Download What is the World Made of?

The end of classical physics:
photons, electrons, atoms
PHYS117B
People have long asked,
"What is the world made of?"
and
"What holds it together?"
What is the World Made of ?
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Why do so many things in this world share the same
characteristics?
People have come to realize that the matter of the world is made
from a few fundamental building blocks of nature.
The word "fundamental" is key here. By fundamental building
blocks we mean objects that are simple and structureless -not made of anything smaller.
Even in ancient times, people sought to organize the world
around them into fundamental elements, such as earth, air, fire,
and water.
What is fundamental ?
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Today we know that there is something more fundamental
than earth, water, air, and fire...
The atomic idea has been around for a long time:
By convention there is color,
By convention sweetness,
By convention bitterness,
But in reality there are atoms and space.
-Democritus (c. 400 BCE)
There is a principal difference between the way the
ancient philosophers approached the world and what
scientists do.
The EXPERIMENT is the ultimate judge of any theory !
We’ll discuss some fascinating experiments that brought
the present scientific idea of atoms, nuclei, elementary
particles
The main questions in the beginning of
1800s
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What is light ?
What is electricity ?
Electric and magnetic field ?
Maxwell’s theory explained electric and magnetic
phenomena. It combined electric and magnetic field
into ONE theory of the electromagnetic field. It also
showed that light is an electromagnetic wave!
… and then in the 1900s people came to the
question of atoms (particles) again.
But accepting that light is a wave was
difficult:
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In most everyday
experiences light
behaves as rays or
stream of particles
To see the wave
properties you need
to deal with sizes
that are comparable
to the wavelength!
Wave nature of light
OK, if you use slits that have small width and
small spacing you will see it !
 Last time we used
Slit width = 0.040 mm
Slit spacing = 0.25 mm
 Just when everybody
was convinced that
light is a wave …
EXPERIMENTAL facts that
showed that light comes in quanta!
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The experiments that lead to quantum physics
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Discovery of the electron ( charge quantization).
Photo-electric effect (light is a particle)
Atomic emission and absorption lines (atomic
energy levels) – angular momentum is
quantized
The discovery of the nucleus (Rutherford
scattering experiment)
Electron diffraction ( wave-particle duality)
X-rays, radioactivity
Faraday: electrolyses and cathode rays
~1850
Electrical conduction through gases:
cathode glow, colored glow (depending on
the gas in the tube)
Faraday: Water is not fundamental. Atoms exist. Charge is somehow associated
with atoms. Electricity is not some unique type of substance. It flows through liquids
and gases.
Cathode rays
Reduce the pressure: cathode glow
extended towards the anode.
Electricity behaves as stream of
particles ( shadow of cathode glow)
Collimate the beam of cathode rays,
place a collecting electrode =>
Measure current ! Cathode rays have
charge !
Charged particles are deflected in
magnetic field
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Another proof that
cathode rays are
charged particles
Determine the sign (-)
Thomson measured the q/m for the
cathode rays: crossed field experiment
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r= mv/qB
No deflection :
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FB = FE
v = E/B
q/m = v/rB
J.J.Thompson found:
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the q/m for cathode rays
was 1000 times higher than
that of the hydrogen ion =>
either q is very large, or m is
very small !
Cathode rays do not
depend on the cathode
material
How to measure q and m separately ?
Millikan oil-drop experiment
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mg = q E
q = m g/E
m=?
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Use density of oil
Measure diameter
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D ~ 1 micron TOOO small to see
Let the drop move: (not a free fall
Measure terminal speed:
depends on radius of the drop
Charge is quantized ! Year 1906
All charges are multiples of a
certain minimal value, e
The discovery of the electron and Thomson’s
“raisin cake model” of the atom
Measured q/m for cathode
rays (1000 times larger than
for H ion)
 Charge is quantized
(multiples of e)
 Subatomic particle – the
electron
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How are electrons emitted from the
cathode ?
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Cold cathode in Crooks tube: high voltage – some
of the electrons are expelled from the cathode. The
phenomenon is now known as field emission,
which is a form of quantum tunneling in which
electrons pass through a barrier in the presence of a
high electric field . That’s how old TV’s worked
Heated cathode: thermionic emission ( discovered
by Edison in 1883). Heat the cathode => some of
the electrons have enough thermal kinetic energy to
overcome the attraction from the nuclei.
BUT: you could get electrons out of the cathode if
you shine light on it!
Photo-electric effect: year 1900
Hertz discovered:
Phillip Lenard (Hertz’s student) studied
the photo-electric effect
Photo-electric effect: experiment
And the explanation came from Einstein
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The energy transfer
is all or nothing
process in contrast
to the classical
theory of continuous
transfer of energy
eV0 = Kmax = hf –E0
Is light particle or wave?
phenomenon
wave
particle
reflection
yes
yes
refraction
yes
yes
interference
yes
no
diffraction
yes
no
Photoelectric
effect
no
yes
How to probe the atom ? Radioactivity
was just discovered
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In the late 1800s the German physicist, Wilhelm Röntgen, discovered a
strange new ray produced when an electron beam struck a piece of
metal. Since these were rays of an unknown nature, he called them "x
rays".
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Two months after this discovery, the French physicist, Henri Becquerel,
was studying fluorescence, when he found that photographic plates
were exposed in the presence of some ores, even when the plates
were wrapped in black paper. Becquerel realized that these materials,
which included uranium, emitted energetic rays without any energy
input.
Becquerel's experiments showed that some natural process must be
responsible for certain elements releasing energetic x rays. This
suggested that some elements were inherently unstable, because these
elements would spontaneously release different forms of energy. This
release of energetic particles due to the decay of the unstable nuclei of
atoms is called radioactivity.
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Rutherford found that:
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3 types of rays: a,b,g
q/m for beta rays
was the same as in
cathode rays
Measured the
spectrum of alpha
rays : it turned out to
be the same as for
He
Rutherford’s experiment
The Rutherford atom
Measured the distance of closest approach ~ 10-14 m
Key discoveries in the beginning of the
20th century
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Electrons – subatomic particles that carry the
smallest possible charge.
Atoms exist and they have a small extremely
dense positively charged nucleus. The
electrons balance the charge to get an
electrically neutral atom.
Light behaves both as a wave and a particle (
photo-electric effect)
Radioactivity: some atoms are unstable and
emit different types of rays