# Physics307L:People/Giannini/Millikan

Purpose

• To use an electric field to determine the charge on an electron.
• To understand the operation of the Millikan Oil Drop experiment and the difficulties associated with it, as well as how to resolve these difficulties.

Summary This experiment was very annoying, in that the particles would either disappear or spontaneously change charge within 10 measurements of the particle. Our procedure can be found here and our data can be found here. My partner was Richard

Charge of an Electron

• Accepted Value: $\displaystyle 1.60 \cdot 10^{-19}$ C
• Droplet 1: $\displaystyle 3.3196 \cdot 10^{-17}$ C
• Droplet 2: $\displaystyle 6.7295 \cdot 10^{-16}$ C
• Exposed Droplet: $\displaystyle 4.7932 \cdot 10^{-16}$ C

Obviously the error in this experiment was quite large, as I do not believe that we had droplets with 100-1000 usable electrons.

Error

Our error is +/- .01 for all times, +/- .1 V for all voltages, +/- .001*10^6 Ohms for all resistances, +/- 1 centigrade for all temperatures, +/- .001 for all viscosities. Other sources of error are: (i)The loss of mass and the spontaneous change in charge of our particles, (ii) fogging of the reticule while being used, (iii) reaction time of the person timing the rise and fall, (iv) possible equipment bias, (v) possible water interactions with oil droplets (since we cleaned our equipment).

Standard Deviation

Fall times (seconds)

• Droplet 1: 7.0861
• Droplet 2: 5.6139
• Exposed Droplet: 2.9223

Rise times (seconds)

• Droplet 1: 0.7684
• Droplet 2: 1.2231
• Exposed Droplet: 0.4176

Rise Velocities (m/s)

• Droplet 1: $\displaystyle 1.6298 \cdot 10^{-4}$
• Droplet 2: $\displaystyle 2.7223 \cdot 10^{-4}$
• Exposed Droplet: $\displaystyle 5.9116 \cdot 10^{-5}$

Fall Velocities (m/s)

• Droplet 1: $\displaystyle 7.9089 \cdot 10^{-6}$
• Droplet 2: $\displaystyle 1.5964 \cdot 10^{-5}$
• Exposed Droplet: $\displaystyle 4.521 \cdot 10^{-6}$

As you can see, there is an overall trend in the reduction of error as Richard and I continued to take more data. Because of this, I believe this experiment can become much more accurate as the people taking the measurements continue to do so (we decreased our standard error to $\displaystyle 1/3$ in only 30 observations, on average)

What Did I Learn?

• I learned that, the second you get your equipment working correctly START TAKING MEASUREMENTS, because although it may be working then, it may not be working next time we try to use it.
• I also learned how to take quick, accurate measurements (as I had to do so or my droplets would disappear before I could finish).

What Did I Explore Outside the Procedure?

• Not much, just stared at the particles we saw and adjusted the focus to see them better.

Citings