I. CATHODE RAYS / RADIOACTIVITY Crookes tube / Roentgen / Becquerel/ Curies : X-rays and radioactivity.
1869-1875 cathode rays inside Crookes tubes are discovered by William Crookes and others. An electric voltage is applied to a tube (crookes tube) with a gas inside. The gas is thinned (air was pumped out) and when the voltage is applied electrons, (not discovered yet at the time) are pushed by one electrode (cathode) , and pumped in by the positive electrode (see picture). The electrons are accelerated by the electric field. . You can't see the electrons but when a fluorescent screen (cross, see picture) is placed on their way, the screen glow producing a green light. (see video below). The electrons were not yet discovered and the rays were called cathode rays. The screen glow because the fluorescent material absorb the energy of the electrons and re emit visible light. None of that was known at the time. The apparatus show that the rays were traveling in a straight line since the shadow of the cross can be seen on the other end of the tube
In 1895 X-rays were discovered by Wilhelm Roentgen , by chance. Roentgen was studying cathode rays. Roentgen
decided to envelope the tube and was surprised to notice that a screen
near the tube, covered with a fluorescent material, was glowing. Some
kind of rays were going through the glass, through the paper and were
impressing the screen. He called this radiation X-rays. (for x the
unknown). He also noticed that if a hand was placed between the source
(tube) and the tube, an image of the skeleton appeared on the screen. (see image below). What was happening was that the electrons were decelerating when reaching the glass or when bumping into
the gas molecules, they were emitting the lost energy as photons of
x-rays. X-rays are absorbed by the fluorescent screen and re emit visible light.
In 1895 Becquerel was studying the phenomenon of luminescence. Some crystals absorb UV light (from the Sun) and re emit visible light. At the time the process was not understood but well known. (The phenomenon was explained later by quantum mechanics) He observed that salts of uranium can impress photographic plates wrapped in black paper. Since Xrays were just discovered and since he was studying luminescence, he thought that, may be, crystals can, not only absorb UV light to re emit visible light, but can also absorb UV (from the Sun) and re emit X-rays. (instead of visible light). One day the sky of Paris was covered so no UV could reach the uranium salts. There was a wrapped, unused, photographic plate in a drawer, next to the salts. Becquerel was surprised as , even if the salts could not get any UV, they were still emitting some radiations (radioactivity) that were impressing the plate. He called this new radiation U rays. He passed his discovery to Marie and Pierre Curie (who were working a few block south). Marie Curie, with the help of Pierre, found materials even more radioactive : polonium (she was from Poland) and Radium. They both received the Physics Nobel prize in 1903 for the discovery or radioactivity and Marie was the first woman to
receive , alone, a Nobel Prize in 1911. She was able, after the
death of Pierre, to isolate metallic radium using electrolysis.
uranium salts impressed a photographic plate. A metallic cross was placed between the salts and the plate and radioactivity didn't not go through.
Roentgen x-ray of the hand of his wife. wise man.
Learn more about radioactivity. A very complete story II - Plank - black body radiation - E = h f - quantization of energy of light (photons) Note: light here (radiation / EM wave ) can be, not only
visible light, but any part of the electromagnetic spectrum from radio waves to
gamma rays. Every
radiation (Infra red, red, yellow, blue, UV, xrays ... ) is
characterized by its wavelength L or frequency f . Maxwell found that
the light has a definite speed c such as f = c/L. The frequency of the light is like its ID, like the number of protons for the atoms. The frequency gives the " color" or " flavor" of
the light. We will see that Plank also proposed a relationship between
the frequency of the light and its energy. Higher the frequency, higher the energy. Gamma rays were not yet discovered at the time and xrays were not yet considered as part of the EM spectrum. The
black body radiation phenomenon could not be explained by classical
Physics. When a very hot body like a star emits light , classical Physics predicts than light of all frequencies (or wavelengths) should be emitted and
that more high frequency light should be emitted. The atoms/molecules
emitting the lights were thought to behave like small
oscillators (small vibrating springs) so there was no reason why the oscillators could not
vibrate at any frequency. On
the contrary, is should be easier to fit oscillators vibrating with a
high frequency into the hot body. Think music. An organ pipe, (or soprano) of definite length, can contain more
waves of small wavelength (high frequency) adding up to
make a sound, than waves of large wavelengths. (the length of the pipe limits the size of the wavelength). However, this was not observed in the spectrum of a hot body. Not
all frequencies were emitted. It was observed that, as the frequencies increases, less
light was emitted. When we sit next to a fire, we feel the heat (infra
red ) , we see color (visible light) but luckily for us we are not
exposed to UV , xrays or gamma rays. and no radio waves are emitted either (low frequency)
1900 - Although Plank didn't want to renounce to classical physics, he introduced a new constant, now called Plank
constant, h such as E = h f. E is the energy carried by a
light (radiation) of frequency f. So if the energy of the oscillators (models for the atoms/molecules at the time) is not large enough, the
corresponding radiation of frequency f = E/h can not be produced. Or
higher frequency can not be produced because the energy is too high. The energy produced by a hot body (or energy of light) comes into discrete energy chunk His new formula could explain the spectra of hot bodies. The formula shows that light can be radiated only by discrete chunks of energy.
