Physics307L:People/Rivera/Notebook
| Mondays |
Millikan Oil Drop Experiment
Purpose
The purpose of this experiment is to find the charge of an electron, using the charge on a drop of oil drop. We assume that this charge is a multiple of a constant that is equated with the charge of one electron.
Equiptment and Setup
The setup can be found in the Pasco manual for the Millikan Experiment
Data Collection
We found that it takes a few practice runs to get the hang of timing the drops correctly. Sometimes the drops don't do what you want them too. They will jump the wrong way when an electric field is applied. After a few tries me and Brian got the timing down and began to be able to manipulate the drops better. After we got it down we seemed to get better numbers and more measurements per drop.
Our data from the first day and the second day show that as we went along we were able to get more numbers per drop.
Calculations
Here are our | calculations. I had to upload them to my web page because for some reason every time I tried to upload to wiki I got an error. Below are our numbers the formulas and data can bee seen on our | calculations page.
Values need for calculations
From Bradley's page
Known (given to as many significant figures as are reasonably certain):
- [math]\displaystyle{ d=7.59\times 10^{-3} m }[/math] (plastic spacer width using micrometer)
- [math]\displaystyle{ \rho=8.86\times 10^2 \frac{kg}{m^3} }[/math] (density of oil given on bottle)
- [math]\displaystyle{ g=9.8 \frac{m}{s^2} }[/math] (gravitational acceleration)
- [math]\displaystyle{ b=8.20\times10^{-3} Pa\cdot m }[/math] (some stupid constant)
- [math]\displaystyle{ l=1.0\times10^{-3} m }[/math] (length droplet will be measured over)
To be found when taking data:
- [math]\displaystyle{ p }[/math] (air pressure in Albuquerque. Changes by day will be read each day data taken)
- [math]\displaystyle{ T }[/math] (temperature from thermistor in °C)
- [math]\displaystyle{ V }[/math] (Voltage between plates in viewing chamber in volts)
- [math]\displaystyle{ t_f }[/math] (time droplet takes to fall in no field in seconds)
- [math]\displaystyle{ t_r }[/math] (time droplet takes to rise in field in seconds)
To be calculated later:
- [math]\displaystyle{ \eta }[/math] (viscosity of air as a function of T found in a table in Pa*s)
- [math]\displaystyle{ v_f=\frac{l}{t_f} }[/math] (average velocity of oil droplet falling in no field in m/s)
- [math]\displaystyle{ v_r=\frac{l}{t_r} }[/math] (average velocity of oil droplet rising in a field in m/s)
- [math]\displaystyle{ a=\sqrt{\left(\frac{b}{2p}\right)^2+\frac{9\eta v_f}{2g\rho}}-\frac{b}{2p} }[/math] (radius of droplet in meters)
- [math]\displaystyle{ q=\frac{4}{3}\pi\rho g d\frac{a^3}{V}\frac{\left(v_r+v_f\right)}{v_f} }[/math] (charge of oil droplet in Coulombs)
Final Numbers
| Measurment # / | q (per electron) |
|---|---|
| 1 | 1.60183E-19 |
| 2 | 1.60013E-19 |
| 3 | 1.60054E-19 |
| 4 | 1.77331E-19 |
| 5 | 1.93057E-19 |
| 6 | 1.7327E-19 |
| 7 | 1.84478E-19 |
| 8 | 1.20668E-19 |
| 9 | 1.68785E-19 |
| 10 | 1.7939E-19 |
| 11 | 1.5606E-19 |
Mean 1.66663E-19
Std. Dev. 1.92246E-20
- [math]\displaystyle{ s = \sqrt{\frac{1}{N-1} \sum_{i=1}^N (x_i - \overline{x})^2} }[/math]
Std Error 5.79644E-21
- [math]\displaystyle{ SE = \frac{s}{\sqrt{N}} }[/math]
Accepted Value 1.60E-19
Exp diff (|accepted-mean|) 6.66E-21 (within our standard deviation)
Relative Error (|accepted-mean|/|accepted|) 4.16%
Lessons Learned
First thigh I have learned is not to throw out data. I thought that our data from the first day wasn't very good but after doing the analysis those numbers seem to fit quite well with all the rest of our data and give us a better argument for our number since we have more numbers to work with making our estimate better. I was very excited when I saw the results from our data.
