Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: switchable binding

Learn more physics!

Most recent answer: 09/20/2013
Q:
Hello. I want to hear what material you can use if you have to have something to "hang together well (attracting each other)" and then be able to take it apart easy. It's a bit hard to explain but if you imagine that (I do not know if it is possible) But if you mix magnetic powder into some glue and you then spread it out on a flat surface and let it dry. And then do the same on another surface and, be able to get the magnets to attract each other with a "command" then they suddenly hung well together, then so do some other "command" that makes the magnets be not attracted to each other any more. (It should be without power). My question is actually: are there a different material you control like that? Or if you do it with magnets is there any subject that can get the magnets to be attracted to each other and then repel each other. So you can control the magnets or other material to attract each other on and off? From Mia I hope you can understand my engglish because im actually from Denmark, and of couse I hope you can answer my question.
- Mia Dall (age 20)
Denmark
A:

It's easy to understand your near-perfect English, but it's not completely clear what sorts of things would be allowed in the switch controlling this binding. Is there an actual application in mind?

No "command" can be made using exactly zero power, but we can try to think of ones in which a switch can be made with very low energy input, much less than the energy that would be required to actually pull the objects apart. With that constraint, I don't think it's possible to make something that can be switched back and forth the way you want. 

Here's an argument. Let's say the switch can be on ("N") or off ("F"). The two objects can either be bound ("B") or apart ("A"). So we have four states : NB, NA, FB, and FA.
Each state will have some energy*:
ENB, ​ENA, ​EFB, and ​EFA.

We want the on switch to bind the things, so we need:
 ENB < ​ENA,
because things go toward the low-energy state, just like things roll downhill.
Likewise we want the things to be apart when the switch is off, so we need:
 EFB  > ​EFA.
If we want to be able to throw the switch either way without much energy input, we need 

ENB ≈ EFB  and ​EFA ≈ ENA​.

Putting those together we get:
 ENA ≈ EFA > EFB  ≈  ENB, so ENA > ENB.
But we started with the opposite requirement: 
 ENB < ​ENA.

So I don't think there's any such system.

Mike W.

* free-energy, for the picky.


(published on 09/20/2013)

Follow-up on this answer.