III. Einstein 4 papers + general relativity 1905 Brownian motion is explained by Einstein (and also by Perrin). He will won the Nobel prize for his paper/ Brownianmotionis
the random movement of microscopic particles suspended in liquids or
gases. This movement is caused by the impact on these particles by the molecules in the surrounding fluid. This showed the existence of atoms as individual units. The
explanation of the Brownian motion was the second of 4 papers that
Einstein published the same year and that changed our view of the
universe.
The second paper explained the photoelectric effect. Light is made of photons (name given by Einstein), , chunks of energy, capable of knocking over electrons from the metal. The energy of each photon is given by Plank formula E = h f. (frequency corresponds to a UV photon). The
energy of the photon must be large enough so the electron can
make a quantum jump from its energy level to freedom. If you shine a
red light, the energy is not large enough and nothing happens. If
the energy is higher than the " freedom jump " , the left over energy
goes into kinetic energy of the electron. Increasing the intensity
of the red light does not help, it is just sending more photons with
the wrong energy. If you increase the energy of the UV light, you
get more electrons free to go but with the same kinetic energy after
the collision. (than less of it)
Classical Physics could
not explain this phenomenon because light was considered as a
wave as demonstrated by Hans Huygens. (light
show the same properties than waves: reflection/refraction ,diffraction /interferences , doppler effect (red shift of galaxies) ) Newton had suggested that light was made of corpuscles but Huygens description of light prevailed until this paper. Einstein paper shows that light can be absorbed or radiated (as shown by Plank ) by discrete bundle of energy. Light is made of " corpuscles" as suggested by Newton/ learn more here.
He built the third paper on an essay he wrote when he was 16 years old. It was his theory of special relativity. Since
Galileo Galilei 's experiences, it was accepted that speed
is relative. It depends on the frame of reference it is measured in. If you walk while shooting a gun , the speed of the bullet is 3000 feet per second in your moving frame. (a coordinate system and a clock is attached to you). But if you walk at 4 feet per second, the speed of the bullet is 3004 feet per second in the frame of reference of the lab. In the frame of reference attached to the Sun, the speed is yet another number since the EArth in moving around the Sun. Or yet another number in a frame attached to center of the milky way .. The big idea of Einstein is to explain that other physical quantities are also relative. The time an event lasts also depends on the frame of reference it is measured in. Not only the time but also mass, length , momentum and energy. If
a spaceship accelerates away from your frame of reference, its time
slows down when measured in the Earth's frame of reference. The mass
of the moving object measured in the frame of reference of the EArth increases and along the
direction of the velocity, the linear dimensions shrinks.
These weird effects are a consequence of Einstein;s genial insight after the famous failed experiment of Michelson-Morley. This experience failed to measure an increase or decrease in the speed of light as it was expected. see details here.
Einstein came with the explanation:the speed of light is constant no matter the frame of reference it is measured in. So if you walk at 4 feet per second shining a beam of light, the speed of the light is c in the moving reference frame Intuitively, you expect the speed of light to be 4 +c in the frame of reference of the lab (considered at rest). But Einstein understood that the speed of light was still c . Which is counter intuitive. source: Cutnell and Johnson. publisher = Wiley.
The
only way for that to be true is for the duration of an event and its
size to (length) be dependent of the frame of reference. c= (larger length in moving frame )/ (larger time in moving frame) = (smaller length in lab frame) /( smaller time in lab frame) see here for SR effects on physical quantities. In mathematics language. source: Cutnell and Johnson, Wiley
The fourth paper is an addition to the previous paper. It introduced E = m c2. mass and energy are interchangeable. Because c is large, a small amount of mass can produce a huge amount of energy. Einstein understood that as the speed on an object increases it mass (as measured in the frame at rest) increases and the amount of energy you need to put in the system to accelerate it also increases. (more inertia). E = mc2 explains the energy released during fusion or fission. some of the
mass of the reactants (that fuse or split) becomes pure energy. The equation is connected to the previous paper because it says as you go faster, you get heavier as energy of motion becomes mass so you can't reach the speed of light. In leap of intuition, Einstein also predicted that the rest energy of a particle is equal to its rest mass x speed of light square. E = mo c2
Example : Suppose 2 opposite charges attract each other. They have a binding energy called potential energy. To separate these charges, you need to put energy in the system. The binding energy is negative = - B. so total energy of the system = moc2 + Moc2 - B = Mtotal c2 mo and Mo rest masses of the charges. So the mass of the system is smaller than if the charges are not binded. You need to put energy in the system to free the charges.
