Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thomson's cathode ray

  1. Oct 11, 2014 #1
    Hi! I've been watching this video http://ocw.mit.edu/courses/chemistr...l-science-fall-2008/video-lectures/lecture-2/ on the discovery of electrons , and I have some doubts about it.
    The lecturer explains the cathode ray experiment performed by J.J Thomson.
    This is what I understood. The cathode ray is produced between a cathode and an anode. The anode has slit through which the ray passes through. Then a difference of voltage is applied by two plates on the side of the tube and the ray is deflected and hits the detector.So Thomson concludes that the ray is composed of negative charged particles which are much smaller than the atom, the electrons. In the lecture, it is also said that something else hits the detector, another part of the beam and that is deflected in a much smaller angle and in the opposite direction,the positive charges.
    So,what is the cathode ray made of? It is suppose to be made of electrons, but according to the lecture there is also this positive charge that also gets deflected.
    This experiment proves the existence of the electron, but doesn´t it also proves the existence of the proton?. Why did Thomson conclude that the electrons are particles(plum) and the positive charge is a pudding.?
    Thanks!
     
  2. jcsd
  3. Oct 12, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Cathode rays are electrons, but those electrons can ionize atoms on their way.

    You can remove electrons from an atom, but not individual protons (not with the methods available at that time). He could not know if the hydrogen ion (which is a proton as we know now) is a single particle, or just some small amount of whatever. He also could not know that the same proton is part of other nuclei.
     
  4. Oct 12, 2014 #3
    Thanks for the answer! I still don´t understand it completely. Quote from the lecture:
    "So, Thomson didn't stop here, he actually continued experimenting with different voltages. And what he found was if he really, really ramped the voltage up between those two plates, he could actually detect something else. And what he could detect here is that there was this little spot of luminescence that he could see on the screen that was barely deflected at all -- certainly in comparison to how strongly this first particle was deflected. The second particle was deflected almost not at all. But what he could tell from the fact that there was a second particle at all, and the fact that it was in this direction, is that in addition to his negative particle, he also, of course, had a positive particle that was within this stream of rays that were coming out.

    "So, of course, he can use the same relationship for the positive particle, so delta x now of the positive is proportional to the charge on the positive particle all over the mass of the positive particle.

    So, this is interesting for several reasons. What did he manage to pull out information-wise from using these two relationships? And actually to do this, he made a few more observations. The first, which I just stated, is that the deflection of that negative particle was just far and away more extreme, much, much larger than that of the positive particle. The other assumption that he made here is that the charge on the two particles was equal.

    So, how could he know that the charge on the two particles was equal? And actually he couldn't exactly know it -- it was a very good assumption that he made, and he could make the assumption because he, in fact, did know that what he started with was this hydrogen gas. So, he was starting with hydrogen. If some negative particle was popping out from the hydrogen, then what he must be left with is h-plus, and since hydrogen itself is neutral, the h-plus and the electron had to add up to be a neutral charge. So, that means the charges of the two pieces, the positive and negative particle, must be equal in terms of absolute charge.

    So, using this relationship, he could then actually figure out by knowing, which he knows how much each of them were deflected, he could now try to think about whether or not he could make some relationship between the masses -- between the mass of the positive and the negative particle.

    So, this relationship he was looking at was starting with the deflection, and the absolute distance that the particles were deflected. So, what he could set that equal to is he knows what x is proportional to in terms of the negative particle, so that's just the absolute value of the charge over the mass of the negative particle. He could divide all of that by the absolute value of the charge of the positive particle, all over the mass of the positive particle. And as we said, he made the assumption that those two charges were equal, so we can go ahead and cross those right out. So, what that told him was if he knew the relationship between how far they were each displaced, he could also know something about the relationship of the two masses. So essentially, there was an inversely proportional relationship between how far the particles were displaced, and what the mass of the two particles turned out to be."

    So, according to this.. Thomson assumed that the hydrogen ion or proton was a part of the same atom as the electron . Which leads me to think that the beam is not only made of electrons, but it is also made of protons. If not, where did the protons come from?
    You said:
    If he removed the electrons ,all that was left was the H+ ,which was also deflected , just like the negative. There must have been something about the experiment which led him to conclude there was a negative charged particle within the atom but not a positive one.

    Thanks!
     
  5. Oct 12, 2014 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    From the remaining gas in the vacuum tube.
    Protons with their positive charge would not get accelerated in the right direction by the electric field between the electrodes.

    The fact that you can extract electrons by heating an electrode, but not positive ions?
     
  6. Oct 14, 2014 #5
    Right, I get that. So, how did the protons got accelerated and hit the detector?
     
  7. Oct 14, 2014 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Maybe from the collisions, maybe from some additional potential difference somewhere, I don't know.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Thomson's cathode ray
  1. Thomson scattering? (Replies: 3)

Loading...