DISCOVERY OF ELECTRON
In the mid-1800’s scientists wanted to know whether the atom was really indivisible. They also wanted to know why atoms of different elements had different properties. A major breakthrough came with the invention of the Crookes’ tube or cathode-ray tube. What is a cathode-ray tube and how does it work?
Discovery of electron is so attractive . . .
Everybody knows that some substances conduct electric current; that is, they are conductors, while other substances do not. But with enough electrical power, a current can be driven through any substance – solid, liquid, or gas. In the cathode-ray tube, a high voltage electric current is driven through a vacuum. The tube contains two pieces of metal, called electrodes. Each electrode is attached by a wire to the source of an electric current. We are going on the discovery of electron .
The source has two terminals, positive and negative. The electrode attached to the positive electric terminal is called the anode; the electrode attached to the negative terminal is called the cathode. Crookes showed that when the current was turned on, a beam moved from the cathode to the anode; in other words, the beam moved from the negative to the positive terminal. Therefore, the beam had to be negative in nature. Discovery of electron has not finished yet.
The German physicists in Crookes’s time favoured the wave theory of cathode rays because the beam travelled in straight lines, like water waves. But the English physicists favoured the particle theory. They said that the beam was composed of tiny particles which moved very quickly – so quickly that they were hardly influenced by gravity. That was why the particles moved in a straight path. Discovery of electron is still to continue.
Notice how an experimental observation led to two different theories. Crookes proposed a method to solve the dilemma. If the beam was composed of negative particles, a magnet would deflect them. But if the beam was a wave, a magnet would cause almost no change in direction. Particles would also be more easily deflected by an electric field. The discovery of electron is about to come to the end.
In 1897, the English physicist J. J. Thomson used both these techniques – magnetic and electric – to show that the rays were composed of particles. Today we call these particles electrons. (The term electron was suggested by the Irish physicist George Stoney, in 1891, to represent the fundamental unit of electricity.)
In 1911, a 35 young American physicist named Robert Millikan determined the mass of the electron: 9.11 x 10-28 grams. (To get an idea of how small this is, notice that minus sign up there in the exponent, and think of all the zeros we would have to put before the 9 if we wrote the entire number as a decimal.) OK. The discovery of electron has been completed. But of course not at all . . .
Next, someone had to prove that the electrons weren’t coming from the electricity, but were being given off by the metal electrodes. Proof that metals do give off electrons came from the laboratories of Philipp Lenard, a German physicist. In 1902, he showed that ultraviolet light directed onto a metal makes it send out, or emit, electrons. This effect, 45 known as the photoelectric effect, indicated that atoms contain electrons.
You should continue for the discovery of electron on your own.
A. Mark the statements as True (T) or False (F).
1. If there is sufficient electrical power, even a solid or liquid may conduct electricity.
2. J.J. Thomson named the electron.
3. In the number 9.11×10-28, the -28 tells us how many zeros to add before the number.
4. Photoelectric effect has shown that all elements contain electrons.
B. Mark the best choice.
1. Lines 4-5, “A major breakthrough came with the invention of the Crookes” tube, or cathode-ray tube’ can be rephrased as
a) “The invention of the Crookes’ tube was a successful development.”
b) “The invention of the Crookes’ tube prevented people from learning more about the cathode-ray tube.”
c) “The cathode-ray tube was invented by Crookes.”
d) “Despite the invention of the cathode-ray tube, a lot about atoms remained unknown.”
2. Line 19, ‘favoured’ means
a) supported b) proved c) acquired d ) resisted
3. Line 26, ‘dilemma’ means .
a) the difficulty of making an experimental observation
b) the basis of most experiments in physics
c) the problem of making a choice between two theories .
d) the technique used for moving particles in a straight path
4. Line 27, ‘deflect’ means .
a) turn into iron c) electrify b) cause to change direction d) make photoelectric
5. The function of this passage is to .
a) describe how a cathode tube works
b) show how the electron and its properties were discovered
c) show the reader the superiority of German scientists
d) inform the reader about the contributions of Robert Milükan