Static-E Attraction on Knitting Needle

[Nehmo]

https://www.youtube.com/watch?v=qHrBhgwq__Q&feature=player_detailpage
Don Pettit, unfortunately, narrates, but doesn't explain much. The main needle is Teflon and the other needle by the injection of the water droplets (seen near the end of the vid) is Nylon. The site asks about the choice of materials.
It seems the Nylon needle imparts a charge to the droplets. Is that possible without touching the droplets?

 

[NascentOxygen]

I think he is probably touching the end of the teflon dispenser with the charged knitting needle before releasing some droplets. It would be next to impossible to rub the dispenser with a cloth to try to charge it while it contains water, without spilling water and upsetting static electricity experiments. So he charges a dry rod, then shares some of its charge with the droplet dispenser. I think he may bring the other charged rod into contact with the water in the dispenser, and the dispenser being an insulator, the water holds the charge. We don't see this part of the procedure, but I think that should work.

 

[Greg-ulate]

The presence of the electric field form the static charges can polarize the charge on the water droplet. Opposite charges are attracted and move to the face closest to the needle and like charges are repelled and move to the far side of the droplet.

Since the electric field gets weaker farther from the shaft of the needle, like charges on the far side are not repelled with equal force as the opposite charges are attracted, so the net force on the droplet is attractive. The droplet itself stays together for a number of interesting reasons including magnetic binding properties, otherwise the like charged water molecules would just fly away and the opposite charged molecules would be attracted even faster.

The divergence of the electric field is what makes this experiment work. The same is true of magnets attracting a piece of iron, the iron is magnetized and the far side is repelled less than the near side is attracted. In either case, if the field were uniform, the material would polarize and the forces would cancel evenly, resulting in a net force of 0

 

[NascentOxygen]

The presence of the electric field form the static charges can polarize the charge on the water droplet. Opposite charges are attracted and move to the face closest to the needle and like charges are repelled and move to the far side of the droplet.

I think your first sentence might explain it. The charged rod is brought near to but not touching the end of the droplet dispenser. This attracts charge in the water towards that end of the dispenser. As droplets are dispensed, they carry with them some of this charge, making them charged droplets. So simple!

 

[Greg-ulate]

Yes, very cool and a great demonstration. Also look up "Kelvin water dropper" which relies on that principle. If you see a video of it on youtube or whatever, they will probably point out that when it gets highly charged, the drops on one side (but not the other) of the generator start to explode and spray out as they drop. I wonder which charge makes them spray, my guess is positive, since negative electrons could probably escape the drop easier.

 

[Nehmo]

Teflon and Nylon react differently to being rubbed.

Teflon acquires electrons from other materials when rubbed, thus, becoming negatively charged.
Nylon gives up electrons to other materials when rubbed, thus, becoming positively charged.

The droplets come out of the syringe near the positively charged nylon needle where they give up some electrons to it becoming positive themselves.This makes the water droplets attracted to the negatively charged teflon needle because opposite charges attract.