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LAN_403

IntraWinding

Tue Jun 29 2010, 10:11PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I've always fancied having a go at making strong Electrets, the electrostatic equivalent of magnets.

I'd like strong monopoles and dipoles I can handle so I can get a sense of how they differ from the familiar effects of magnets. Presumably it's hard to make them that strong or we'd see them more often.

ShawnLG

Wed Jun 30 2010, 08:58PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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Posts: 399

How do you make electrets?

Pinky's Brain

Wed Jun 30 2010, 09:59PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Charge metal powder in a container (so it doesn't fly off everywhere) and then put in liquid paraffin, mix with one of those magnetic beads until it solidifies?

Kilham

Wed Jun 30 2010, 10:15PM

Registered Member #2814 Joined: Wed Apr 21 2010, 12:28PM
Location: Scotland, UK
Posts: 24

Heating and cooling a piece of plastic whilst in an electrostatic field. I tried this about 25 years ago with some limited success, it's difficult to make it last any length of time though.

IntraWinding

Wed Jun 30 2010, 11:52PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

The classic electret is made by cooling a mixture of melted waxes in a high voltage field. It solidifies with polar molecules aligned. As I understand it that gives you a dipole electret in that one side will be +ive and the other -ive, in the same way magnets have a north and south pole.
Unlike magnets though, you can make monopoles. The sort of thing I've imagined is a ball bearing encased in PTFE. You could charge the ball bearing to a high negative potential with an electron accelerator and hopefully the charge would remain trapped. More easily I can imagine placing a rounded metal shape in a liquid plastic and before it sets, dipping in a wire and charging it to a high voltage.
Here's the old Amateur Scientist article

Here's something I hadn't considered. The way magnets and electrets interact is unusual:

"If you take one electret and one magnet you will get a surprise. When not in motion, these two differing objects will have no effect on one another. It is only when you move them that anything happens ... and ... it is not the familiar attraction-repulsion. When a pole of the magnet is in relative motion to a "pole" of the electret they push each other at 90 degrees to the direction of motion. The effect is entirely odd and immediately unfamiliar (unless you are a physics student)."

klugesmith

Thu Jul 01 2010, 02:02AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Sounds like an interesting endeavor. You found the Amateur Scientist column; here's another DIY reference.

[edit]Please note that that reference shares a domain with everything from worm composting to free energy machines.

What properties are you going to measure to confirm that the finished product is an electret? Planning to make a teslatika?

I think electrets are present in the majority of the world's microphones today, when we count the ones in telephones, headsets, etc.

Does a statically charged object embedded in insulator count as an electret? Then "many" (a significant understatement) are found in nonvolatile semiconductor memories! I thought the term means an insulating material that retains electric polarization in the absence of an applied electric field.

radiotech

Thu Jul 01 2010, 04:07AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

The electrets used in microphones would have a model that would equal a insulator that gave evidence of a charge difference when a means to cause the oppositely charged faces to acquire the same charge. However whatever electrical energy is obtained by conducting from face to face is derived by the work (in the case of the microphone) of moving the diaphram. In other words it is as useless as a magnet for storing energy

Old capacitors sometimes show microphonics in amplifiers if you tap them gently.

This reflecting galvanometer works when probing very weak currents.when the reflected light beam is 10 M long, and nothing is vibrating the floor, it works great. We've used it to prove the current from a 12 inch electrophoresis disk can to a bit of work.

Leeds and Northrup--sensitivity 0.0041 uA/MM

IntraWinding

Thu Jul 01 2010, 05:20PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Klugesmith wrote ...

Sounds like an interesting endeavor. You found the Amateur Scientist column; here's another DIY reference.

I think electrets are present in the majority of the world's microphones today, when we count the ones in telephones, headsets, etc.

Looking at the Wiki Article on electrets, I think perhaps an my insulated metal monopole idea doesn't count as one. It looks like the charge is supposed to be distributed within or on the surface of a dielectric. It would still be an interesting artifact to create though.

I'm not planning to make electrets yet - I'm interested to know if it's possible to make them strong enough to handle and experience strong attraction/repulsion forces in the same way you can with magnets (see original post). And now I'm also interested to see how they interact with magnets. It looks like PTFE is the stuff to use, which is handy as it's easy to get hold of on eBay these days.

klugesmith

Thu Jul 01 2010, 06:37PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

IntraWinding wrote ...
I'm not planning to make electrets yet - I'm interested to know if it's possible to make them strong enough to handle and experience strong attraction/repulsion forces in the same way you can with magnets (see original post).

There's a reason most practical motors are electromagnetic rather than electrostatic.

At reasonable magnetic flux density of 1 Tesla, the attraction between two pole faces in air is 400 kPa (4 bar or about 60 psi).

At an electric field strength of 3 MV/m (threshold of breakdown) in air, the attraction between two parallel plates is just 40 Pa (0.4 mbar or 0.16 inches of water column), in my unreviewed calculation.

In both cases, the forces are proportional to the square of the field strengths.
In dielectric oil at an aggressive E-field of 15 MV/m, we're up to 3 kPa (0.4 psi), still more than 130 times weaker than the magnet system.

Anybody got engineering data on the strength of commercial electrets, such as microphone film?

-Rich

...

Mon Jul 05 2010, 12:01AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

That was a very interesting article, and it seems that if it were possible to get 40pa it would be easy to observe the effect (40pa over a 1in^2 area is about 2g of force).

I found the comment that the charge falls to 0 and then reverses to be the most interesting, the time scale of a few weeks seems to rule out any possible explanations I can think of. Who is up to build one and connect it to a high impedance voltmeter and observe the process

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