confused about milikan oil drop exp

[premedicine555]

Okay, so TBR says that before the oil is hit by the beam, "an electron must be added to the oil drop or the oild rop must be ionized before it is placed into the electric field." So, does the oil drop have a negative or positive charge before it is hit by the x ray beam? I thought when you ionized you had a positive charge for the element.

Also, for the capacitaors, is the first one (where the oil drops from the pore) a + and second a - charge?

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[Erythropoietin]

Okay, so TBR says that before the oil is hit by the beam, "an electron must be added to the oil drop or the oild rop must be ionized before it is placed into the electric field." So, does the oil drop have a negative or positive charge before it is hit by the x ray beam? I thought when you ionized you had a positive charge for the element.

Also, for the capacitaors, is the first one a + and second a - charge?

The oil drops are neutral before it get hits by the beam

 

[premedicine555]

Then why did TBR write that "an electron must be added to the oil drop or ionzed by impact [....] BEFORE it is placed into the electric field"?

 

[Erythropoietin]

Then why did TBR write that "an electron must be added to the oil drop or ionzed by impact [....] BEFORE it is placed into the electric field"?

Because the uncharged oil drops will be unaffected by the electric field, so they will just fall like nothing happened; they will just fall by gravity. The whole purpose is to suspend a charged oil drop into this field. Since they will be unaffected inside the electric field, the only way to charge them will be before entering the field. The purpose of this experiment is to form some type of applied force to oppose gravity, and you do this by charging the droplet by an electron beam or x-ray beam.

Once a "sufficient amount of charge" is present in an oil drop, it will fall at constant speed..and the net force will just be qE = - mg
and that "sufficient amount" came out to be 1.6 x 10^-19 C

 

[premedicine555]

Okay, so before entering the field, the droplets would be charged via an x-ray beam?

 

[Erythropoietin]

Okay, so before entering the field, the droplets would be charged via an x-ray beam?

x-ray or electron beam, either one

 

[Ssina]

yeah so the oil is not charged at the beginning, you then zap it to get an electron on it so now it's negatively charged... then it passes through the hole into the space in between the capacitor plates.

as you mentioned the top plate is positive, so to pull the oil drop upwards, and then you can change the magnitude of the electric field until the oil is floating inbtw the plates.... now qE=mg for the oil and then you can calculate the magnitude of the charge because you know everything but the q

 

[Erythropoietin]

yeah so the oil is not charged at the beginning, you then zap it to get an electron on it so now it's negatively charged... then it passes through the hole into the space in between the capacitor plates.

as you mentioned the top plate is positive, so to pull the oil drop upwards, and then you can change the magnitude of the electric field until the oil is floating inbtw the plates.... now qE=mg for the oil and then you can calculate the magnitude of the charge because you know everything but the q

Yes and it makes sense that the top plate is positive because if it was the other way around (the top being negative, and bottom plate being positive), then 1) it will be very hard to pass the charged drops through the negative plate since they both have the same charge and would repel, and 2) the charged drops would not really reach a constant velocity if the positive plate was on the bottom because both the gravity force pointing downward and the magnetic attraction from the positive plate would pull the electron downwards...

so positive plate is on top, and negative plate on bottom

 

[premedicine555]

i guess I was confused by tbr's wording. thank you both!

 

[BerkReviewTeach]

i guess I was confused by tbr's wording. thank you both!

What might be confusing is that couple versions of the experiment are presented in the book, one in the text and then the other as a variation in one of the passages. If you read about the Millikan Oil Drop Experiment historically, you'll find that the charge was attained via collision with the spout of the atomizer, resulting in oil drops taking on a negative charge. This can also be achieved when they pass through an electron stream.

But the experiment in the passage using plastic spheres and X-ray beams is based on the same theory, but instead of using negative oil drops they use positive plastic spheres of known mass.

The important thing is that you recognize that charged oil drops (or plastic balls) can be slowed and/or suspended in an electric field.

 

[ilovemedi]

Hi, i had a more technical question about milikan oil drop. I was looking http://www.fi.edu/learn/case-files/millikan/oil.html for more clarification and it discusses that the drops should be "completely spherical" -- how and why does this occur? does the gravity or ionization of the drop make the drops spherical? i know it's technical but i'm confused. BerkTechRev also mentions the "spherical-ness" of the drop..

 

[MedPR]

Hi, i had a more technical question about milikan oil drop. I was looking http://www.fi.edu/learn/case-files/millikan/oil.html for more clarification and it discusses that the drops should be "completely spherical" -- how and why does this occur? does the gravity or ionization of the drop make the drops spherical? i know it's technical but i'm confused. BerkTechRev also mentions the "spherical-ness" of the drop..

Needs to be completely spherical so the electron can theoretically be in the exact middle of the drop. If it's not a uniform shape (e.g not completely spherical), the charge won't be the same at all points. For example, it can't be shaped like this:

 

[ilovemedi]

Thanks MedPr 🙂. So it's spherical because of the ionization (electrostatic charge equal everywhere)?

 

[sciencebooks]

Hey everyone, I honestly don't remember this experiment being presented in the TPR texts. Is it something we should understand prior to entering the exam or should we be able to use our general knowledge and apply it to the passage information in order to answer the questions? Thanks.

 

[californiamed]

for this experiment:
on this website http://www.fi.edu/learn/case-files/millikan/oil.html it says "the centimeter of travel path for the oil drop was obtained from 22 cm diameter plates positioned 16 mm apart and the strength of the applied charge was about 6,000 volts per cm." so the length of the capcitator plates needs to be larger than the height between the plates (distance between them)??