2.1: Examples of PDE
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Partial differential equations occur in many different areas of physics, chemistry and engineering. Let me give a few examples, with their physical context. Here, as is common practice, I shall write ∇2 to denote the sum
∇2=∂2∂x2+∂2∂y2+…
- The wave equation: ∇2u=1c2∂2u∂t2This can be used to describes the motion of a string or drumhead (u is vertical displacement), as well as a variety of other waves (sound, light, ...). The quantity c is the speed of wave propagation.
- The heat or diffusion equation, ∇2u=1k∂u∂tThis can be used to describe the change in temperature (u) in a system conducting heat, or the diffusion of one substance in another (u is concentration). The quantity k, sometimes replaced by a2, is the diffusion constant, or the heat capacity. Notice the irreversible nature: If t→−t the wave equation turns into itself, but not the diffusion equation.
- Laplace’s equation: ∇2u=0
- Helmholtz’s equation: ∇2u+λu=0This occurs for waves in wave guides, when searching for eigenmodes (resonances).
- Poisson’s equation: ∇2u=f(x,y,…)The equation for the gravitational field inside a gravitational body, or the electric field inside a charged sphere.
- Time-independent Schrödinger equation: ∇2u=2mℏ2[E−V(x,y,…)]u=0|u|2 has a probability interpretation.
- Klein-Gordon equation ∇2u−1c2∂2u∂t2+λ2u=0Relativistic quantum particles,|u|2 has a probability interpretation.
These are all second order differential equations. (Remember that the order is defined as the highest derivative appearing in the equation).