Why does a charged comb bend a stream of water?

[sophiecentaur]

okok, I misunderstood your posts then. Sorry

I'd say that the term polarisation applies whenever charges are displaced from an equilibrium state. My comments are mostly applicable for all situations with that.

There's such a lot of EM that doesn't involve 'actual contact' and flowing charges for energy transfer.

 

[shjacks45]

Summary:: The effect isn't permanent and there's no clear discharge, so where do the electrons go after bending the stream of water?

I suppose the stream isn't being charged!

Usually rub comb with silk or combing dry hair, so that's where the electrons went. Old rubber combs had a positive charge. The reason the water stream bends is due to charges being induced by the comb on the water stream.

 

[shjacks45]

Usually rub comb with silk or combing dry hair, so that's where the electrons went. Old rubber combs had a positive charge. The reason the water stream bends is due to charges being induced by the comb on the water stream.

Duh, (unless it is exceedingly pure) water conducts electricity.

 

[sophiecentaur]

Duh, (unless it is exceedingly pure) water conducts electricity.

Whether the water is pure or has ions in it, charges will still be displaced and a drop or stream will be 'polarised'.

For impure water, the resistivity is also a factor and the dielectric constant is frequency sensitive (not relevant here). The capacitance of the drop is the equivalent of an ideal capacitor with a resistor in parallel

 

[DrDu]

Once I saw a nice experiment where the experimentator used a high voltage power supply and compared the effect with and without an electric shield on the outlet of the water pipe, so that drops could form in field free space. The deflection of the drops was gone once the shield was employed which lead the experimentators to the conclusion that it is due to influence when the water droplets are formed and not due to the dielectric properties of the water drops. With a jet of water instead of droplets, influence should be even more important. Nevertheless, I tried to estimate the relative size of the two effects. When a water droplet of radius r forms at height l and transversal distance d < l over a point charge q, the influenced charge is of order of magnitude $qr^2/l^2$. The relevant maximal force will act on it when it is at the same height as the charge, so that the distance is d. Then the force will be of magnitude $F_i \sim qr^2/(l^2 d^2)$. On the other hand, the dielectric constant of the water droplet (epsilon=81) is very large. If we set it to infinity, the droplet behaves like a metallic sphere whose dipole moment can easily be calculated. The maximal force on the drop due to the charge-induced dipole moment interaction is $F_d \sim q r^3/d^5$. So their ratio is $F_d/F_i \sim r l^2/d^3$. In experiments where $d \approx l$, the dipole force is smaller due to $$r \ll d\approx l$$. This seems to be the case, when a rubber globe is used as charge. In cases, where a charged rubber rod is used, d is considerably smaller than l, and both effects may be important. It would be interesting to do this experiment with an AC high voltage charge source of few kHz, as the deflection of charged drops should vanish in the AC field, while the interaction with the induced dipoles should remain unchanged.

 

[sophiecentaur]

I've always assumed they would. I doubt that many of the demo's we've seen involve deionised water. I guess a similar experiment could be done with falling ballbearings or metal powder, which would be an extreme case.

I have to wonder about that (re-reading it after years).
The charging by induction involves two steps. You have to polarise the object and then earth part of it, to make one charge flow away then you have to disconnect the earth connection. The drops carry a permanent net non-zero charge once they have separated. This could be hard to do with iron filings or at least it would all be different. If you induce a polarisation of a mass of filings, in contact with each other, (say in a funnel) the ones exiting the bottom of the funnel would fly apart and not form drops like water.
I'm reminded of a technique of spray painting which keeps charged individual tiny droplets apart and they are then attracted 'round corners' as they approach the target, giving a good uniform covering.