Download The discovery of the electron Thomson (1897) Let`s take into

The discovery of the electron
Thomson (1897)
Let’s take into consideration a long bulb containing a gas that can be gradually pumped out.
to the vacuum pump
If we apply an electric tension between the end-sides of the bulb, we’ll see a luminescence.
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to the vacuum pump
This luminescence will change as the gas pressure goes down. Finally we see nothing else than a
little green spot at the end-side where there is the anode; it seems that mysterious rays, coming from
the cathode, hit the glass near the anode.
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to the vacuum pump
Crookes, who first discovered such a phenomenon in 1879 , gave it the name of “cathode rays”.
In 1897 Thomson discovered that cathode rays are made up of the same corpuscles. He built an
apparatus provided with deflecting electric and magnetic fields (such as the one that we saw in our
lab last year), and succeeded in determining the value of the ratio charge-to-mass (e/m), the so
called specific charge of what he named “electron”., the so called specific charge of what he named
“electron”.
The discovery of the X-rays
Roentgen (1895)
The luminescence that we can see when the cathode rays hit the glass or the sheet of ceramic foil is
due to the rapid deceleration that electrons suffer (they are stopped in a relatively short time). The
electromagnetic energy irradiated can be up to 3 keV (this is the energy that the electron gun
supplies to the electrons). If the energy has increased up to 30 keV the electromagnetic energy is
irradiated in the X-ray range of the electromagnetic spectrum (these rays are also invisible and
mysterious, so Roentgen, who first detected them in 1895, called them X-rays)
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The discovery of the ,, rays
Bequerel (1896)
Bequerel casually discovered some other mysterious rays coming from minerals of uranium. The X
letter was already used by Roentgen, so he thought of the Greek alphabet to name these new rays.
There are three kinds of rays and they can be discerned by their interaction with a magnetic field.
field.
-rays
Minerals of Uranium
-rays
-rays
Now we know that -rays are a flow of helium nuclei (i.e. two protons and two neutrons), -rays
are a flow of electrons, and -rays are electromagnetic waves (they are actually a flow of photons,
as we’ll see later)
The Rutherford experiment and his model of an atom
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Electrons orbit around circles whose radius is about 10-10 m . At the centre of the circles there is
the nucleus, a small sphere whose radius is about 10-15 m.
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This model permits us to understand why most of the alfa-particles pass right trough the atom:
there is a lot of empty space there! Instead, the unlikely particles which move just towards the
nucleus, suffer a very strong repulsion: they can be bounced right back!
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