Integration by parts of derivative of expectation value problem

In summary, the conversation discusses the integration process in a given problem and clarifies the presence of a negative sign in the final equation. The confusion arises when the speaker mistakenly assumes that the integration has already been done, leading to a misunderstanding of the resulting equation. The conversation ends with the clarification that the negative sign is always present in integration by parts.
  • #1
Normalization
23
0

Homework Statement


I don't know how the writer of the book took integral of the first statement and got the second statement? Can anybody clarify on this?
Problem.png

Homework Equations


Given in the photo

The Attempt at a Solution


When I took the integral I just ended up with the exact same statement but without the negative sign behind the (ihbar/2m)
 
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  • #2
Welcome to PF!

Hi Normalization! Welcome to PF! :smile:
Normalization said:
When I took the integral I just ended up with the exact same statement but without the negative sign …

But that negative sign is always there in integration by parts. :confused:

(see the last equation in your photo)
 
  • #3
Oh w8 (facepalm) they haven't actually integrated yet? Wow... I feel so stupid right now. I thought they already integrated both products inside the brackets in which case -[itex]\int[/itex][itex]\frac{\partial\Psi^*}{\partial\ x}[/itex][itex]\Psi[/itex]dx-[itex]\int[/itex][itex]\frac{\partial\Psi}{\partial\ x}[/itex]×-[itex]\Psi^*[/itex]dx Which would be the exact same thing. Alright thanks I guess...
 
  • #4
I'm not convinced you've got this. :confused:

They have integrated that big bracket, and so they also differentiate the x (to get 1) …

the […] term is 0 (so they haven't written it), and that only leaves the ∫ term, which always has a minus in front of it. :smile:
 
  • #5
tiny-tim said:
I'm not convinced you've got this. :confused:

They have integrated that big bracket, and so they also differentiate the x (to get 1) …

the […] term is 0 (so they haven't written it), and that only leaves the ∫ term, which always has a minus in front of it. :smile:

Yes,yes,yes I know but I thought they had already taken the integral of -int(df/dx g dx)
 
Last edited:
  • #6
Which by nature should be equal to the left side of the equation. So it's fine I was just being daft
 

Related to Integration by parts of derivative of expectation value problem

What is integration by parts?

Integration by parts is a method used in calculus to evaluate integrals that involve products of functions. It involves using the product rule from differentiation to transform the integral into a different form that is easier to solve.

How does integration by parts work?

To use integration by parts, you must first choose two functions, usually denoted as u and v, and apply the product rule to find their derivatives. Then, you can rewrite the original integral as a combination of these derivatives and solve it using the integration by parts formula.

Why is integration by parts useful in the derivative of expectation value problem?

The derivative of expectation value problem often involves integrals that can be difficult or impossible to solve using basic integration techniques. Integration by parts allows us to transform these integrals into a more manageable form, making it easier to find the solution.

What is the formula for integration by parts?

The integration by parts formula is ∫u dv = uv - ∫v du, where u and v are the two functions chosen and du and dv are their corresponding derivatives.

Are there any limitations to using integration by parts?

While integration by parts is a powerful tool, it does have its limitations. It may not be applicable to all integrals, and it can sometimes lead to more complex integrals that are still difficult to solve. It is important to carefully choose the functions u and v to ensure that integration by parts is the most effective method for solving the integral.

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