Example: if an object has kinetic energy KE Einsten found : Etotal = mc2 = moc2 + KE if the speed is small KE = 1/2 mV2 The mass of a system in motion increases. m = mo / gamma factor
Einstein published his theory of general relativity in 1916. Masses wrap space around them. So he dismissed the force of gravity introduced by Newton. To explain the motion of planets, you don't need force of gravity + inertia principle anymore. You can imagine the Sun as wrapping the space around and the planets are trapped in the curved formed. So it is like you have a 5th dimension and the space -time is curved into that 5th dimension.
III. models for the atom from classical to quantum Physics.
(460?–370?BC). The first known theory ofatomism—that matter is composed of elementary particles that are minute and indivisible— was originated by the ancient Greek Leucippus. His pupil Democritus substantially developed and systematized this theory. In some ways, his work anticipated the findings of physicists more than 2,000 years later.
Lavoisier's
results gave chemists their first sound understanding concerning the
nature of chemical reactions. The next milestone was the atomic theory
advanced in 1805 by an English schoolteacher, John Dalton. This theory states that matter is made up of small particles calledatoms, that each chemical element has its own kind ofatoms(in contrast to earlier ideas thatatomsare essentially alike), and that chemical changes take place betweenatomsor groups ofatoms. To support his theoryDaltonset about calculating the relative weights of theatomsof several elements. Still atoms are thought to be static, like buttons.
1896
JJ Thomson discovers the electrons thanks to his
3 experiments with cathode rays. He found particles smaller than the
atom. They
will be called electrons later. (he called them corpuscles.) Before him
Jean Baptiste Perrin had demonstrated, using cathode rays, that the rays were made of negative charges but the scientific community disagrees with that theory. J J THomson took a step further by
showing that the the cathode ray was deflected by a magnet (magnetic
field) and by an electric field. (voltage). The rays are made of
negatively charges particles. He was able to find the ratio between the mass of the electron and the mass of a proton (H + or ionized hydrogen). He
found that an electron is 2000 less massive than a proton. So these
particles not only are charged but also are smaller than the nucleus. He is the
inventor of the spectrometer. (you can deal out particles/atoms according to
their mass). learn more on wikipedia
1905 Einstein explains the brownians motion. the existence of atoms are confirmed. The model of the atom is plum pudding. The electron are like the plums, homogeneously distributed inside the atom and the positive charges are the pudding. Until this paper the existence of atoms was controversial. Boltzmann hanged himself in 1906. He was a believer of atoms and of the fact that the matter is not in definitively divisible. 1911 Rutherford introduces another model of the atom after his gold leaf experiment (source: encyclopedia Britannica). Rutherford shot alpha particles (helium
nuclei) at a gold leaf. He expected the alpha particles to go through the leaf. To his surprise, a few alpha particles bounced back. (1 in 8000) Geiher his student was in charge of this tedious job. (counting the alpha particles in all directions). He developed a new model for the atom. THe nucleus is tiny and positive. The electrons orbit around but a lot of space is left between the electron and the nucleus. (if the nucleus is an apple, the electrons would be 1km away or if the nucleus is a dice, the electrons would be 1 block away 100m ). His
model looks like a tiny planetary system. The Sun is the nucleus and
the planets are the electrons.
Interestingly, he got the alpha particles using the
radioactive element radium discovered by the Curies earlier. When the radium decays it produces alpha particles. He was the first atom prober shooting particle at atoms to find out about the structure of atoms. Later, He continued his experiments, shooting particles at atoms. In 1932 protons and neutrons are fund in his lab. The planetary model for the atom replaced the plum pudding model.
