Integrating Charge Conservation Equation with Dirac Delta Function

  • Thread starter jmwilli25
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In summary, the conversation is about solving an equation involving charge conservation and the Dirac delta function. The equation is -\int_{V}d\vec{r}e\frac{\partial}{\partial t}e\delta(\vec{r}-\vec{R}(t))=\int_{V}d\vec{r}\vec{\nabla}\bullet(e\frac{d}{dt}\delta(\vec{r}-\vec{R}(t))). The person is looking for help with the left hand side of the equation and is trying to show that it is equal to the right hand side. They mention that they may need help applying the divergence to a full derivative of time.
  • #1
jmwilli25
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Homework Statement



I just need help integrating this equation.
e is just the charge of an electron so it is constant



Homework Equations




[tex]
-\int_{V}d\vec{r}\frac{\delta}{\delta t}e\delta(\vec{r}-\vec{R}(t))
[/tex]

The Attempt at a Solution



[tex]
-e\frac{d}{dt}\int_{V}d\vec{r}\delta(\vec{r}-\vec{R}(t))
[/tex]
[tex]
-e\frac{d}{dt}4\pi
[/tex]

I don't believe my solution is correct or complete
 
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  • #2
Some additional context would be helpful/necessary. Is the derivative an ordinary derivative like this?
[tex]-\int_{V}d\vec{r}\frac{d}{dt}e\delta(\vec{r}-\vec{R}(t))[/tex]

or is a partial derivative, like this?
[tex]-\int_{V}d\vec{r}\frac{\partial}{\partial t}e\delta(\vec{r}-\vec{R}(t))[/tex]

Is [itex]\delta[/itex] just a constant, or are you indicating an impulse function?
 
  • #3
I think he maybe referring to functional differentiation, not too sure though.
 
  • #4
Sorry, it is the partial derivative inside the integral and
[tex]
\delta(\vec{r}-\vec{R}(t))
[/tex]
is the dirac delta function.

Context: I am trying to show that the equation of charge conservation holds when
[tex]
\rho(\vec{r},t)=e\delta(\vec{r}-\vec{R}(t))
[/tex]

The entire equation that I am trying to solve is
[tex]
-\int_{V}d\vec{r}e\frac{\partial}{\partial t}e\delta(\vec{r}-\vec{R}(t))=\int_{V}d\vec{r}\vec{\nabla}\bullet(e\frac{d}{dt}\delta(\vec{r}-\vec{R}(t))).
[/tex]
I have to show that the two sides are equal.
But I figured if I could get help just with the LHS then I might be able to do the RHS myself. Granted, I still need to figure out how to apply the divergence to a full derivative of time, but one step at a time. Also, the equation that I have just given is exactly what is in the book. Classical Electordynamics, Schwinger 1998.
 

1. What is integration of an equation?

Integration of an equation is a mathematical process that involves finding the antiderivative of a given function. It is the reverse process of differentiation and is used to solve problems involving area, volume, and motion.

2. Why is integration important?

Integration is important because it allows us to solve a variety of real-world problems, such as finding the area under a curve, determining the volume of irregular shapes, and predicting the motion of objects.

3. What are the different methods of integration?

The main methods of integration are substitution, integration by parts, trigonometric substitution, and partial fractions. These methods are used to solve different types of integrals and are chosen based on the complexity of the given equation.

4. How do you know when to use integration?

Integration is used when we need to find the total amount or quantity of something. It is also used when we need to find the rate of change of a quantity, such as velocity or acceleration.

5. Can all equations be integrated?

No, not all equations can be integrated. Some equations are not continuous or do not have a well-defined antiderivative. In these cases, numerical methods may be used to approximate the solution.

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