In the last lesson we said that the history of human research on electrical phenomena began from friction to generate electricity, through the study of friction electricity we have some qualitative understanding of electrical phenomena, for example, we stipulate that glass electricity is positive electricity, resin electricity is negative electricity, this provision directly led to today's electrons with negative electricity, atomic nuclei with positive electricity, and even put forward the very advanced concept of charge conservation.
But because the nature of frictional electricity is very complex, to this day we don't quite understand why some materials are more hungry for electrons than others.
So in the 18th century, people at that time wanted to conduct quantitative research on electricity, wanted to know more details about electricity, and it was certainly not possible to study friction to generate electricity. We need to find a simple, intuitive electrical phenomenon to continue to study.
Then some people think, what about the lightning phenomenon of nature? In 1752, Franklin also proved that lightning in the sky, like static electricity on the ground, was an electric current.
However, the lightning in the sky is too far away from us, and the time and place of their occurrence are irregular and difficult to control, so it is not convenient to study, so we still need to find some simple and easy-to-control electrical phenomena to study.
In 1709, the Englishman Hawksby, who found that electricity had a repulsive phenomenon, found that the air in a glass tube was pumped away, but the vacuum pump had limited capacity at that time, only pumping the air pressure in the tube to 1/60 of the standard atmospheric pressure, and then connecting the electrodes at both ends of the glass tube, and then connecting the friction power supply.
Note that the word electrode was later invented by Faraday. In later articles we will talk about electric fields and electric field forces, and we will talk about Faraday.
Hawksby saw a wonderful flash of light in the device above, and for a long time afterward, many scientists, including Watson, discovered this phenomenon.
However, at that time, people did not know how this flash came from? I only know that this phenomenon is very magical, which has aroused great interest from everyone.
In fact, this phenomenon was not explained until 1913 when Bohr published his "trilogy" paper, which we now know is that when the cathode and the anode in the glass tube are discharged, the electrons in the current hit the atoms of the thin gas in the glass tube, and from these atoms knock out its ground-state electrons, and the electrons in the upper layer emit electromagnetic radiation when they jump downwards, so the flash of light is formed. This phenomenon is actually the basic principle of the luminescence of today's fluorescent lamps and neon lamps.
However, at that time, the fact that electrons could make inert gases emit light did not make much sense, and even if people knew this principle at that time, he could not create a brilliant neon light, in fact, the most important phenomenon for human beings was the discharge phenomenon between the cathode and the anode. Why?
Because this means that we can study the current directly in the vacuum tube, the previous current is inside the conductor, you can't see it, there is no way to study, now we can put the current in a transparent bottle, we can conduct a controllable analysis of its properties in the laboratory. That's the key.
Therefore, in order to obtain pure current, we should eliminate the secondary phenomenon of electric current letting the gas emit light, so what should we do? It must be the gas in the pipe, then without the gas, there is no reaction.
Secondary
However, making a vacuum is still more challenging for human beings, I believe you have heard such a sentence "nature hates vacuum", so it was not until 1858 that we invented a vacuum pump in the true sense, which can make the air pressure in the glass tube as low as a few tens of thousands of the standard atmospheric pressure, this level.
"This is a more modern cathode ray tube"
As soon as the air pressure was low, there was a new discovery immediately, and this year, Prück of the University of Bonn in Germany found that the luminescence of the gas in the low-pressure vacuum discharge tube disappeared, but a green glow was indeed seen on the glass wall at the end of the anode.
So where do these green glows come from? Is it related to the cathode, or is it related to the anode? In 1878, the British physicist Crooks improved the vacuum discharge tube, which is what the picture below looks like.
It placed a metal foil in front of the glass tube at the end of the anode, and coated the inner wall of the end glass tube with fluorescent powder, and the shadow of the metal foil could be seen casting on the end glass tube, which fully proved that there was an invisible thing emitted from the cathode, and then hit the end glass tube, and a green glow appeared. It can also be seen that this kind of thing has a limited penetrating ability. That's why it was blocked by metal foil.
Since it is emitted from the cathode, people will call it cathode rays in the future, and this tube will be called a cathode ray tube. So the next question is, what exactly is the cathode ray?
Plück once said that cathode rays are small particles splashed out of cathode material because it saw a thin layer of cathode material on the glass tube at the end, Crookes once said that cathode rays are electrobolic ions, and Wally once said that cathode rays are negatively charged particles because it deflects cathode rays with a magnetic field.
The latter two are both British, and although they have different opinions, they have one thing in common, both are charged physical particles. In Germany, a different theory came out, and the representative figure was the famous Hertz, which confirmed the existence of electromagnetic waves in 1888.
