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#1
11-20-2006, 09:53 PM
 jekch85 Junior Member Join Date: Nov 2006 Posts: 6
electric fields

Hi all. I'm new here and I'd appreciate any help I can get!

1) In a trial of Millikan's oil drop experiment, we find that the electric field required to suspend a tiny oil particle with mas s 1.46x10^014 kg is 2.98x10^5 N/C upward. We can infer that the

a) is missing three electrons
b) is missing two electrons
c) has three extra electrons
d) has two extra electrons

I determined that the particle has a charge of 4.8x10^-19 instead of 1.6x10^-14. I just can't figure out if they are missing or extra. I assumed extra, but I think I'm missing something. Any help is very much appreciated!!
#2
11-20-2006, 10:50 PM
 OfficeShredder Senior Member Join Date: Jun 2006 Posts: 191

First, if the electric field points upwards, and the force on the oil drop is upwards (to fight gravity), does that mean the oil drop has a positive or negative charge?
#3
11-20-2006, 11:21 PM
 jekch85 Junior Member Join Date: Nov 2006 Posts: 6

Well, the electrons have a negative charge, but I guess the oil droplet would have a positive charge because the electric field has a positive charge. Since the number increases in negative charge, would this mean there are extra electrons pulling it downward?
#4
11-21-2006, 12:31 AM
 OfficeShredder Senior Member Join Date: Jun 2006 Posts: 191

Quote:
 Well, the electrons have a negative charge, but I guess the oil droplet would have a positive charge because the electric field has a positive charge. Since the number increases in negative charge, would this mean there are extra electrons pulling it downward?
Electrons have a negative charge. So if you have a neutral oil droplet, and remove an electron, does the new oil droplet have a positive or negative charge?

I don't understand the second half of your post at all
#5
11-21-2006, 12:41 AM
 jekch85 Junior Member Join Date: Nov 2006 Posts: 6

I don't really think I understand it either.

If you remove an electron from the oil droplet, it now has a positive charge. So..that would mean that there are extra electrons, making the oil droplet more positive?
#6
11-21-2006, 12:44 AM
 OfficeShredder Senior Member Join Date: Jun 2006 Posts: 191

I don't understand this idea about removing an electron giving extra electrons. Here's how it works

You have an oil droplet. It has an even number of protons and electrons, say 10 of each. You then remove 2 electrons. It has 8 electrons, and 10 protons, so its electric charge is equivalent to having 2 protons. Where the other 2 electrons went is irrelevant for the problem
#7
11-21-2006, 12:47 AM
 jekch85 Junior Member Join Date: Nov 2006 Posts: 6

I don't think it's irrelevant, the question is asking if there are missing or extra electrons. What I am trying to figure out is which one it is.
#8
11-21-2006, 02:30 PM
 OfficeShredder Senior Member Join Date: Jun 2006 Posts: 191

We already answered the question. The oil drop has a positive charge. Hence, it is missing electrons. If it had extra electrons, it would have a negative charge

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