Sample Calculation on Milikan's Data Analysis

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Homework Help Overview

The discussion revolves around calculating the charge on an oil drop using Millikan's data analysis, focusing on the electric and gravitational forces acting on the drop. The subject area includes concepts from electromagnetism and fluid mechanics.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to calculate the electric field and mass of the oil drop, leading to questions about the relationship between electric and gravitational forces. Some participants inquire about the appropriate values to use for gravitational force, particularly regarding the distance involved.

Discussion Status

Participants are actively engaging with the problem, exploring the relationships between forces and questioning the assumptions regarding distances in gravitational calculations. There is a recognition of the need to clarify the values used in the calculations, but no consensus has been reached on the approach.

Contextual Notes

Participants note confusion regarding the use of distances in gravitational force calculations, specifically whether to use the radius of the drop or twice the radius. The original poster has provided specific values for voltage, distance between plates, oil density, and drop radius, which frame the problem context.

realslow
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Homework Statement


Find the charge on the oil drop in Coulombs, and the number of electrons it is "missing".
Vplates= 2400 V
Dplates = 0.020 m
Oil Density = 850 kg/m3
Oil Drop Radius = .000051 m (from Stokes’ Law)



Homework Equations


E = V/d
Volume = 4/3πr3
m= density x volume


The Attempt at a Solution


\Sigma = (2400 V)/(0.020 m) = 1.2 x 105
Volume = 4/3 π(0.000051)3 = 1.7456 x 10-12 m3
m = (850 kg/m3)(1.7456 x 10-12 m3) = 1.4838 x 10-19 kg
And this is where I got to... now I know that Fe = Fg and I was thinking I could find Fg, but am stuck :( I would appreciate it if you could help me out!
 
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Sorry about the electric field equations.. they came out weird haha
 
How much is the electric force on the charged oil drop in terms of its charge? And how much is the force of gravity if the mass is m?

ehild
 
I know that Fe = Q\Sigma so if I find Fg I can solve for Q. However, I am confused as to what values I use for Fg. I know the gravitational constant and the two masses (which are equal?), but am confused about the distance between them. Should I be using the radius of the drop or twice the radius?
 
Sorry about that.. Fe = QE
 
okay I realized I just need to use Fg = mg :)
 

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