Download J. J. Thomson Determined the charge to mass ratio

J. J. Thomson
Determined the charge to mass ratio of the electron
He used a A cathode Ray tube (beam of electrons)
1856–1940
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We are now ready to begin our Journey into the ATOM.
Many scientists had ideas about what the atom is made up of. They were just ideas, because
nobody actually could see the atom!
Electricity and Magnetism and gave us some ideas as to what it
might look like!
The Project
You need to start a timeline with your partner
At the end of the course you will hand this timeline in illustrating your understanding
of the history of the atom
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Dalton's Atomic Theory
We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen. His model
of the atom
John Dalton (1766­1844) His main conclusions
1. All matter consists of tiny particles.
The Atom is a Solid Sphere !
Pull
2. Atoms are indestructible and unchangeable.
Pull
3. When elements react, their atoms combine in simple, whole­number ratios.
Pull
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Dalton’s theory, it is a cornerstone for modern atomic theory.
This theory could explain the observations made by Dalton
and other chemists, but their experiments did not provide any
direct evidence that atoms actually exist. At the end of the
19th century, there was still some doubt about whether all
matter was made up of atoms. By 1900, experiments were
providing more direct evidence.
Along comes the Cathode Ray Tube and J.J. Thomson
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During the 1800s, scientists discovered that high voltage connected
at electrodes created a mysterious green beam. Here is a simple explanation of the cathode Rays
Thomson did not have a way of measuring the mass of an electron
nor did he have a way of measuring the charge of an electron. Thus he did the best he could, and he measured the
charge to mass ratio of the electron
Here is what he did
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First recall the electric force and how we calculate it
Fe = qE
Now we also have the magnetic force
Fm = qvB
Now suppose if we superimpose perpendicualar fields. That is we could set
up a magnetic field and set up an electric field, and we could make this
green beam actually go undeflected through the space. like below
++++++++
ld
e
i
F
­
e
What happens?
X X X X X
M
ag
n
eti
X X X X X
c Fie
tric
d
X X X X X
­­­­­­­­­­­­­
Fm = qvB
c
e
l
E
l
Fe = qE
or
Fe =
qV
d
Fe = Fm
put them together and make electron go straight
or undeflected
Therefore we now have a way of knowing the speed of the electrons!
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Example #1
A beam of electrons passes undeflected through a 0.50 T magnetic field combined with a 0.50 kN/C electric field.
How fast are the electrons moving?
Concept Check
What would happen if the electrons slowed down to 50 m/s?
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Now here is where Thomson was extremely brilliant. He now new
the exact speed of the electrons, so he made them go into another
magnetic field, causing them to move in a circle. He would measure
the radius of the circle with a fancy ruler, and he was able to determine
the charge to mass ratio of the electrons
Then he used our classic idea of charges in magnetic fields move in circles
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Example #2
When a beam of electrons, accelerated to a speed of 5.93 x 105 m/s, is directed perpendicular to a uniform 100μT magnetic field (directed in to the page), they travel in a circular path with a radius of 3.37 cm. Determine the charge to mass ratio of the electron.
Hint
#1
Hint
#2
Generally Thomson found electrons have q/m ratios of x 1011 C/kg
you should know that!
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Concept Check How could we make the radius of the circles bigger for an electron?
Hint
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Dalton
Thomson
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Check and Reflect 15.1
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