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General Science => General Science => Topic started by: neilep on 13/10/2007 14:58:06

Title: Why does Cling film stick to itself ? (a question about static charge)
Post by: neilep on 13/10/2007 14:58:06

Dear Peeps in Charge !!

see this cling film ?

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Nice isn't it ?.....notice how all wrapped up and clinged to itself it is ?

It does this when unrolled too.

now, my daughter tells me that the reason why cling film sticks to stuff is because of the static charge......so, with regard ti cling film specifically...how does something with the same charge stick to itself ?...should it not repel ?

Title: Why does Cling film stick to itself ? (a question about static charge)
Post by: Karen W. on 15/10/2007 09:44:33

I wonder if it has to do with all the air being pushed out from under it! so when air inters the pictures as you unroll the plastic is sort of recharged so to speak! I am pulling thing out of my hat, maybe not my hat!!! LOL

Title: Why does Cling film stick to itself ? (a question about static charge)
Post by: lyner on 15/10/2007 14:39:17

I think it's because, when you separate the layers on the roll, they get charged - by induction- in the same way that an uncharged piece of dust or paper is held by a piece of charged plastic; the object gets polarized.
The plastic, used, is a very good insulator, so the charges are held on the surface.
I am not sure whether the manufacturing process encourages this by polarizing it as it is made.
I would be interested to know why you always seem to get attraction between the pieces.  - when we know that like charges will repel.  Is there a charged layer inside the sheet?
It works best first time you try to stick it - once it gets damp or used, it loses its properties.

Title: Why does Cling film stick to itself ? (a question about static charge)
Post by: Karen W. on 15/10/2007 19:19:15

Quote

I would be interested to know why you always seem to get attraction between the pieces.  - when we know that like charges will repel.

Quote

Do you mean like two magnets of the same charge repelling each other?

Title: Why does Cling film stick to itself ? (a question about static charge)
Post by: lyner on 15/10/2007 19:51:42

Except that magnets have two poles -  or, at least, you need North and South poles for any magnetic system ALWAYS. The field lines are loops.
Charges exist on their own. You CAN have an isolated positive or negative charge.. (Ignore the problem of saying charges can be at a point with no size).
You can still say that 'like repel' and 'unlike attract'.
Have a look at this:
http://www.glenbrook.k12.il.us/gbssci/phys/class/estatics/u8l2b.html (http://www.glenbrook.k12.il.us/gbssci/phys/class/estatics/u8l2b.html)

Title: Why does Cling film stick to itself ? (a question about static charge)
Post by: eric l on 16/10/2007 08:22:29

I don't know for sure about cling foil, I know a bit about plastic foil in general, and also about papermaking.
In paper making, you also have a winder at the end of the machine, and sometimes the paper comes out so dry that it shows electrostatic charges due to friction from felts and the like.  Sometimes this electrostatic charges are removed by passing the paper over a metal rail before the winder.  This is done to avoid the paper clinging too  much to the roll when it has to be unwound for printing or any other further use.
I can imagine that they make the foil pass over a rail covered with some material that would induce electrostatic charges by simple friction, just like we used to rub the plastic parts of our pens to make them attract small pieces of paper.
The foil would remain charged until being discharged by passing it over metal or by wetting.

Title: Why does Cling film stick to itself ? (a question about static charge)
Post by: neilep on 16/10/2007 12:06:10

THANK YOU SophieCentaur and Eric as always for your informative and educational replies.

It really is very much appreciated.