to sure could you help me out? Also could i show how to calculate coulumbs constant from that equation, but what is the E thing? and is there a value for it?


A formula involving work (energy) and force?
Reply 8
Avatar for Itsmoe07
Itsmoe07
OP
wow, workdone is force x distance :redface:
Reply 9
Avatar for teachercol
teachercol
9
Electrical and grav PE is no longer on most A level syllabuses.

Yeah - I know

(:biggrin: Drummy got it wrong ....:smile: )
Reply 10
Avatar for CubeDude
CubeDude
Itsmoe07
wow, workdone is force x distance :redface:


Right, and if potential energy is the negative of work done (if only conservative forces are acting) then you should be able to work out a formula for the electrostatic potential energy.
Neither is integration ....
Electrical PE = q1q2 / 4 pi eo r

(Yeah - I'll learn LateX one day)
Reply 13
Avatar for CubeDude
CubeDude
teachercol
Electrical and grav PE is no longer on most A level syllabuses.

Yeah - I know

(:biggrin: Drummy got it wrong ....:smile: )


Really? I did mine a few years ago but even at the time it was one of the most interesting topics. Oh well...
Reply 14
Avatar for CubeDude
CubeDude
teachercol
Neither is integration ....


I thought that little problem was avoided by the assumption that this was all one dimensional.
Has anyone come up with Cold Fusion yet?
Reply 16
Avatar for Itsmoe07
Itsmoe07
OP
Yeah im doing the calculations and getting stupid answers for the force: i got 1.15x10^-13 N this cant be correct any ideas? Also i think it is r^2 because all formulas have it like that.
Reply 17
Avatar for Drummy
Drummy
10
errr yeah, I meant over r, not r^2

apart from that my logic is impeccable
Reply 18
Avatar for Drummy
Drummy
10
and just for Teachercol

EPE=eV=14πϵe2rEPE = eV = \frac{1}{4\pi\epsilon} \frac {e^2}{r}
Reply 19
Avatar for CubeDude
CubeDude
Itsmoe07
Yeah im doing the calculations and getting stupid answers for the force: i got 1.15x10^-13 N this cant be correct any ideas? Also i think it is r^2 because all formulas have it like that.


The 1r2 \frac{1}{r^2} appears in the force equation, yes. Another formula of use to you is provided by drummy and contains 1r \frac{1}{r} , this gives you the potential energy which is what you want to that you can equate it to 3kT2 \frac{3kT}{2} .

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