When he applied an electric field to the cathode rays, he found that the cathode rays did not seem to be deflected, so he concluded that the thing was not charged, something similar to electromagnetic waves. In 1891, Hertz further proved that cathode rays have a certain ability to penetrate, can pass through a very thin gold leaf, and further confirmed the theory that cathode rays are electromagnetic waves.
So the Germans think it's an electromagnetic wave, the British say it's a negatively charged particle, so what is it? The key is why can this thing be deflected by a magnetic field, but not by an electric field?
It can be said very clearly that there must have been a problem with one of the experiments, and now we know that Hertz's experiments are not rigorous enough, and the reason why he cannot use the electric field to deflect the cathode rays is because the gas in his vacuum tube is not pumped clean, resulting in the weakening of the electric field force, coupled with the fast speed of the cathode rays, so he did not see the deflection of the cathode rays.
The gas is not pumped clean mainly affects the amount of charge on the charged metal plate, as we said earlier, the cathode rays can ionize the gas, then the ionized gas will be attracted to the charged metal plate, resulting in the strength of the electric field between the metal plates becomes low, so that the cathode rays cannot be effectively deflected.
Thomson was the first to deflect cathode rays with an electric field, and it was with a better vacuum pump that he deflected cathode rays, thus proving that cathode rays are a stream of negatively charged particles emitted from the cathode.
So now the problem becomes the property of this negatively charged particle, then the next method of Thomson's research is also very simple, that is, in the vacuum tube, the cathode rays are applied to the electric field and magnetic field respectively, to see the cathode rays are deflected, and then calculate the charge-mass ratio of the cathode ray particles.
Next, we will talk about the relationship between the cathode ray under the action of force and its mass, speed, flight distance, and deflection distance.
The picture above is the cathode ray tube used by Thomson at that time, it can be seen that c is connected to the negative electrode, ab is connected to the positive electrode, and there is a large potential difference between c and a, so the electron is repelled at c and flies out of the negative pole straight to the positive pole, leaving a small hole in the center of the positive electrode, so a cathode ray is formed at ab.
Cathode rays will pass through de after coming out of b, de can apply an electric field or apply a magnetic field, in short, in de here cathode rays will be subject to a force perpendicular to the direction of motion, resulting in an upward or downward acceleration.
The magnitude of the acceleration is equal to the force divided by the mass of the particle, and then we can calculate the speed that the particle obtains after flying out of the deflection zone by the time the particle flies out of the deflection zone. So how is this time calculated? In fact, the distance of the deflection zone is divided by the flight speed at the particle level.
This is understandable, which is the content of Newton's second law, when the particle goes out of the deflection region, until it hits the end of the glass tube, and the distance it flies is called the drift zone.
Since the particle has a velocity above or below in addition to its velocity in the horizontal direction, it will definitely shift its center position when it hits the end glass tube.
How is the amount of this offset calculated? It is also very simple, that is, to use the speed of the particle up or down by the time it flies in the drift zone, which we have just calculated, and the rest of the time is actually divided by the distance of the drift zone by the speed of the particle at the level.
"The pipe that Thomson used to use"
Then we calculate the distance of the particle at the end of the offset, so we put together the above formula and it is like this, "the displacement of the ray at the end" = "the force on the particle under action" × the "deflection zone length" × the "drift zone length" / the mass of the particle × the particle horizontal velocity ^2.
The formula is like this, very simple, in this formula, the distance the particle deflects at the end can be measured, it is known, the length of the deflection region and the drift region is known, it can be measured when the imprint ray tube is designed, and the unknown amount is the mass and velocity of the particle.
There is also a force that acts on the particle, and we know that in the electric field force, the magnitude of this force is proportional to the charge of the particle, so if we apply an electric field to the cathode ray tube, then the displacement at the end is related to these unknown parameters of the particle: the charge is divided by the mass multiplied by the square of the velocity.
Three unknown quantities, one equation, which is certainly not OK, so at least an equation is needed, so we add a magnetic field to the cathode rays, and the magnitude of the magnetic field force is proportional to the charge of the charged particles, as well as the velocity, so after applying a magnetic field to the cathode rays, we have obtained a relationship between displacement and charge, mass, and velocity.
Now that we have two different sets of parameter combination values, we can calculate the speed of the cathode rays, as well as the charge and mass ratio.
Therefore, it can be seen that with the mechanical content of Newton alone, we can study the important properties of cathode rays and discover the first subatomic particles.
In the next lesson, we will talk about the electric field, and the electric field force, and the deflection of cathode rays under the action of the electric field force. Then we are talking about magnetic fields and magnetic field forces, as well as the deflection of cathode rays under the action of magnetic fields. Finally we are calculating the charge ratio of the cathode ray, which is to be done step by step, don't worry.
So in the content that follows, when we talk about how the nucleus was discovered, we are talking about two other major discoveries triggered by cathode ray tubes, X-rays and radioactivity.