1913
Bohr was a theoretician from Copenhagen, who went to work with Thomson and Rutherford in Manchester, England. He combined the concept
of the planetary atom with the quantum theory of Max Planck . (energy of light is quantized) and Albert Einstein, departing radically from classical physics.He studied the emission lines of hydrogen and understood that the lines had to do with the atom structure. Each line corresponds to an energy or quantum jump between 2 orbits. learn more In his model, The
electron travels in circular orbits around the nucleus. The orbits are quantized according to the level of energy of the electrons. Energy (as photons) is emitted from the atom when the
electrons jumps from one orbit to another closer to the nucleus. The
model of Bohr explained the data (the spectrum of hydrogen). Each
time the electron jumps from a higher energy level to a lower energy
level, a photon is emitted. The color the the photon (frequency) match E = hf where E is the energy difference between the 2 levels. This
jump expresses itself as an emission line in the emission
spectrum. The color of the line defines the frequency of the
photon emitted. But the theory needed to be refined to explain the spectra of other atoms. (note:
To get an emission spectrum from hydrogen gas, the gas is excited by an
electric arc and the light produced by the hydrogen gas go
through a prism. The prism separates the light produced into its
components. Inside the atom, when the gas is excited, the electrons moves to a higher level of energy. When they comes back to a lower level of energy, they emit a photon corresponding to the energy jump they make. Each line corresponds to one of this jump ) learn more here (note:
Bohr was so excited to be on the verge of a breakthrough that he
postponed his honey moon and his fiance helped him to write his paper)
IV- model of the atom : Heisenberg and Shrodinger Bohr
model was the transition between classical Physics and the new physics,
quantum mechanics, developed by Shrodinger, Heisenberg, Born and Dirac.
In 1924, while witting his PhD paper, Louis de Broglie suggested
that particles such as electrons shows also a wave nature like photons. The energy of electrons can be written as E = h f where f is the frequency of the electron-wave. Furthermore, the momentum of
the electron is p = m V = m c = E /c = h f/c = h / L or p = h/L
where L is the wavelength. So an electron has a wavelength like a
photon has one. Large
objects, like us, have wavelengths but the wavelengths are
really small because the momentum is very large (large mass). We can't resolve wavelength smaller than about the size of a nucleus atom. (around 10-14 m ). We can't measure it. so
we behave more like a particle than a wave. We can't detect the wave
properties of a baseball because its mass is too large. (and so is its
momentum). For example a yellow photon has a wavelength of 600 nm (1nm = 10-9m) an electron has a wavelength of 6 nm for a velocity of about 100,000 m/s a
sodium atom, at 80K (temperature is related to speed), has a wavelength
of 0.06nm for a velocity of about 300m/s. starts to be hard to detect. a baseball , 170g, has a velocity of 40m/s and a wavelength of about 6 10-26m. We can't detect these small wavelength
WE don't experience quantum effect. (when you observe a school of fish, you see the group and not the individual fish, likewise we can't detect the small wavelengths we are made of )
This was confirmed in 1927 when Clinton Davisson and Lester
Germer found that beams of electrons could be diffracted bycrystal. This was a big breakthrough. It means that electrons could behave like a wave. See this video to understand:
We can use photons and their particle properties to slow down atoms by bumping into them. By slowing down the atoms, their speed decrease and the temperature drop. This is called laser cooling. It resulted in a nobel prize in 1996. 1926 Shrodinger built on the De Broglie breakthrough. Electrons can behave like waves if they are confined. (this is called quantum confinement). Meaning if they have to stay in a bounded space like around atoms. The space they are confined needs to have the same order of magnitude than their wavelength (nanometers). . Electrons are
confined because of the binding energy between electrons (negative) and
nuclei (positive). We say Electrons are trapped in an " energy
well" . (they have a negative total energy, to free them, kinetic
energy has be given from an oustide source like a photon hitting them = photoelectric effect). So
around an atom or molecules, electrons are confined and behave like
standing waves. In his new formalism electrons are described by a wave function Ψ. To find this wave function, he used the classical mechanics equation that describes classical waves but the electron, in the equation, has a wavelength L = h/p inversely proportional to the momentum p. (relationship between wavelength L and momentum p derived by De broglie). The equation takes in account the boundary conditions (like for a classical wave, like a wave on a rope tied at 2 ends). These boundaries conditions are described by the potential well.
The wave equation is
called the shrodinger equation. The solution of the equation gives the quantized energies of the electron as well as the function Ψ.the function does not represent any physical quantity. But as shown later by Born, the amplitude squared represent the probaility of finding an electron in a given region of space.
