Calculating the Attraction Speed of Magnets

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Discussion Overview

The discussion revolves around the question of whether there is a formula to calculate the speed at which magnets attract each other. Participants explore the theoretical aspects of magnetism, including force calculations and potential energy, while considering practical applications and measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about a formula for the speed of attraction between two neodymium magnets, noting their specifications and initial distance.
  • Another participant suggests that while a specific velocity formula may not exist, the force between magnets can be calculated, emphasizing that this force is highly dependent on the distance between them.
  • A different participant proposes using the concept of potential energy and kinetic energy to derive the velocity just before impact, indicating that this could be repeated to find velocities at various distances.
  • One participant mentions the possibility of measuring the force between magnets as a practical approach, referencing an external method for doing so.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence of a formula for the speed of attraction. There are multiple competing views regarding the calculation of forces and energies involved, and the discussion remains unresolved.

Contextual Notes

Participants express uncertainty regarding the dependence of force on the distance between magnets and the implications for calculating speed. There are also references to potential energy and kinetic energy without resolving the mathematical steps involved.

Electrica
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Hi PF Pros... I wonder if you can help me...

I am trying to see if there is a formula for how fast magnets attract. I know that there is the inverse square in terms of power loss and that magnetism is a field that travels at or near light speed.

But, for example, if I have 2 cyllander NIB magnets (.5" wide x 1" long) with 5,000 Gauss each and they are 3 inches away from each other... at what speed will they meet? If I have the speed I can calculate the time/distance. (as for weight or other forces like friction, etc, please feel free to make something up for the equation).

Thanks!

~Electrica
 
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No, there probably isn't a formula for this velocity.

If anything, you might be able to find the force between two magnets, however it is very dependent on the spacing between the magnets.
So, you would only have to move the magnets closer to each other by a small amount and they would be attracted to each other a lot more.

There is a formula for this attraction here: (formula taken from Wikipedia article on magnets)

ea0c076f1c59249aba590d07b31da41e.png


where

B0 is the magnetic flux density very close to each pole, in T,
A is the area of each pole, in m2,
L is the length of each magnet, in m,
R is the radius of each magnet, in m, and
x is the separation between the two magnets, in m

8534cbb9499a30dfa335ad6b0f01400b.png
relates the flux density at the pole to the magnetization of the magnet.

Probably easier is to just measure the force with the following method:
http://www.exo.net/~pauld/activities/magnetism/forcebetweenmagnets.html
 


Well, there will be a potential energy due to that force that looks like the integral of Fdx, so you can set that integral equal to the kinetic energy when they collide (1/2 mv^2) and you will find the velocity right before impact (neglecting friction)... should be able to repeat this to find the velocity after traveling half the distance, etc. etc.

But if you are holding them close, and you want to find the velocity right when you let them go, then... I'll think about it...
 


Thanks a lot! I'll try that. Please do let me know if you come up with the other. -Electrica

mordechai9 said:
Well, there will be a potential energy due to that force that looks like the integral of Fdx, so you can ...
 


Thank you very much for this info! I have some reading to do! - Electrica

vk6kro said:
No, there probably isn't a formula for this velocity.

If anything, you might be able to find the force between two magnets, however it is very dependent on the spacing between the magnets...
 

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