J.J. Thomson's Cathode Ray Experiment: Discovery of Electrons

In summary, J.J. Thomson's cathode ray experiment demonstrated that the ray was composed of negatively charged particles, which he called electrons. However, he also observed a positive charge within the stream of rays, which led him to assume that it came from the same atom as the electrons. He could not conclusively determine the nature of the positive charge, but he assumed it was equal in charge to the electrons and therefore a part of the same atom. This experiment proved the existence of electrons, but it could not prove the existence of protons, which were not yet known at the time.
  • #1
Genaro
3
0
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!
 
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  • #2
Cathode rays are electrons, but those electrons can ionize atoms on their way.

Genaro said:
Why did Thomson conclude that the electrons are particles(plum) and the positive charge is a pudding.?
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.
 
  • #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:
mfb said:
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.
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!
 
  • #4
Genaro said:
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?
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.

Genaro 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.
The fact that you can extract electrons by heating an electrode, but not positive ions?
 
  • #5
mfb said:
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.
Right, I get that. So, how did the protons got accelerated and hit the detector?
 
  • #6
Maybe from the collisions, maybe from some additional potential difference somewhere, I don't know.
 

Related to J.J. Thomson's Cathode Ray Experiment: Discovery of Electrons

1. What is J.J. Thomson's Cathode Ray Experiment?

J.J. Thomson's Cathode Ray Experiment was a series of experiments conducted by physicist J.J. Thomson in the late 19th century to study the properties of cathode rays. This experiment led to the discovery of electrons as subatomic particles.

2. How did J.J. Thomson discover electrons through this experiment?

Thomson used a cathode ray tube, a sealed glass tube with a cathode (negative electrode) and anode (positive electrode) at either end. When a high voltage was applied, the cathode emitted rays of particles that were attracted to the anode. By measuring the deflection of these particles in an electric and magnetic field, Thomson was able to determine the charge-to-mass ratio of the particles, which he found to be much smaller than that of an atom. This led to the conclusion that these particles were in fact, electrons.

3. What were the major contributions of J.J. Thomson's Cathode Ray Experiment?

J.J. Thomson's Cathode Ray Experiment revolutionized the understanding of the structure of atoms by providing evidence for the existence of subatomic particles. This experiment also paved the way for further discoveries in the field of particle physics and led to the development of the modern model of the atom.

4. What were some of the challenges faced by J.J. Thomson during this experiment?

One of the major challenges faced by Thomson was the lack of knowledge about the properties of cathode rays at the time. He had to carefully design and conduct his experiments to accurately measure the properties of these particles. Additionally, the equipment used for the experiment was not as advanced as those available today, making it a difficult task to accurately measure and analyze the data.

5. How did J.J. Thomson's Cathode Ray Experiment impact the scientific community?

The discovery of electrons through this experiment had a significant impact on the scientific community. It challenged the existing model of the atom, which stated that atoms were indivisible and led to the development of the modern model of the atom. Thomson's experiment also opened up new possibilities for further research in the field of particle physics and contributed to the advancement of our understanding of the fundamental building blocks of matter.

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