His
theory was well accepted by the scientists (Plank, Einstein, De
Broglie.. ) because it was easy to visualize the new atom. The electrons were still distributed on different orbits of given energy (like for the Bohr mode) . But now electrons were waves (or harmonics) wrapping around the nucleus. Only given energy (frequency) were allowed for the wave
to exist and not to annihilate itself. (the wave interferes with
itself and if the interference is not constructive, the wave can't exit. (it cancel itself) ). see below: ------------------------------------------------------------------------------------------------------------------------------------------------ more about the wave function / quantization of energy / orbitals (optional) - you can skip If you consider an electron as a wave of given energy (frequency), it becomes easier to understand the quantization of energy. Imagine a
rope held by 2 person. 1 person shake the rope, putting energy into the system. (could be an electron trapped in an 1D space , along a wire for example) For a given energy that is for a given frequency of the shaking (by one person), you get a given standing wave. (or harmonic) . see image below. Other patterns can't exist because the reflecting wave anihilate the coming wave. TRy this animation for a better undertanding. Try 1st harmoinc alone, then 2nd alone .. don't add them. The energy is quantized because the wave is confined. Each harmonic = discrete energy = given wavelength. No other patterns can happen, because the waves traveling along the rope are negatively interfering. (the crests destroy the troughs) For the lowest energy, You gets, 1 bump (half of a wavelength). See picture. If you increase the energy by increasing the shaking you get 2 bumps. (1 wavelength). but there is no pattern between these 2 harmonics. Next you get 3 bumps. (1 wavelength and 1/2). So the energy of the rope is quantized . Same thing with a flute. The air vibrates inside the flute and forms standing waves. The frequency of the sound does not increase continously but in a discrete manner.
By solving the equations, you get the quantized energies and the wavefunction (sine function). We represent only the amplitude squared because probability has to be positive. So for example, for the 2nd harmonic we get: The horizontal axis is the wire where the electron is confined. It means it is very unlikely to find the electron in the middle after measurement. (of its location). It is very likely to find it where the maxima occur. But You can't ask the question "How the electron can move from one crest to the other , since there is a node in the middle " . This
question does not make sense because, as long as you don't measure its
position, the electron behaves like a wave, taking all the space. Only
when you measure the position, the function collapses and you will find
the electron most likely to be where the max are. Strange, isn't it ?
The analogy given here with harmonics along the rope is true only if the electrons can only move in a 1D space. In
atoms, electrons are confined in a 3D environment. The wave function
becomes a function in 3 variables (spherical coordinates). When you solve the wave function, you still get harmonics but they are more complex. They are called the orbitals of atoms. These orbitals are obtained by solving the shrodinger equation. Again we represent only the amplitude squared of these orbitals. Each electron is assigned to an orbital (probability) characterized by a unique set of 4 numbers called quantum numbers. n,l,ml,ms.
n is for the energy level (orbital) , given n l is for the shape
of the orbital (you have s orbitals (l=0) sphere, p orbitals(l=1), d
orbitals, f orbitals ..) for a given ml is for the orientation of the orbitals (given shape but different orientation in space) and given ml,ms can be up or down.
Note about molecular orbitals: To form molecules, the atomic wave functions combine together to produce molecular orbitals. The atomic wave functions interfere with each other constructively or negatively to give the molecular orbitals. (probability to find electrons around the atoms). These molecular orbitals define the bonding between the atoms. If a combination between orbitals results in a higher energy state for the molecule, then it won't happen in nature. For
example H + H = H2 = lower energy state, so this happens. But He + He =
He2 = higher energy state. Does not happen in nature. Like in music, the orbitals combined together like harmonics combines to make a sound given a mix of harmonics. check this animation -------------------------------------------------------------------------------------------------------------------------------------------------------------------- For new forms of atomic theory Shrodinger shared the 1933 Nobel prize for physics with the British physicist P.A.M. Dirac Shrodinger equation is today the main tool to predict the behavior of particles.
1925. Heisenberg a protege of Bohr developed an all new formalism and a new type of mathematics using matrices to explain the atom. He worked with Max Born to develop this new theory. His matrix mechanics predict the behavior of
atoms using pure mathematics. He was not using classical mechanics
tools anymore. He didn't think atoms where " things " This new theory was not welcomed by other physicists like Plank or Einstein because it was not possible to visualize the
atom ANYMORE. The atom became matrices. Although very
complicated, this new formalism was able to predict the bEhavour of
atoms. Heisenberg
was criticizing Shrodinger formalism with fierce (he was arrogant and passionate) In 1926 he introduced his uncertainty principle. You can't know the position of an electron or particle if you know its momentum and you can't know where a particle is going if you know where it is. This was not accepted by Einstein , Plank and Shrodinger. Einstein said his famous " god doesn't play dice" .
(note: there were some controversy about Heisenberg. He worked on the nazis project to design the bomb before the american. He worked with Otto Hahn at the Kaiser Wilhelm Institute. Heisenberg said later that he was trying to slow down the nazis from getting the bomb first. Which is very suspicious. )
1927 Brussel Copenhagen interpretation conference The
state of every particle is described by a wave function which is a
mathematical representation used to calculate the probability for it
to be found in a location or a state of motion. According to this
interpretation, the act of measurement causes the calculated set of
probabilities to " collapse " to the value defined by the measurement.
