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B Ampere and Coulomb

  1. Sep 10, 2016 #1
    ok, so i am taking an auto electrical class this semester and i can not continue reading the book/doing the homework until i understand this..my question is this: if andré marie ampére (1775-1836) discovered the electron flow, how the hell did he do it back then? how did they discover atoms and the way they worked with virtually nothing? also, the book says: "one ampere is the movement of 1 coulomb (6.28 billion billion electrons) past a point in one second." how the hell did they measure this? i can understand now a days with the technology we have now: computers, simulators, electronics etc...but back then??? i guess the way you guys can help answer my question is by telling me how i can reproduce this finding for myself using only the tools they had in the 1800's...
     
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  3. Sep 10, 2016 #2

    Charles Link

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    The measurement of the electron charge wasn't performed until 1909 by Robert Millikan with his famous oil drop experiment for which he won the Nobel Prize in 1911. Until then, the electron charge was a complete unknown. Even the charge to mass ratio of the electron had only been known for a few years with J. J. Thomson's cathode ray tube measurements. Any observations and experiments on electrical currents prior to about 1880 were done without knowing what it actually consisted of. Also, I don't think much was known about the magnetic fields from electrical currents prior to about 1850. Thomas Edison's long lasting electric light bulb didn't even come around until 1879...
     
  4. Sep 10, 2016 #3
    Don't know how your book tells the story, but things get named after people long after they're dead usually, and it's a matter of something needing a name rather than any necessarily specific contribution by the person being honored to what gets named. Newton didn't discover or invent the Newton, neither did Faraday discover or invent the Farad.

    You can google to find out exactly what Ampere or anyone contributed. That history is interesting in and of itself. But you'll find there wasn't any direct or compelling reason to name the Tesla after Tesla, as another example.
     
  5. Sep 10, 2016 #4
    ok how was the 1909 measurment taken? and can sumbody reproduce the experiment and get the same result?..i was mainly just curious hth they came up with 6.28 billion billion when u cant visually see the electrons to measure them, like im sure they had microscopes but even to count that high of a number in one second is impossible with a microscope alone..what is the math here?
     
  6. Sep 10, 2016 #5

    Charles Link

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    We (my classmates and I) did the 1909 Millikan oil drop experiment as sophomore physics majors at the University of Illinois Champaign-Urbana in 1975. (It was one of the experiments that we did in the laboratory part of the advanced E&M course. We worked in pairs. In a class of about 20, they had about 10 such experimental set-ups.) Experimentally, my lab partner and I got electrical charges on the oil drop in multiples of 1.59 E-19 Coulombs, if I remember correctly=quite close to the 1.602 E-19 number. Most of the droplets cleared themselves upon turning the switch off and on of the voltage between the plates of a capacitor a couple of times, but several persisted, and it is these that we observed, if I remember correctly... The free fall of the oil drop was measured and the result put into a computer program=it used some mathematics involving Stokes law(for terminal velocity of the oil drop in a free fall, taking account of viscosity of the air, etc...) to determine the mass of the oil drop... Then the voltage was turned on, and an electric field from the capacitor plates (of known voltage) caused the drop to go upwards...The time was measured for the oil drop to travel a few millimeters , etc... We observed the oil drops with a microscope that had a scale on it, and as I recall, the oil drops were illuminated so that we could see them...Additional details=we sprayed the oil from a little spray bottle (atomizer) and a typical oil drop that survived switching the voltage off and on a couple of times might contain 3 or 4 or 5 electron charges... The capacitor plates, as I recall, had a separation of about an inch, and I seem to recall the voltage of the capacitor plates to be at about 500 volts, but I'm not sure of this last number...
     
    Last edited: Sep 10, 2016
  7. Sep 10, 2016 #6
    You can't see electrons with a microscope or any device. Millikan figured out a clever way to determine the charge of an individual electron. Once you know that, and that all electrons are of equal charge, then you can take any macroscopic charge, divide by the individual charge, and know how many electrons are present.

    You can google the oil drop experiment, which is how Millikan determined the individual charge, but the principle could be applied to large numbers of anything. Say you had various containers all full of different amounts of marbles of equal weight, but you can't open the containers or see into them. There is no way to isolate an individual marble and weight it. But, by weighing a lot of these containers you will eventually find that they are all multiples of some weight, which is the weight of an individual marble. Knowing that, you could determine the number of marbles in any container by its weight. Millikan was measuring charge, not weight, but since all electrons have the same charge, you can use that to count them.

    edit: I should also stipulate you do know the weight of the containers in the marble example.
     
