Diffusion formula


Diffusion is the net movement of molecules or atoms from a region of high concentration to lower concentration. It can also be referred to movement of substances towards the lower concentration.The diffusion equation is the partial differentiation equation which indicates dynamics in a material which undergoes diffusion. It also describes processes exhibiting diffusive behavior. The diffusion equation is ∂ψ/∂t = 2ψ

The diffusion equation can also be written as




ϕ(r, t) is the density of the diffusing material

D(ϕ, r) is the collective diffusion coefficient

∇ shows the vector differential operator del

If the diffusion coefficient depends on density, then the equation is nonlinear, else it’s linear.


Example 1

Solve the diffusion equation ∂ψ / ∂t = kΔ2ψ with the u(x, 0) = f(x), u(0,t) = 0 and u(L, t) = 0

f(X) = 6 sin(πx / L)


Given equation is

F(x) = 6 sin (πx / L)

 U(x, 0) = F(x), u(0,t) = 0 and u(L, t) = 0

Let n = 1

B1 = 6

Using formula,

Un(x, t) = Bn sin(nπx / L) e-k(nπ / L)2 t

 U(x, t)  = 6 sin (pix / L) e –k(π / L)2 t      


Example 2

Solve the diffusion equation

∂ψ / ∂t = kΔ2ψ with u(x, 0) = f(x), u(0, t) = 0 and u(L, t) = 0

F(x) = 12 sin (π x / L)


u(x, 0) = f(x), u(0, t) = 0 and u(L, t) = 0

Let n = 1

B1 = 12

From the formula,

Un(x, t) = Bn sin (n π x / L) e –k(2 π / L)2 t

U(x, t) = 12 sin(π x / L) e –k(π/L)2 t

Practise This Question

At a point P in space, the disturbance created by a travelling wave which is not sinusoidal is +10 cm, while the disturbance created by another sinusoidal wave at the the same point in space is +5 cm. What is the net displacement at the point P ?