THis feature of the mathematical representations is known as wave function collapse. (see below wave-particle duality) All
the great physicists of the time were there to discuss the new physics.
Including Marie curie. Heisenbereg seems to have won the round
at the end. His formalism was strong and could predict the
observations. Shrodinger, Plank, Einstein were disappointed. to learn more to learn even more click on picture
V ,. reconciliation Shrodinger and Heisenger formalisms / probability functions
Max Born reconciles the 2 formalisms by showing , mathematically , that the wave function developed by shrodinger was actually a probability function. The function gives the probability to find the electron at a given point in space.
probability
also comes into play into the energy jumps from one level to the other
inside the atom. If you consider the spectrum of an element, some lines are more intense than others. This is because some energy jumps are more likely to happen than others. The black body radiation is now completely explained. The left part of the spectrum, has lines with smaller intensity because the energy jumps that produces the corresponding photons are less likely to happen.
The Shrodinger cat story illustrates the probabilistic nature of particles. A cat is in a box (we can't observe it) and has
a 50% chance to be dead (depending if a radioactive element has decayed
or not), From our point of view, the cat is half dead and half alive.
The probability function will collapse only once we open the box to
look inside...
quantum mechanics was born. Both formalism (shrodinger and Heisenberg) were equivalent if you consider the wave function as a probability function.
3 models:
planetary model of the atom. based on classical Physics. Problem : the electrons should collapse on the nucleus. source: Encyclopedia Britannica.
Bohr model of the atom. Electrons orbits are quantized according to energy level. This model is both based on classical Physics and quantum mechanics. The model predict the frequencies of light emitted by an excited atom. source: Encyclopedia Britannica.
quantum physics model orbits are now represented as clouds. Denser is the cloud , higher is the probability to find the electrons. The probability is given by Shrodinger equation. It depends on the potential that binds the electrons to the atom or make them move. source image : encyclopedia Britannica. learn more
VI - particle waves - uncertainty principle of Heisenberg quantum mechanics is the Physics of the very small
Dirac a genius mathematician shows the dual nature of electrons. They are particle-wave. They can behave like a particle or a wave, depending on what you are measuring. Dirac also predicted the existence of anti matter using mathematics. Like positrons are the antimatter of electrons. He also, mathematically, reconciled quantum mechanics and special relativity into 1 theory. (note: Dirac is considered as the second most brilliant physicist of England/ He was a student at Cambridge when he developed a formula that shows the dual nature of electrons.)
The
dual nature of electrons or particles and the uncertainty
principle is not that hard to understand if you think of particles as
particle-waves. if
you don't interact with the electron (or particle) , it behaves like a wave:
(like when they orbit the nucleus or when they interfere with
themselves)
This
model also explain the slit interference experience that shows that 1
electron can produce an interference pattern, because it is interfering with itself. (see video above) so
you can tell which way it is going because it has given a frequency
(wavelength) and that means a given momentum (see de Broglie
equations, the momentum depends on the wavelength). But, You can't tell its location because a wave is
spread in space.
If you interact with the electrons, the wave
function collapses and becomes a wave-packet. Now the wave is made of many frequencies (sum of waves of different frequencies). So you don't know its momentum anymore ( you get a wave-packet by adding waves of different wavelengths/momentum. You can only compute the probability of having this momentum rather than this one) However, now you know
where it is. The wave function collapse because you can't observe small particle (using a detector) without interacting with it and disturbing it. quantum mechanics made possible the use of TV, integrated circuits, lasers .. pet scans (use of antimatter) nuclear reactors (and nuclear weapons). see this chapter for more.
1932 James Chadwick designed an experiment that proves the existence of the neutron, predicted by his professor Rutherford. Next Fermi used the neutron newly discovered to make the atomic bomb. video about Chadwick video about the experiment another video
Lisa Meitner is the scientist who understood how to trigger a fission reaction./ However, she didn't get full credits for it. Instead her ex colleague Otto Hahn (from Kaiser Wilhelm Institute, Germany) got the Nobel prize (chemistry) in 1944 and didn't share it. Lisa Meitner was jewish and began her career in Germany before the rise of the Nazis. She was safe for a while because of her austrian citizenship but was forced to flee to Holland and then Sweden (when the Nazis occupied Austria). She did regret to have stayed in the institute while the Nazis were in power. Otto Hahn took full credits for the discovery and didn't mention her name when he received the prize. Letters between Otto and Lisa showed that she is the one who understood the process. She was offered to work with Oppenheimer on the Manhattan project but declined since she didn't want to be involved in the building of an atomic bomb. Oppenheimer, director of the Manhattan project said looking at the explosion: "God bless us, we have created something more awful than hell". See movie: Lisa Meitner / NOVAE Einstein big idea/ She came with the idea that shooting a neutron at an unstable atom like uranium will produce fission fregments (other elements / particles) and a huge amount of energy. This is because uranium is unstable and will decay in another element. In the process, some mass will be transformed into energy . The energy can be computed thanks to Einstein formula E = m c2 . The formula shows that matter (like a particle or a piece of an atom) can become pure energy (like radiation) and pure energy (like after the big bang) can become matter.
