What is the relationship between force and potential in particle interactions?

Click For Summary

Discussion Overview

The discussion revolves around the relationship between force and potential in the context of particle interactions, specifically focusing on how to derive force vectors from a given interaction potential between two repelling particles. The scope includes theoretical considerations and mathematical reasoning related to vector calculus and potential energy.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant proposes that the x-component of the force vector F_12 can be expressed as -du/dx, and similarly for the y and z components, suggesting that F_21 would simply be the negative of F_12.
  • Another participant cautions that typically, a potential describes a particle moving under an external force, implying that there may be different potentials for each particle involved.
  • A participant clarifies that the discussion pertains to an interaction potential specifically between the two particles.
  • It is noted that as long as the definition of the distance r is correct, the initial expressions for the force vectors appear valid.
  • One participant expresses confusion regarding the nature of potentials as scalar functions and their relationship to vector quantities.
  • Another participant confirms that the gradient of a scalar function can yield a vector function, which relates to the forces derived from the potential.
  • A later contribution states that the interaction potential typically takes the form V(𝑥1−𝑥2), and provides the mathematical expressions for the forces on each particle, affirming that Newton's 3rd Law applies due to the dependence of the potential on the relative position vector.

Areas of Agreement / Disagreement

Participants express some agreement on the mathematical relationships between force and potential, but there is also a recognition of the need for careful definitions and considerations regarding the nature of the potentials involved. The discussion contains elements of uncertainty and varying interpretations of the concepts presented.

Contextual Notes

There are limitations regarding the assumptions about the potentials being discussed, particularly the distinction between interaction potentials and external potentials, which may affect the derived force expressions.

Tim667
Messages
12
Reaction score
0
TL;DR
Slight confusion about vectors from a potential
Suppose I have some interaction potential, u(r), between two repelling particles. We will name them particles 1 and 2.

I want to find the force vectors F_12 and F_21. Would I be correct in saying that the x-component of F_12 would be given by -du/dx, y-component -du/dy etc? And to find the components for the other force vector, would this simply be the negative of the first vector?

So F_12 would be given by (-du/dx, -du/dy, -du/dz) and F_21= (du/dx, du/dy, du/dz)?

Thank you
 
Last edited:
Physics news on Phys.org
Tim667 said:
Summary: Slight confusion about vectors from a potential

Suppose I have some potential, u(r), between two repelling particles. We will name them particles 1 and 2.

I want to find the force vectors F_12 and F_21. Would I be correct in saying that the x-component of F_12 would be given by -du/dx, y-component -du/dy etc? And to find the components for the other force vector, would this simply be the negative of the first vector?

So F_12 would be given by (-du/dx, -du/dy, -du/dz) and F_21= (du/dx, du/dy, du/dz)?

Thank you
You need to be careful. Normally with a potential you have a particle moving under the influence of an external force. In this case technically you have a different potential for each particle.
 
  • Like
Likes   Reactions: Tim667
PeroK said:
You need to be careful. Normally with a potential you have a particle moving under the influence of an external force. In this case technically you have a different potential for each particle.
I see, I should specify that this is an interaction potential between the two particles
 
Tim667 said:
I see, I should specify that this is an interaction potential between the two particles
Okay. Of course. As long as you define ##r## the right way round, what you have looks right. I must be getting tired.
 
  • Like
Likes   Reactions: Tim667
PeroK said:
Okay. Of course. As long as you define ##r## the right way round, what you have looks right. I must be getting tired.
That's okay. I think I was confused because potentials are usually scalar functions, and I wasn't sure you could get vectors from them
 
Tim667 said:
That's okay. I think I was confused because potentials are usually scalar functions, and I wasn't sure you could get vectors from them
Yes, the gradient takes a scalar function of position and generates a vector function of position.
 
  • Like
Likes   Reactions: Tim667
An interactian potential usually takes the form ##V(\vec{x}_1-\vec{x}_2)##. The force on particle 1 is
$$\vec{F}_{12}=-\vec{\nabla}_1 V(\vec{x}_1-\vec{x}_2)$$
and on particle 2
$$\vec{F}_{21}=-\vec{\nabla}_2 V(\vec{x}_1-\vec{x}_2)=-\vec{F}_{12}.$$
Newton's 3rd Law holds, because the potential only depends on the relative vector ##\vec{r}=\vec{x}_1-\vec{x}_2##.
 
  • Like
Likes   Reactions: dlgoff, PeroK and Tim667

Similar threads

Replies
5
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
7K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 28 ·
Replies
28
Views
3K
Replies
1
Views
5K
  • · Replies 6 ·
Replies
6
Views
11K