- #1
mliuzzolino
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Homework Statement
A long rectangular metal plate has its two long sides and top at 0°. The base is at 100°. The plate's width is 10cm and its height is 30cm. Find the stead-state temperature distribution inside the plate.
Homework Equations
∇2T = 0
T(x,y) = X(x)Y(y)
X(x) = Acos(kx) + Bsin(kx)
Y(y) = Ceky+De-ky
The Attempt at a Solution
Using boundary conditions to obtain X(x): T(0,y) = T(10,y) = 0
[itex] X(x) = Bsin(\dfrac{n\pi x}{10}) [/itex]
Using boundary conditions to obtain Y(y): T(x, 0) = 100; T(x, 30) = 0
Y(0) = C + D = 100
D = 100 - C
[itex] Y(30) = Ce^{3n\pi} + De^{-3n\pi} = 0 [/itex]
[itex] Ce^{3n\pi} = -De^{-3n\pi} [/itex]
[itex] C = -De^{-6n\pi} [/itex]
[itex] C = -(100 - C)e^{-6n\pi} [/itex]
[itex] C = (C - 100)e^{-6n\pi} [/itex]
[itex] C = Ce^{-6n\pi} - 100e^{-6n\pi} [/itex]
[itex] C - Ce^{-6n\pi} = -100e^{-6n\pi} [/itex]
[itex] C(1 - e^{-6n\pi}) = -100e^{-6n\pi} [/itex]
[itex] C = \dfrac{-100e^{-6n\pi}}{1-e^{-6n\pi}} [/itex]
[itex] C = \dfrac{100e^{-6n\pi}}{e^{-6n\pi} - 1} [/itex]
Then by D = 100 - C
[itex] D = 100 - \dfrac{100e^{-6n\pi}}{e^{-6n\pi} - 1} [/itex]
[itex] D = \dfrac{-100e^{-6n\pi}}{e^{-6n\pi} - 1} [/itex]
So
[itex] Y(y) = \dfrac{100e^{-6n\pi}}{e^{-6n\pi} - 1} (e^{\dfrac{n\pi y}{10}} - e^{\dfrac{-n\pi y}{10}})[/itex]
[itex] Y(y) = \dfrac{200e^{-6n\pi}}{e^{-6n\pi} - 1} sinh(\dfrac{n \pi y}{10}) [/itex]
So by T(x,y) = X(x)Y(y)[itex] T(x,y) = B \dfrac{200e^{-6n\pi}}{e^{-6n\pi} - 1} sinh(\dfrac{n \pi y}{10}) sin(\dfrac{n \pi y}{10}) [/itex]
but then...
[itex] T(x,y) = \sum_{n=1}^{\infty} B \dfrac{200e^{-6n\pi}}{e^{-6n\pi} - 1} sinh(\dfrac{n \pi y}{10}) sin(\dfrac{n \pi y}{10}) [/itex]
Up until here, aside from the nastiness of the problem, I decided to read the book and see if I was on track, but they went a completely different way and ended up with something else.
They didn't solve it analytically at all, and just said that we can notice a solution, namely when [itex]C = -\dfrac{1}{2}e^{-30k} [/itex] and when [itex]D = \dfrac{1}{2}e^{30k} [/itex].
This makes sense to me, and when we analyze the summation and check the condition when T(x,0) = 100, my solution shows that T --> 0 rather than 100.
What did I do wrong, and how do I analytically acquire the solution they achieved? I'm not well versed enough to just pick a solution out of a hat like that, so how could I achieve it along the methods I was using by applying boundary conditions to the Y(y) ODEs?