She first thought that shooting neutrons at uranium (the heavier known atom at that time) would increase the mass of Uranium and
change it to a new and larger atom. But when the experiment was done,
by Hahn in Germany, some mass was missing after the collision and radium
was one of the product. (less mass than uranium). Lisa was in Sweden,
with her nephew , a physicist, when she got the results of the
experiment by
mail from Hahn. There, she understood that Uranium instead of
growing in size, broke down into a smaller atom and in
radiant energy (gamma rays). Some of the mass lost was changed into energy (radiation) as predicted by Einstein equation E = m c2. They published a paper but Hahn took the credits for this discovery. more about that story
Early
in 1942 Fermi transferred to the University of Chicago. In a squash
court under the football stadium he developed a method of causing
nuclear fission. With this method he and his group produced a chain reaction that released the explosive nuclear energy.
We use fission in a nuclear reactor to produce electricity. We collect the energy produced due to the transformation of some of the mass of Uranium235. (conversion of mass to energy) The advantage is that you just need a tiny bit of uranium to get a huge amount of energy. There is no pollution in form of gas unlike when fossil fuels are burnt Here is an example to grasp the difference: To prepare popcorn you need to give the microwave 200 KJ. To get that much of energy you need 10 to 20 g of coal but only 2 micrograms of uranium.
You can use uranium 238 (natural one) or uranium 235 (processed, enriched). If you use U 238, you need 140g of it to sustain an average household in electricity during 1 year. If you use coal, you need 3 tons (3000kg) to sustain the same household. (same time) If you use enriched uranium U235, you need only 0.3 g of it to sustain the household for 1 year. .
The draw back are the waste products that are also radioactive. However, if stored properly, the radiations are contained. The radiactivity will decrease with time. (as plutonium also decay). The worry is that plutonium can be used to build atomic weapons. (Nagasaki was destroyed by a plutonium bomband Hiroshima by a uranium bomb, a result of the Manhattan project lead by Oppenheimer). (although it is harder to build a plutonium bomb than an uranium bomb with U235)
1 g of coal produces 29KJ 1g of Uranium 235 produces 96 400 000KJ
VIII. nuclear Physics, the standard model
In the 1940s Richard Feynman started to develop QED or quantum electrodynamics. This theory combines the theory of relativity (what
happens when particles reach high speed, close to the speed of
light) quantum mechanics and electromagnetism. He also
developed his diagrams that sim
plify the description of interactions between particles. The
diagrams take in account time . His diagrams describe the
interactions/forces between particles as an exchange of particles
like the gluons. (exchange particles for the nuclear force) and the
virtual photons (exchange particles for the EM force)
After the war, modern Physics was very productive in finding new particles and in refining nuclear physics based on QED . Using colliders, new particles were found all the time. It was called a zoo of particles.
Then the quark theory was developed by Murray Gell-Mann in 1964 . The quark is a building block of matter that make up many other particles like the proton and the neutron. electrons are another type of building blocks (leptons). So
now instead of a zoo of particles, you have only 2 building blocks of
matter. the quarks (12 in total but only 2 are stable and make up matter) and the leptons (electrons are leptons and make up matter).
