-
2147 - Ernst Rutherford, discovers the atom. - In 1908 Rutherford devised the experiment to
fire alpha particles at a sheet of gold foil that was only 1/50,000 of an inch
thick. Alpha particles. Beta particles,
Cathode Rays and Gamma Rays were early names given to these mysterious
particles when they were first discovered.
-
-
-
---------------------------------- 2147 - Ernst
Rutherford, Discovers the atom
-
- Ernst
Rutherford was a Kiwi. His grandfather
migrated from Scotland to New Zealand in 1842.
Rutherford was also the physicists that discovered the structure of the
atom. The atom is only 10^-10 meters
thick. That is 0.000,000,0001 meters in
diameter. Rutherford discovered that the
nucleus of the atom was 1/10,000th that size. Here is how he did it:
-
- He started as
the 2nd child out of 12 children growing up on a farm in New
Zealand. He worked hard in school and
when he was accepted to college he proclaimed, “ I have dug my last
potato.” He went to Cambridge
University in England in 1895.
-
- He went to work for J.J. Thomson who was the
inventor of the Cathode Ray Tube. The
CRT is what all our TV’s and computers used for a display before the flat
screen liquid crystal display was invented.
-
- J.J. Thomson
was using the CRT to study the cathode ray, which was a stream of
electrons. He shot the electrons through
the tube at a phosphorus screen and ran them through a magnetic field to see
how much they were deflected with their negative charge. From the data he collected he determined that
the electron was 1/1836 the mass of the atom, or ion, which were atomic nuclei.
-
- He got the
Nobel Prize in 1906 for his discovery.
This idea must have had quite an influence on Rutherford’s thinking
because he devised a similar experiment to determine the structure of the
atom. At the time J.J. Thomson and other
scientists thought the atom was similar to a very small ball of positively
charged pudding full of negatively charged raisins. This is how science visualized the atom. The proof was in the pudding.
-
- In 1908 Rutherford
devised the experiment to fire alpha particles at a sheet of gold foil that was
only 1/50,000 of an inch thick. Alpha
particles. Beta particles, Cathode Rays and Gamma Rays were early names given
to these mysterious particles when they were first discovered. Beta particles and Cathode Rays turned out to
be electrons. Alpha Particles turned out
to be the ions of helium atoms.
-
- Ions are
atomic nuclei that have lost their electrons.
In the case of helium the nucleus is two positively charged protons. So, I will refer to Alpha Particles as helium
nuclei. Rutherford is firing these
helium nuclei at the gold foil and to his amazement most all of them passed
straight through the foil and hit the photographic on the other side. The gold foil was about 2,000 atoms thick so
he had to conclude that the structure of the atoms was mostly empty space since
most of the time the helium nuclei did not hit anything.
-
- However, a
few of the helium nuclei did scatter.
And, some even bounced straight backward. This was totally amazing to Rutherford. He said it was like firing a canon ball and
having it bounce right back at you. From
this he had to conclude that somewhere inside the atom there was a massive
positive electric charge that repelled the positively charged helium nuclei
with a tremendous electric force.
-
- He announced his theory in 1911 that atoms
were made up of a massive nucleus of positive charge in the center with very light electrons orbiting the nucleus. Most of the atom being empty space. This theory is how most of us visualize the
atom today.
-
- Rutherford died in 1937 three years before I
was born. He was buried in Westminster
Abbey in London next to J.J. Thomson, Isaac Newton, and Lord Kelvin. Kathy and I visited his grave in 1967. Debbie was home with our English nanny
babysitter. Debbie was one year old.
-
- Now comes the
math, sorry. We can calculate the size
of the atomic nucleus using the data from Rutherford’s experiment. It is fun to do. We will use the formulas for Kinetic Energy,
Coulomb’s law for electric charges and Newton’s 2nd law of motion:
-
- A helium ion
is two protons. Each proton has a
positive electric charge and a mass of 1.67*10^-27 kilograms. The Kinetic Energy is mass times velocity
squared.
-
------------------
K.E. = ½ m*v^2. (See note 1)
-
- The velocity
of the helium ions fired at the gold foil was 34 million meters / second.
(See note 2) Therefore, the Kinetic Energy is 10^-12 joules
of energy for each ion.
This is the
energy of each helium ion that is entering the thin wall of gold atoms.
