# Guessing what a simple partial differential equation is describing physically

Let us try to rewrite the equation in approximate form of finite differences:
$$\frac{A(x,t+\Delta t)-A(x,t)}{\Delta t} = C_3\frac{A(x+h,t)+A(x-h,t)-2A(x,t)}{h^2} +$$
$$+ C_2 \frac{v(x+h,t)A(x+h,t)-v(x-h,t)A(x-h,t)}{2h} + C_1 A(x,t)+C_0$$
Where $\Delta t$ -- is a time step, and $h$ -- space step.

The expression becomes your PDE, in the limit $\Delta t\to0, h\to0$.

The left hand side describes how much the quantity $A$ changes during a time step at a given point (hope that is obvious). Let us see what is on right hand side term-by-term:

- $C_3$: If $A$ is larger in total at "neighboring" points than in the point itself: $A(x+h,t)+A(x-h,t)>2A(x,t)$ then $A$ will increase at $x$. Otherwise it will decrease. So the term "forces" $A$ to some kind of local equilibrium.

This is a standard term for description of various diffusion or heat distribution processes that forces your system to some equilibrium. - $C_2$: Here one have to be careful with signs. Let us call $x+h$ "the point to the right" and $x-h$ will be "the point to the left". If $v>0$ then it "flows to the right" and if $v<0$ it "flows to the left". Now you check that if your flow goes into your point $x$ (say, flows to the right from point to the left) then it increases $A$ at $x$. And decreases if flows goes away from your point.

That is a standard term that describes stuff carried with the flow $v$ and usually derived in terms of substantial derivative - $C_1$ and $C_0$: These two are trivial, because they are local. $C_0$ just gives a constant contribution to growth of $A$, and $C_1$ changes (inhibits in your case) the growth rate proportional to the value of $A$.

You can also understand them in terms of local ODE for a given point: $\dot{a} = C_1a+C_0$

To sum it up: let us say that $A(x,t)$ describes density of, say, bacteria in a tube. Then $C_3$ describes how they diffuse around, $C_2$ describes how they are carried by a stream $v(x,t)$ of liquid in the tube, $C_0$ -- is a growth rate of new bacteria, and, finally, $C_1$ -- is responsible for the growth to slow down from the overpopulation.

As an alternative to Christian Blatter's heat interpretation, $A$ might describe the concentration of particles adsorbed onto a one-dimensional substrate surface (or a two-dimensional one, where we ignore one of the dimensions).

- New particles are adsorbed at rate $C_0$ per unit length.
- Adsorbed particles detach from the surface at rate $-C_1$ per particle.
- The particles move along the surface (or the surface itself moves) with mean net velocity $-C_2 v$.
- While moving, the particles also diffuse over the surface with diffusion coefficient $C_3$.

In any case, this equation describes the dynamics of some quantity $A$ undergoing diffusion and advection over a one-dimensional space, while also undergoing constant (i.e. zeroth-order) local accumulation and first-order decay.

(As a mathematical ecologist, my first thought was to interpret it as a spatial population model, but it doesn't really fit that interpretation very well: there's no $A^2$ term that could describe local density regulation.)

You have a thin cylindrical tube along the $x$-axis that is filled with some gas of density $\rho(x,t)$. The temperature of the gas is $A(x,t)$, and the gas is moving along with "mass flux" $m(x,t):=\rho(x,t)v(x,t)$, where $v$ denotes the actual speed of individual particles. (The $\rho$ is missing in your equation). Heat is transported through heat conduction and by means of convection. In addition the $x$-axis is an electrical wire which produces heat hat a constant rate, and at the surface of the tube we have a heat loss towards outer space, the latter being a temperature $0$.

Your equation describes the temporal change rate of temperature in a "length element" at $x$ at time $t$. The individual terms on the right side account for the contributions of conduction, convection, surface loss to outer space, and heating.