# Solving Partial differential equation , distinguishing the meaning of boundary condition and Initial condition.

I have started my Differential equation course recently. And My professor was interchangeably using the term boundary condition and initial condition. I'm wondering what are the differences of their meaning in this context when dealing with the partial differential equations. I thought one would not need a boundary condition if a system extends to infinity, but the case of wave on a string would need, since it's a finite system For the initial condition, it always mean the initial state of the system. Can anyone tell me if my understandings are correct. Thank you!

## 1 Answer

If you think of a differential equation being defined on the coordinates of a certain object, that is mathematically like a number or a time line, and physically like a boundary.

So if $y(0)$ is given, in terms of time, that would be an "initial condition."

But if $y(0)$ and/or $y(L)$ are given for a differential equation that determines the temperature of a rod of length $L$, then $y(0)$ is more appropriately understood to be the "boundary condition" because it gives the temperature of the rod at the boundary of the rod.

Boundary conditions can be understood philosophically as being able to see only outside effects of something, but not directly measure the internal state of an object. The laws of physics governing the internal state of the object are used to formulate differential equations which can be solved to infer the internal state of the object based only on the external state that is known. This is like finding a criminal based on the evidence he leaves behind when committing a crime, and the large field of solving such problems is known as "Inverse Problems."

• So in this case, boundary is to do with The boundary of a system* and Initial condition * is to do with how system evolves with the time * right? And boundary means external state? Thanks for your answer – el psy Congroo Dec 5 '13 at 0:03
• Yes, boundary basically is like the external state which can be measured without entering the object being considered. In a way, the "future" is like the internal state of the object which we cannot penetrate. So the initial condition when time is 0 (the present time), is like the boundary of the object that represents the future. So initial conditions are very closely related to boundary conditions. The initial condition when time equals zero is simply the boundary condition at the edge of the object that represents the future. – Joseph Myers Dec 5 '13 at 2:51