- #1
tim9000
- 867
- 17
Hi,
This is overwhelmingly more of a maths problem than a physics problem, because it's all theoretical. I'll give some background to modle it incase the math's isn't enough.
Say you've got a planar structure of thickness 'd', lying on the z plane. Also say the upper and lower surfaces are y = 0 and y = -d, respectively.
The structure has scalar potentials inside it as so:
As you can see the vector fields cancel out on one side, As it says below, there is a Poisson equation of:
BUT I HAVE NO IDEA WHY that is the poission equation, I get that Fiinside is a scalar potential, but why is mok.cos(kx) the vector field?, not like mx+my or something instead? It looks like they've just differentiated mx and that's the vector function, maybe just a coincidence?
I also have no idea how that is the general solution? Specifically the homogenious part.
I get that for the part of the particular you can solve the Poisson equation of using method of undetermined coefficients with a guess of (Asin(kx) + Bcos(kx)) and just differentiate that twice for del2:
(Asin(kx) + Bcos(kx))'' = mok.Cos(kx)
therefore: -A.k2sin(kx)-Bk2cos(kx) = mok.Cos(kx)
therefore: -A.ksin(kx)-Bkcos(kx) = mo.Cos(kx) and equating coefficients yields:
B = - mo/k
A = 0
so Yp = - mo/k * Cos(kx)
But why does the homogenous part have exponentials and y in them? I thought they'd just be zero.
If someone could explain that or even just why the Poisson equation is what they say I'd be greatful.
THANKS!
P.S I also wonder, which point is chosen as x = 0 on the diagram...?
This is overwhelmingly more of a maths problem than a physics problem, because it's all theoretical. I'll give some background to modle it incase the math's isn't enough.
Say you've got a planar structure of thickness 'd', lying on the z plane. Also say the upper and lower surfaces are y = 0 and y = -d, respectively.
The structure has scalar potentials inside it as so:
As you can see the vector fields cancel out on one side, As it says below, there is a Poisson equation of:
BUT I HAVE NO IDEA WHY that is the poission equation, I get that Fiinside is a scalar potential, but why is mok.cos(kx) the vector field?, not like mx+my or something instead? It looks like they've just differentiated mx and that's the vector function, maybe just a coincidence?
I also have no idea how that is the general solution? Specifically the homogenious part.
I get that for the part of the particular you can solve the Poisson equation of using method of undetermined coefficients with a guess of (Asin(kx) + Bcos(kx)) and just differentiate that twice for del2:
(Asin(kx) + Bcos(kx))'' = mok.Cos(kx)
therefore: -A.k2sin(kx)-Bk2cos(kx) = mok.Cos(kx)
therefore: -A.ksin(kx)-Bkcos(kx) = mo.Cos(kx) and equating coefficients yields:
B = - mo/k
A = 0
so Yp = - mo/k * Cos(kx)
But why does the homogenous part have exponentials and y in them? I thought they'd just be zero.
If someone could explain that or even just why the Poisson equation is what they say I'd be greatful.
THANKS!
P.S I also wonder, which point is chosen as x = 0 on the diagram...?
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