Dot Physics

Lab: The Charge of an Electron

Not really. Here are the details (and some data) for the Millikan Oil Drop Experiment without the oil drop that I talked about previously (originally from The Physics Teacher – lucky you, it was a featured article so it should still be available (pdf)).

The basic idea that Lowell McCann and Earl Blodgett from U of Wisconsin propose is to do an experiment similar to the oil drop experiment, but not so squinty (if you have done the oil drop experiment, you know what I mean). Instead of dropping charged oil in an electric field, they drop containers with metal nuts in water. The goal is to find the mass of a nut.

Here is what I wrote up for my lab students. After that, I will include videos with some data so you don’t have to set this up yourself. Feel free to use this and modify if you like (and however you like).

Lab: Charge of an Electron (but not really)

This is a picture of Robert Millikan. (from Wikipedia)

i-ac41e323ec3abcb4d59f235f823e0668-millikan.jpg

He measured the electric charge on the electron and this is basically how he did it – by dropping oil drops. He shot some drops of oil (very tiny) into an area with a constant electric field. The drops then moved at a constant speed in the presence of this constant electric field. Maybe this diagram will help.

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Hopefully, you remember from the last lab that the drag force depends on the speed of the object. So, at terminal velocity the electric force, the drag and the gravitational force all add up to zero. To find the gravitational force, Millikan turned off the electric force and let the drop fall. In that case, the forces looked like this:

i-95bb22a42dd0e9d7dca7a33ddbd82bd9-untitled_78.jpg

This allowed him to find the weight of the drop based on the terminal velocity (he calculated the coefficient of drag based on some assumptions). Here are the steps to his experiment:

  • Shoot in oil drop
  • Measure terminal velocity while falling to determine the mass and weight
  • Measure the terminal velocity while rising (with the electric field on) to determine the electric force
  • Use the electric force to determine the charge of the oil drop

And then something cool happened. Millikan found that all the values of electric charge on the oil drop were multiples of the same value. This is the value of the electric charge of an electron because you can only have 1, 2, 3, 4, 5, 6 … electrons on the drop of oil. You can’t have 1.34 electrons on there.

Now for this lab

We are not going to do the oil drop experiment. It really is a pain to get it to work. We are going to do something similar. Here is a container. It has some metal nuts in it (who knows how many).

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We are not going to do the oil drop experiment. It really is a pain to get it to work. We are going to do something similar. Here is a container. It has some metal nuts in it (who knows how many).

i-9a3ad8723f83c55925d1528f06cebf17-untitled_79.jpg

The buoyancy force is due to the water. Here are the expressions for the magnitudes of these forces.

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When rising, the following would be true:

i-bdea416349739bb58efb5aba3fad9dc5-la_te_xi_t_1_195.jpg

And for falling:

i-1b5648f077cd67f1390c48f0e4f34bfc-la_te_xi_t_1_196.jpg

The Plan

First, you need to determine the drag coefficient. I have some containers that do not have nuts in them. You can find the mass of these and do something similar to the air resistance lab to determine C. For uncertainty, I would just take one container and find the terminal velocity 5 times to see what kind of uncertainty you have. To measure the terminal velocity, just use a stop watch.

Next, you need to get a value for the buoyancy. I measured the volume at roughly 75 cm3, but you might want to try and get a better value for this. The density of water is 1000 kg/m3.

Now for the fun part. Take the other containers with nuts. DO NOT FIND THE MASS (that would be cheating and ruin all the fun). Drop them and measure the terminal velocity. From that, you can calculate the total mass. Do this for all the containers and see if you can group them according to mass.

Video Data

Here are the videos I made so you can do this lab without the set up. You might need to know that the volume of the container is about 75 cm3. Also, the distance between the blue lines on the tube of water is 0.5 meters. I am not sure how well the data turned out. I probably should have made sure all the bubbles of air were off the container – but I was afraid the containers would start leaking.

This first video is four known mass containers. You can use this video to find the drag coefficient. Note that I used a v2 dependent drag force. Not sure if this is the best model in this case (it might be better to use a linear drag force – but the article in the Physics Teacher used v2).

Oil Drop Lab – Finding drag coefficient from Rhett Allain on Vimeo.

This next video is for some unknown masses.

Oil Drop Lab Data II from Rhett Allain on Vimeo.

Comments

  1. #1 Fran
    November 14, 2009

    This is awesome! I would love to do this with my AP class! Thanks so much for providing awesome videos! I HATE doing the real Millikan experiment. I consider it worse than squinty–it is painful. I don’t actually have the equipment to do it anyway. And this is so much better than doing an online simulation!

  2. #2 Rafael
    November 18, 2009

    One quick question… How come the containers in Video 1 are all falling down and the ones in video 2 are going up? Were the first containers filled with stuff heavier than the nuts?

  3. #3 Rhett
    November 18, 2009

    @Rafael,

    It just has to do with the number of nuts in the container. For most of them, the total weight is less than the buoyant force so they float. I only used the four “sinkers” because I ran out of nuts.

  4. #4 syakti
    April 19, 2011

    i am interested in this lab. I’ve tried it but i got a difficulty in analizing the data. For finding N value,
    May be, you have any suggestions by grouping the data. I do hope foe your help!

    Nice to know you, Allain

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