It took a lot of practice to get our data collection down but once we got it there were not many problems. We did find that we had to clean out the apparatus after each set of drops and sometime couldn't get usable drops into the viewing area each time. It took patience to get it right and to keep a drop in place doing what we wanted it to do.
Speed of Light Lab Summary
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From left: power supply for photomultiplier, delay box, TAC (time amplitude converter)
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Power supply for LED light emitter.
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oscilloscope
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On top tried different time delay to try and get better signal.
Setup
see comment
Start with cardboard tube with photomultiplier in one end and LED emitter in the other. Connect DC power supply (150V-200V) to the LED emitter. Connect BNC cable from the LED emitter to the start in the TAC. Connect DC power supply (1800V-2000V) to the photo multiplier. Connect photo multiplier to the time delay input. Connect from photo multiplier output to stop on the TAC. Connect TAC output to oscilloscope.
Procedure
We spent the first day setting up the experiment and testing the individual parts to make sure they were working. We were unable to get any data that day but did get some ideas and things to think about for the next week.
Before the next Monday we looked at Anne's Notebook and got some ideas from what they did on Wednesday.
On Monday we began taking data but noted a problem with the readings we were getting from the scope. We tried switching out the time delay but were still getting the anomalies. Next instead of using a delay we added more BNC cables to create a delay. After this we got better numbers but they were still fluctuating quite a bit.
We averaged the oscilloscope and took average readings using the peak output from the photomultiplier (this is achieved by rotating the multiplier to get the right polarization of the light) and averaging the reading from the scope. There were a lot of problems with this method as it was dependent on our ability to correctly turn the multiplier and taking a visual average from the scope. We also tried splitting the output from the multiplier into channel 2 on the scope but still got inconsistent numbers with what was expected.
Given these numbers we determined the speed of light to be **2x10^8 m/s (+/- 1x10^8)** as the mean of our readings.
Notes and Charts
Problems and Issues
see comment
The first problem we had was the setup. We spent a lot of time trying to locate all the parts need for the experiment. This ended up taking most of the first day.
Next it seemed that some of the equipment wasn't working correctly and had to change some of it out and just live with some of it.
It also seemed that we were getting some feedback from the LED signal that seemed to be causing the fluctuations in our data. Even with terminators the stream was quite erratic.
In averaging the oscilloscope we add in another discrepancy to our data not really knowing how accurate the average is at only 128 cycle averaging. If we could of gotten a reading every tick in a file, and averaged the data ourselves on a spread sheet, I would have felt better about the numbers.see comment
Oscilloscope Lab Summary
- Brief summary of what you did (linking to the lab manual page is OK) Lab
- Add links to all wiki pages that contain your notebook entries. This is likely only one page. Lab
- Report your value for the fall time using AC coupling.
fall time = 57.48ms
- Include error bars!
see comment
should have taken multiple measurements over time the used mean square for value and std deviation for error
- Explain how you measured this (briefly)
This was measured by using the oscilloscope's measurement and selecting "fall time"
- What did you learn?
see comment
I will send you an email with your grade for this lab.
- It's OK if you didn't learn anything. But include things you are still confusing.
I learned a lot about how to read the scope and to make measurements using the tools on the scope. I think it will help when I am doing labs and have to use it to measure data. I have used a multi-meter extensively at work but not an oscilloscope. I am still not full sure how I might use it for measuring values from the actual labs, but think that this will come as the lab progresses.
- What did you explore outside of the standard lab procedure? Anything interesting?
Didn't have time to do more would like to have more time to play. I may pull one out at work and play around a bit when I get time.
- What could make the lab better next year?
shorten to leave more time for play.