Murray Gell-Mann predicted the quark and then it was observed. He won the Nobel Prize for his prediction. The discovery of the quark to simplify the classification of particles. the name " quark " (particle that makes up neutrons and protons) comes from a quote from the book : " Finnegans wake " from James joyce and the quote is " 3 quarks for Muster Mark " Quarks were theorized by
Murray GEll-Mann (1964) who named these particles after the book he was
reading. (james joyce). He got the nobel prize for it when quarks were
actually observed in 1968 in the accelerator of Stanford. (particles
smasher) Also,
the 4 fundamental forces of nature between particles (matter) (gravity,
electromagnetic, strong nuclear and the weak nuclear) were now explained in term of exchange of particles. . The electromagnetic force is due to an exchange of photons (massless). In
1934s Fermi theorized that the weak force (responsible for
radioactivity and fusion in the sun) was due to an exchange of particles later
called the W+, W- and Z particles (found in 1980s, it took a big
accelerator to find these particles, because they are heavy) . The strong force was due to an exchange of gluons (found in 1970s. massless). It is theorized that gravity is due to the exchange of gravitons (not yet found)/ Review this page to learn more about the 4 forces and their job. The
electromagnetic force and the weak nuclear force was unified in 1970 by
Sheldon Glashow and Steven Weinberg. (Nobel Prize in 1979)
Note: Enrico Fermi introduced the weak force when he explained the beta emission during radioactive decay. During a beta decay, an electron is emitted. Physicists could not understand where this electron was coming from. Fermi
explained that, during the radioactive decay, a neutron was
changed into a proton and this was accompanied by the emission of an electron and a neutrino (that way the charge was conserved). This transformation is due to the weak force. In
kind of the same way, during fusion in the Sun, 4 nuclei of hydrogen (4
protons) combine together to give 1 nucleus of helium (2 protons and 2
neutrons). The the process, 2 protons are changed into 2 neutrons so 2 positrons (anti electrons) are emitted. (conservation of charge). This is also the weak force in action. (a force is the agent of change)/
The so called " standard model " combine every thing we know about particle Physics. 3
kinds of particles are the building blocks of matter/energy: (although
some think they are the same particle but in different state) quarks (6
flavors : up, down, top, bototm, charm and strange) . protons and neutrons are made of 3quarks. they are called baryons
or matter particles. mesons like the pions are made of 2 quarks.
A ion is a force particle .The carrier of the strong nuclear force. leptons
(electrons, muons, taus, electron neutrino, muon neutrino, tau neutrino) bosons carrying
the forces (photons carry the EM force, gluons carry the strong nuclear
force that hold the nucleus together , vector mesons (also called W+
W- and Z carry the weak force that is responsible for fission and
radioactivity, maybe gravitons carry the gravity force. .. )
THe leptons are unresponsive to the nuclear force (gluons). They are responsive to the EM force and the weak force.
WE can also classify quarks and leptons into 3 families The fermions are the particles of matter. They have a spin of ½, 3/2, 5/2 …
Baryons and leptons are fermions. Leptons include the electron and
The neutrinos. Baryons have 3 quarks. Example : proton and neutron. to
build the matter you know you only need the particles from the 1st
column = up quark, down quark, electron and electron neutrino. we don't know what the 2 other families are for.
There are 6 quarks. (last one found in 96 in the Fermi Lab) named: up,
down, bottom, top, charm and strange !(not as original names). But you have also 6 antiquarks. so 12 quarks altogether. quarks are the only particles with a non integer charge. A quarks has a charge of + 2/3 (like up quark )or - 1/3 (like the down quark) protons are made of 3 quarks. (2 up and 1 down) so the charge of a proton is 2 (2/3) - 1/3 = + 1
Matter particles are called fermions. They have spin 1/2, 3/2, 5/2 ....
Example: baryons have 3 quarks like the protons and the neutrons.
matter particles take place. they can't pile on each other.
we have also the bosons = force particles. the bosons have a spin of 0,1,2,3 ...
the bosons can pile on each other. see below for the force carriers.
hadrons are particles made of quarks. Baryons have 2 quarks and are matter particles. Example: protons and neutrons.
Mesons like the pions have 2 quarks. They are bosons because their spin is an integer.
Pions are carriers of the strong force.
SEE HERE FOR a review:
SOURCE: http://www.pha.jhu.edu/~dfehling/ The
missing piece is why the W and Z bosons have a mass whereas gluons and
photons are massless. In the 1960s Peter Higgs and other
described the mechanism that could explain why some particles have mass
and some don't. This mechanism is called the higgs mechanism and
involves a particle called the higgs boson. This particle has not been observed yet but hopes are, that the Large Hadron Collider, will find it. The analogy for the higgs bosons is a group of students scattered around in front of the Physics building. If
their famous professor want to pass through the group, to get in the
building, he will be slowed down because students want to talk to him. This
gives the professor inertia or mass. If a insignificant , insignificant
for the students, person wants to get through, he/she will have no problem
as students don't pay attention to this person. He/she is massless.
Physicist think a particle has a mass when higgs bosons response to its presence and make its motion slow down. Massless particles don;t trigger any reaction from the higgs bosons.
The actual theory is that the vacuum just after the big bang had different properties than now. Today when a particle-wave like a proton or an electron travel in the vaccum and interacts with it, they get a mass. They interact with the higggs field and they acquire a mass. It is believed that just after the big bang, the vacuum was different and particle waves didn't have a mass because there was no higgs field to interact with.
slides here too. from the book: cosmic perspective, Pearson, 6th edition
TRy this animation for a better undertanding. Try 1st harmoinc alone, then 2nd alone .. don't add them. 
The horizontal axis is the wire where the electron is confined. It means it