  8. Sep 10, 2016 #7

    Charles Link

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    See also my post #5. It's rather remarkable that Millikan was able to find something that would hold a very small number of elementary electrical charges. If the number of residual elementary charges on the drops had been 100 or more, it would have been very difficult to determine what the elementary unit was. Meanwhile he also needed to know quite a lot about the atmospheric drag that the oil drop would experience=he needed to be able to accurately determine the mass of each oil drop...
     
    Last edited: Sep 10, 2016
  9. Sep 10, 2016 #8
    I have always assumed he did digest it somehow from high charges. Not that I knew how he might have done that. Your post #5 is the first time I've heard the elementary charges on the drops could be as few as 3, 4, or 5. I've just spent some time googling and no one seems to put a figure on it.
     
  10. Sep 10, 2016 #9

    Charles Link

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    Somewhere, I think I may still have the printout from that experiment. As I recall, we measured about 10 different drops, and the maximum charge was something like 8 or 9 elementary charges. I seem to recall having a couple of drops that were found to contain 2 or 3 elementary charges. (They had us feed the data that we took, of fall times and rise times of about 10 different drops for each of us, into a computer program that they had.) Our laboratory apparatus was no doubt rather optimized. I do expect on his first attempts, Millikan may have been working with larger numbers of electrical charges. In our case, the drops with the larger charges were pulled out of the mix by flipping the switch on and off a couple of times... The gravitational force on a given oil drop was in the same ballpark as the electrical force on the drop when it contained a couple of elementary charges in the electrical field of the capacitor plates... I seem to recall rise and fall times of about 10 or 15 seconds. I think the distance covered may have been about 1 centimeter or less ... I remember having to watch the drop carefully and keeping my eye on it as my lab partner flipped the switch, etc. I think we may have observed several passes of the same drop and averaged them. Then we would move on to another oil drop, etc. = Sometimes by supplying the apparatus with a fresh squeeze of oil from the atomizer...
     
    Last edited: Sep 10, 2016
  11. Sep 11, 2016 #10

    Charles Link

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    After a little digging, I was able to locate the laboratory procedure we were given (at the University of Illinois- Urbana in 1975) for the Millikan Oil Drop experiment which describes the complete theory including all the Stokes formulas they used in their computer program. I also located my original notebook with the data I took, along with the printout they gave us after we submitted the data. My lab partner and I did measurements on a total of 14 oil drops and we measured approximately 5 rise and falls on each oil drop (and averaged these for improved accuracy). ## \\ ## The distance the oil drop traveled on each of the rises and falls (it was 3 divisions of a scale we calibrated) was 1.46 mm. And I did have the plate voltage correct in the above post=it was 500 volts. The spacing between the capacitor plates was 3.2 mm. Rise and fall times ranged from 5 seconds to 30 seconds for the various oil drops.## \\ ## Of the 14 oil drops, one had 1 elementary charge (e=1.6 E-19),there were Qty. 3 of 2e, Qty. 2 of 3e, 2 of 4e, 1 of 5e, 2 of 6e, 2 of 7e, and one had 8e. (@zoobyshoe you may find this of interest in response to your post #8 above). In any case, I had it correct in the post above that the amount of charge on each drop was less than 10 elementary charges.
     
    Last edited: Sep 11, 2016
  12. Sep 13, 2016 #11
    god damn im lost lol...i need to finish skool/my major and ill come back to it haha
     
  13. Jan 4, 2017 #12
    Ampere did not discover current flow! He did, however, notice that two pieces of metal with current flowing in them would have a force between them. He was inspired by Oerstad who had found that current in a wire would move a compass (remember a compass is only a magnet on a pivot) to point in a circle around the wire. The units were designed far after Ampere did his experiments. Ampere also made a law (Ampere's law) which related how much force was produced with how much current you had in the wires and the distance between the wires. At the time, charge was not measured in Coulombs and of course, current was not measured in Amperes. If you think about it, Ampere's law is a way to measure current by how much force it produces at a certain distance, which is why it is named after him. (although, of course, many of the things have names that are not really related to the person's experiments or discoveries, politics!)

    If you are interested in the history of the electron, I highly recommend you visit the website for the Nobel Prize and read JJ Thompson's talk from 1906 - it is surprisingly easy to read. It also shows that they had an inkling what the charge was on an electron even before Milikin did his infamous oil drop experiment (infamous because it was a total pain in the ass to do :))

    http://www.nobelprize.org/nobel_prizes/physics/laureates/1906/press.html

    Hope that helps a little,

    Kathy
     
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