1. Sep 26, 2010

### fluidistic

I've realized Millikan's experiment to determine the electron's charge.
However I have a very basic question that puzzles me. Why are some (maybe all) oil drops charged?
Basically in the lab we had a very bit of oil and with a device we blow on the flux of oil that leaves a very thin capilar and this creates lots of very, very tiny drops. Why are some charged?!!! (I've realized that some are more charged than others and some are positively charged and some are negatively charged, due to conservation of charge I guess).

2. Sep 26, 2010

### alxm

Static electricity.

3. Sep 26, 2010

### Sakha

There was an x-ray source ionizing the air, which lost electrons and the oil drops gained them, becoming negatively charged.

4. Sep 27, 2010

### fluidistic

What do you mean by that? Static electricity is responsible for the charged drops? Can you explain further please?

Actually in my lab there wasn't anything of x-rays and the oil was neutral and in contact with the air I breathed. I'm really puzzled.

5. Sep 27, 2010

### vorcil

I think (when I did it) I used a vacuum chamber with the drop on the inside, I never blew air into anything.

A better question to ask, is do the oil droplets need to be charged or neutral when balancing the gravitational force and the electric field source? I'm not %100 certain, but I don't think the oil needs to have charge to make it move when you apply an electric field.

You'll have to wait until someone with more experience in this can help you sorry :(

6. Sep 27, 2010

### fluidistic

Ok I see. I'm still 100% sure that in my experiment the oil was in direct contact with the air I breathed. When I pressed on the pump to produce the drops, most of them would just fly in the lab (with my eyes I could only see a "cloud" and not the drops since they were too small), while a very tiny part of the drops would enter between the plates of the capacitor (with air as dielectric, not vacuum).

It's a good question, not sure if "better" however. :)
I do believe they need to be charged. In my experiment, some drops wouldn't almost be affected by a 600 V difference of potential while others would be more affected by this than by gravity. But as you said, let's wait for someone who has more knowledge on this.
So now it makes 2 questions to answer!

7. Oct 4, 2010

### Sakha

Then like alxm said it's static electricity. When the oil goes through the nozzle it 'rubs' with it, gaining charge. But again, I'm pretty sure that on his experiment Milikan did use an x-ray source. I'm not the expert you were waiting for but I tihnk thats it.

8. Oct 7, 2010

### fluidistic

Yes. I asked to a professor (not the lab one though) and he told me he believes that the drops get their charge from friction with the air. And he said that Millikan indeed used an extra "ionazing" source. I guess it gives better results with even a tiny voltage (lesser than 500 V) between the plates.

Thanks guys, problem solved. I'll now start a question arising from this answer, in a new thread.

9. Oct 7, 2010

### fluidistic

Thunders storm, rain drops and Millikan's oil experiment related question

I've basically made Millikan's oil experiment. We "injected" very tiny oil drops between 2 plates of a capacitor. Some drops got negatively charged, other got positively charged and some others weren't charged. Friction due to air caused the drops to be charged.

So I guess that tiny drops from rains also get charged, and probably even larger drops. When there's a thunders storm, I think there's an enormous voltage (compared to the 600 V I had in the lab) between the ground and the clouds. So why don't wee see approximately half the drops going up and the other half going down faster than the ones that aren't charged?

I find it really hard to believe that oil drops are very easily -on not even 1 cm of trajectory in air- charged while rain drops falling sometimes more than 1 km aren't charged at all.

And if it's not too much asked (I could do some research before asking this but... kind of lazy but I'll do it anyway), are thunders all of the same color? If so, then I guess photons are emitted due to some atomic properties. If not, why not? And if so, why exactly? What's the mechanism of light emission in thunders?

10. Oct 7, 2010

### K^2

It's technically static electricity, but it's not caused by rubbing of anything against anything else.

You are taking a continuous fluid and breaking it up into little drops. Now, on average, oil is neutral, but because of the heat energy, electrons don't just sit around in their assigned places. There is a bit of randomness to their distribution. When you break oil up into drops, you don't necessarily end up with the same number of positive charges and negative charges in each drop. Just due to random fluctuation a few electrons that should be in this drop could have ended up in another drop. Hence, charged drops.

And yes, very often, the setup does involve a radioactive source. It makes it easy to randomly charge the drops. But it's not necessary. Even without radioactive source, you'll still have some charged drops.

Oh, and whoever mentioned vacuum, Milikan Oil Drop relies on viscosity of air. You cannot do it in vacuum. You can do it in some gas other than air, if you want, but vacuum won't work.

11. Oct 7, 2010

### K^2

Re: Thunders storm, rain drops and Millikan's oil experiment related question

The drops are too large and heavy for their small charge to be affected by the electric field. I suspect, there might be some very tiny drops that do indeed "fall" the wrong way.

And you could have posted in your other Milikan thread. No reason to have two of them.

12. Oct 7, 2010

### fluidistic

Thanks a lot, that makes sense to me.
Yes, that's how I did my experiment, without radioactive source.

Yeah, all our apparatus was immersed into air.

13. Oct 7, 2010

### fluidistic

Re: Thunders storm, rain drops and Millikan's oil experiment related question

Thanks once again. I think I'm done for now. I will do the research on my question in the last paragraph.
(with respect to my other thread: dully noted and will do something to merge the threads).

14. Oct 7, 2010

### K^2

Re: Thunders storm, rain drops and Millikan's oil experiment related question

I missed that last question, but you might be interested to take a look at these topics:
Spectral Lines

These are two factors that determine the color of the glow of just about anything. Which one dominates will depend mostly on temperature.

15. Oct 7, 2010

### fluidistic

Re: Thunders storm, rain drops and Millikan's oil experiment related question

Thanks again.
A small doubt: What about the electrons being accelerated while moving from ground to clouds, thus emitting EM waves (why not emitting in the visible spectra)? Is this covered by any of the 2 effects you posted? It's an EM radiation, but I don't think it can be called as thermal radiation, although I'm not 100% sure. It wouldn't be related to a spectral line, I believe. What do you think?

16. Oct 7, 2010

### Staff: Mentor

(merged two similar threads at fluidistic's request)

17. Oct 8, 2010

### K^2

Re: Thunders storm, rain drops and Millikan's oil experiment related question

Well, technically, ANY EM emission is due to charge acceleration, so sort of, but it tends to be acceleration associated with various oscillations.

As far as continuous acceleration, the only objects I am familiar with that produce visible or higher energy EM radiation are particle accelerators, neutron stars, and black holes. I might be wrong, though.

At any rate, I'm fairly certain that charge acceleration in lightning will be responsible for RF noise only.