Math 4600, Homework 11
1. Do the following exercises for these functions:
a) f = y 2 sin x,
b) f = exp(−0.1(x2 + y 2 ))
∂2f
∂2f
- find ∂f
∂x , ∂x∂y , ∂y∂x , gradient of f
- (computing) Visualize the surface and the level curves given by the equation
z = f (x, y)
in Matlab. Explore different functions for plotting: mesh, meshz, meshc, surface
and see what they do. Type
help mesh
or similar to see the available options. Here is a sample code to help you:
[X,Y] = meshgrid(-5:.2:5, -6:.2:6);
z=sin(X).*cos(Y);
meshc(X,Y,z)
xlabel(’x’)
ylabel(’y’)
The following three problems refer to the one-dimensional diffusion equation
ut = Duxx
.
2. Find the steady state solution of the one-dimensional diffusion equation
with boundary conditions u(0, t) = c1, u(L, t) = c2, x ∈ [0, L].
3. Consider the diffusion equation with u(0, t) = 0, u(1, t) = 0.
Using the separation of variables approach, we need to consider solutions of the
form
ũ(x, t) = eλt sin(ωx).
a) Find appropriate values of ω using the boundary conditions
b) Find corresponding values of λ by substituting the prospect solutions into
the equation
c) What is the qualitative behavior of ũ(x, t) as t → ∞?
4. In an experiment a substance of concentration A is released into a narrow
tube at x = 0, t = 0. It difuses along the tube with diffusion constant D = 1.
Detectors are setup along the tube at all locations x > 0. They can detect the
substance if the concentration is above 5% of A.
a) What is the furthest location X(t) where the detector will be responding at
time t? (Hint: You will need to solve the diffusion equation before you find
X(t). Use fundamental solution approach.)
b) Sketch the time evolution of X(t)
1