autobiography

Topics: Electron, Oil-drop experiment, Elementary charge Pages: 6 (863 words) Published: May 15, 2014
Millikan Oil Drop Experiment

Purpose
The purpose for this lab is to find the terminal velocity, mass, and radius of the oil drops to find the charge of an electron. Hypothesis
The prediction for this investigation is that an oil drop entering the space between two plates would be affected by the uniform electric, gravitational field and the viscous drag. The oil drops would reach terminal velocity without applying any voltage. So the mass of the oil drop, radius and from the radius the volume of the oil drop can be calculated from the terminal velocity when there is no electric field applied. Once the electric field has applied the oil drop would be standing still when the net force is zero or change its direction. So by applying an electric field we can calculate the charge of an electron which is close to Millikan’s observation. Also it shows that the charge of an electron found by Millikan is credible value and is accepted worldwide. Variables

The major variables in this lab are the volume of the oil drops and their acceleration before reaching the terminal velocity. It is also essential to consider the movement of the oil drops as they are going to stop and their net force would be zero. It is not always completely zero so we have to consider the possible errors. Experimental Design

This is an experiment on the website that is a virtual experiment which is provided by a computer program. In the program oil drops can be sprayed through and atomizer and different voltages can be adjusted to catch a drop. Also the button x-ray can be turned on and turned off to ionize drops. The visual represent one’s view through microscope.
Materials
The only important material used for this lab is the web site which has the animation to catch the oil drop. http://webphysics.davidson.edu/applets/pqp_preview/contents/pqp_errata/cd_errata_fixes/section4_5.html Calculator for calculations.

Procedure
1. Go to the website and make sure the animation works on your device. 2. Click on the spray button to release the oil drops.
3. Choose an oil drop and try to catch it.
4. Use the stop watch and notice the time and distance travelled between the two points of the chosen oil drop. 5. Choose a voltage and click on the Voltage On button to suspend the same oil drop in air. 6. Click on the x ray on button.

7. Record the time taken and the distance travelled between two points by the oil drop. 8. Charge the drop is calculated.
9. Repeat the experiment for four more time with different voltages.

Observations and Analysis
After isolating an oil drop and considering the time and the voltages and by knowing the η = 7.25×10-6 Ns/m2, ρ = 875 kg/m3 from the website we can calculate the mass and the radius of the oil drop. The website also provides the distance between to plates which is 6 mm. Trial

Distance(m)
Time(s)
Voltage(Volt)

1st
1mm
12.3 s
112 V

2nd
1mm
12.5 s
39 V
3rd
1mm
13.2 s
20 V
4th
1mm
12.6 s
32 V
5th
1mm
13.3 s
110 V

The terminal velocity of the oil drop can be calculated by the equation V= Distance/ Average Time taken. So, the terminal velocity is 7.8369 x 10-5m/s
Once we find the terminal velocity so we can find the radius of the oil drop by the equation r = √(9ηv/2ρg) r=√(9(7.25 x 10-6 N*S/m2)( 7.8369 x 10-5m/s) / 2(875 kg/m3)(9.81N/kg)) r= 5.39 x 10-7m.

By finding out the radius, the mass can be find out as well. m = (4/3) πr3ρ
m = (4/3) π (5.39 x 10-7m)3(875 kg/m3)
m=5.73774 x 10-16 kg
Now it is possible to find the charge of the electrons:
#
Charge (C)
#
Charge (C)
1
q=
q=((5.73774 x 10-16 kg)(9.81 N/kg)(0.006m))/112v
q=3.0153 x 10-19c
4
q=
q =((5.73774 x 10-16 kg)(9.81 N/kg)(0.006m))/32v
q=1.0553 x 10-18c
2
q=
q=((5.73774 x 10-16 kg)(9.81...
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