Is the transfer of energy in hydraulics truly instant?

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Discussion Overview

The discussion centers around the nature of energy transfer in hydraulics and its comparison to mechanical and electrical systems. Participants explore whether energy transfer can be considered instantaneous, particularly in the context of a hypothetical scenario involving a long pole and the propagation of force.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant theorizes that energy transfer in hydraulics could be instantaneous, drawing a parallel to the speed of electrons in electricity.
  • Another participant asserts that neither hydraulic impulse nor electricity travels instantly, emphasizing that the effect of pulling on a long pole would propagate at the speed of sound in the material.
  • It is noted that for electricity, the impulse travels at a fraction of the speed of light, which varies based on the characteristics of the wire's insulators.
  • A participant questions whether the pole would appear longer during the energy transfer, leading to a discussion about the stretching of the pole as a result of the propagation delay.
  • Several participants express a need for further research into these concepts before forming conclusions.

Areas of Agreement / Disagreement

Participants generally agree that energy transfer is not instantaneous, but there is ongoing exploration of the implications of this delay, particularly regarding the stretching of materials and the nature of force propagation.

Contextual Notes

Participants have not fully resolved the implications of the stretching of the pole during energy transfer, and there are assumptions about the properties of materials and the speed of sound that remain unexamined.

Who May Find This Useful

Readers interested in the principles of energy transfer in mechanical and hydraulic systems, as well as those exploring the differences between mechanical and electrical propagation of forces.

Helios223
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Hello,

This is my first ever post in an entirely science-dedicated forum. On another forum, another user and I theorized that the energy transfer in hydraulics from one point to another is instant. He initially posted about electricity being instant because electrons move at the same speed throughout a metal, and together we came up with this question:

If I managed to make a pole one light-year long, and I pull on one end of this pole, would a friend standing at the other end of the pole experience this same movement instantly, or would there be a one-year delay?

It's baffled me, and I was wondering if any of you wonderful people could shed some light on this question. :biggrin:

Many thanks!
 
Last edited:
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Neither hydraulic impulse nor electricity travel "instantly".

If I managed to make a pole one light-year long, and I pull on one end of this pole, would a friend standing at the other end of the pole experience this same movement instantly, or would there be a one-year delay?
The effect of the pull would travel at the speed of sound in the pole, much slower than the speed of light--definitely not instantly!
 
For a mechanical pole yank, the force propagates at the speed of sound in the material of the pole. Similarly for hydraulics.

For electricity propagating on wires, the impulse travels at a fraction of the speed of light -- the fraction depends on some of the characteristics of the insulators around the wire, etc.

Nothing is instantaneous. You and your friend should stop speculating and do some reading. Welcome to the PF, BTW. Be sure that you've read and understood the PF Guidelines before getting too crazy with more posts:

https://www.physicsforums.com/showthread.php?t=5374
 
Thank you Doc! :)

You seem very certain about it being the speed of sound. Does this not mean the the pole will be longer for part of the energy transfer?

Berke, thank you for the link. I will be sure to follow the guidelines. I will do further reading about forces, and thanks for your welcome!
 
Last edited:
Helios223 said:
Does this not mean the the pole will be longer for part of the energy transfer?
Absolutely... it will stretch a bit.
 
Doc Al said:
Absolutely... it will stretch a bit.

Ah, that makes this much clearer. It seems I need to research these theories more before I jump to conclusions.
 

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