Understanding the Euler Lagrange Equation and Its Boundary Condition

In summary, the conversation discusses the role of boundary conditions in deriving the EL equation and extremising the functional. It is mentioned that all variations must give zero variation of the functional, including those with fixed and free end points. The BCs may come from surface terms in the action.
  • #1
TimeRip496
254
5
I am trying to derive it but I am stuck at the boundary condition. What is this boundary comdition thing such that the value must be zero?
 
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  • #2
Can you elaborate?
Is this related to the variation being zero at the endpoints?
 
  • #3
robphy said:
Can you elaborate?
Is this related to the variation being zero at the endpoints?
Yes.
 
  • #4
TimeRip496 said:
Yes.

You do not need the boundary conditions to get to the EL equation. In order to extremise the functional, all variations must give zero variation of the functional, in particular those variations with fixed end points.

Now, if you do not have fixed BCs, also considering variations without fixed BCs will give you BCs when requiring zero variation of the functional - so called free BCs.
 
  • #5
the variation can be zero at the endpoints, but it may not be...I think that your BCs are coming from the surface terms (the total-divergent terms in your action).
 

1. What is the Euler Lagrange equation?

The Euler Lagrange equation is a mathematical expression used in the field of calculus of variations to find the extremum of a functional. It is used to find the function that minimizes or maximizes the functional, representing a physical quantity, over a specified interval.

2. What is the importance of the Euler Lagrange equation?

The Euler Lagrange equation is essential in many areas of physics, engineering, and mathematics. It is used to find the motion of a system in classical mechanics, the path of a light ray in optics, and the shape of a soap film in surface tension, among others. It also provides a general framework for solving optimization problems in various fields.

3. What are the boundary conditions in the Euler Lagrange equation?

The boundary conditions in the Euler Lagrange equation specify the values of the function at the endpoints of the interval. These conditions are necessary for finding a unique solution to the equation and determining the extremum of the functional.

4. How is the Euler Lagrange equation derived?

The Euler Lagrange equation is derived by setting the derivative of the functional with respect to the function equal to zero and then applying the fundamental theorem of calculus. This results in a differential equation that can be solved to find the function that minimizes or maximizes the functional.

5. Can the Euler Lagrange equation be extended to higher dimensions?

Yes, the Euler Lagrange equation can be extended to higher dimensions, known as the Euler-Lagrange PDE. It is used to find the extremum of a functional in multiple variables and is commonly used in fields such as fluid dynamics, electromagnetism, and quantum mechanics.

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