-
- The gold
atoms have an atomic weight of 80, therefore, each gold atom has 80 protons in
the nucleus. When the 2 helium protons
approach the 80 gold protons they are repelled due to their like charges. And, if they are repelled straight backward
at the same velocity then the change in their momentum is 2* mass*
velocity.
-
----------------------The Change in Momentum = 2*m*v.
-
- The force
that caused that repelling comes from Coulomb’s Law for electric charges. The force is directly proportional to the
amount of the electric charges, “q“.
And, inversely proportional to the square of the distance between them:
-
----------------------- Electric Force = k*q*q / d^2.
-
- “k” is the
constant of proportionality for the units we are using in the equation which is
9.10^9 kilograms * meters^3 / seconds^2 * Coulombs^2. The charge on each proton is one electron
charge which is 1.6*10^-19 Coulombs. The
charge on the helium ion is therefore 3.2*10^-19 Coulombs and the charge on the
gold ion is 128*10^-19 Coulombs.
-
----------------------
The total Electric Force must be 3.7*10^-26 / d^2.
-
- “d” is the
distance we want to calculate. And, “d”
is roughly the size of the gold atom nucleus.
The thinking is that the largest possible force that repelled the helium
nucleus backward must be when the protons approached each other at the minimum
possible distance, “d“. “d” is the
diameter of the atomic nucleus.
-
The force going in is the Momentum Force of the helium
ion and the force sending them back out is the Electric Force of the like
positive charges of the helium ion and the gold nuclei ion. These two forces must be equal and opposite. Action = Reaction.
-
- We have calculated the Electric Force. To calculate the Momentum Force we need to
use another definition for a force. A
force is defined as a change of momentum over time. We have already calculated
the Change in Momentum = 2*m*v.
-
-------------------------- Momentum Force = 2*m*v / t
-
-------------------------- Since distance = velocity * time, t = d / v
-
------------------------- Momentum Force = 2*m*v^2 / d
-
- Lastly we set
these two forces equal to each other:
-
------------------------ Electric Force =
Momentum Force
-
----------------------- 3.7*10^-26 / d^2 =
2*m*v^2 / d
-
----------------------- d
= 3.7* 10^-26 / 2 *
m* v^2
-
- From our
first calculation for the helium ions, ½ m*v^2
= 10^-12 joules.
-
----------------------- d
= 3.7*10^-26 / 4 *
10^-12
-
----------------------- d
= 10^-14 meters
-
- The diameter
of the atom is 10^-10 meters.
-
- The diameter
of the nucleus of the atom is 10^-14 meters.
-
- Therefore,
the nucleus is 10,000 times smaller than the size of the atom. This would be like a softball in the center of
a football stadium. The softball would
be the nucleus on the 50 yard line and the electrons would be circling the
field. The electrons would be 1 / 1,836
the size of the softball, or about the size of a grain of salt.
-
-------------------------------------------------------------------
-
(1) We can
derive the formula for Kinetic Energy from Newton’s 2nd law of
motion stating that a force is what changes the velocity of a mass. A change in velocity is called acceleration. Therefore, Force = mass * acceleration.
-
---------------------------- F = m * a
-
- A definition
for Energy is the work used when a force operates over a distance. Energy = Force * distance.
-
---------------------------- E = F * d
-
---------------------------- E
= m*a*d
-
- Kinetic
Energy of motion is mass times acceleration times distance. We want K.E. as a function of velocity. Acceleration is the change in velocity per
unit time.
-
--------------------------- a = v
/ t
-
- Distance is
the average velocity times time. d = V
average * t. Average velocity is
½(initial velocity + final velocity). We
are assuming the initial velocity is zero since the helium protons come to a
dead stop and turn around. ( Most of
these formulas should be using differential equations but I am simplifying with
special cases to avoid calculus.)
-
--------------------------- d = ½ v * t
-
- Combining the
last two equations together we get:
-
--------------------------- a*d
= ½ v^2
-
- Substituting
into our E = m*a*d equation we get:
-
--------------------------- E = ½ m*v^2
-
- Since this is
the energy of motion we call it Kinetic Energy:
-
--------------------------- K.E. =
½ m*v^2
-
------------------------------------------------------------------------------------
(2) 34 million
meters / second is 77 million miles per hour , or a little less that 12% the
speed of light.
--------------------------------------------------------------------------------------------------
-
- October 30, 2018. 950
----------------------------------------------------------------------------------------
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--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
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--- to:
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“Jim Detrick” -----------
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--------------------- Tuesday, October 30, 2018 -------------------------
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