Some times text books talk about the particle-wave duality but this is confusing.
Quantum Physics says that every thing is made or particle-waves.
An electron is a fundamental particle-wave. It is a wave but confine in a region of space. You can visualize as being a wave packet :
If the wave packet is small enough, you can think of it as being a particle.
A neutron is made of 3 bundles of particle-waves called quarks. These particle-waves attract each others/
The quark is a fundamental particle-wave although string theory physicists think quarks are made of little strings that vibrate.
An electron around a proton spreads out more because it is lighter. a proton is more compact because it is heavier.
so smaller wave packet.
Much of our high technology is based is on quantum physics. In all the following examples, the key feature is the QUANTUM LEAP or QUANTUM GAP.
The idea of energy being quantized in an atom or in a crystal. Here are some examples:
LASER (based on quantum chain reaction)
SOLAR CELL (based on photoelectric effect )
DIGITAL CAMERA (photoelectric effect)
XEROX MACHINE (light comes and electron is released, photoelectric effect)
LASER PRINTER (XEROX + LASER)
CONDUCTOR AND SEMI CONDUCTOR
TRANSISTOR (fill computers, ipod..)
FINGER PRINT of gas (we can observe the spectral lines emitted by a gas and determine its components. it can be done remotely. We can
find out it a smoke contains pollutants remotely or the constituents of a star).
All this technology is based on the energy gap between allowed orbits in an atom or crystal. SEE HISTORY
electron-waves behave like standing waves that wrap around the nucleus. Only certain orbits are allowed .
Otherwise, the wave will cancel each other. The waves are long enough for that to happen. (long wave because
electrons are light. electrons also carry a charge with them and they are attracted to the nucleus).
electrons can't fall into the nucleus are predicted my classical Physics because there is no orbit available for them to got.
In quantum Physics , not allowed means not happening. only a certain number of electrons are allowed per level of energy (Pauli exclusion principle).
EAch orbit has a given energy. The electron can jump from one orbit to the other. They make a quantum leap and
emit light. The color of the light (frequency) depends on the energy gap between the orbits. the formula is very simple and was found by Einstein:
energy gap = h x frequency of light emitted when electron jumps down
So orbits are quantized/
Energy difference between level = a given color . This explain the emission spectral lines. In the Sun atoms collide and electrons can jump between level
and lose energy in the form of lights (visible or invisible).
Mercury gas emit ultra violet because it corresponds to an energy gap inside the atoms/ Each element has a spectrum of emission lines associated with it.
click here for spectra of some elements.
photo electric effect
application: XEROX PRINTER
The photoelectric effect was explained by Einstein. Light is shined on a metallic surface and kick off an electron
from outer orbit. this was true only if the light had a large enough frequency. Classical Physics could not explain this phenomenon.
The photon is absorbed by the electric . A photon is a quantum of energy or bundle or packet.
The energy is absorbed by the electron and the electron has enough energy to escape
the metal. If the electron get more energy necessary to escape, it leaves with kinetic energy.
You can observe this by shinning an UV light on a negatively charged electroscope.
The strips of the electroscope will come together because some electrons have been kicked off.
you can collect these electrons with a positive terminal and harvest electricity.
Einstein explained how light was made of quanta of energy = frequency x h.
This phenomenon is used in a XEROX MACHINE :
see animation here
A drum is negatively charged. (using a wire that produces sparks.you can see it when you open the machine).
The paper to be copied is focused (with a lens) on the drum and the letters form a shadow on it.
The letters are written in " electrons"
The drum spins and Light is shined on the drum and shoot off the electrons from the drum every where but on
the shadows of the letters. Ink dust or toner is sprayed on the drum and stick to the electrons.
The drum spins and run against o piece of paper. the paper is heated and the letters stick to it.
LASER PRINTER
The drum is still negative charged. But it is a laser, a very focused of light, that
kick off the electrons. The laser is turn on and off to make a dot or no dot.
It moves sideways and updown. The image is made of dots. Where the laser
shines there is a white dot and where it is tuned off there is a black dot.
1 piece of crystal (like silicon) or how to exploit the energy gap .
application:LED
SOLAR CELL / PHOTOELECTRIC EFFECT
Instead of using a metal, you can use a piece of crystal and exploit the energy gap to produce electricity.
It works like the photo electric effect described above but the electrons are not set free.
They are kicked on an conduction band where they can flow.
In a crystal the energy levels look like :
The level of energies are piled up to form bands of energy. Between the 2 bands there is a gap of energy.
The electrons can jump from the low energy band to the higher one only if they are given the right energy.
The lower band is filled with electrons and they can't move . The upper band has some electrons but
also some " holes " or places available for electrons/ If you shine a light on the crystal and if
the light has the right frequency some electrons will jump (quantum leap) from the lower
band to the higher one. (see below drawing) Electrons repel each other in this band and begin to flow. (they move from hole to hole).
This upper band is the conduction band. Unlike for the metal, the energy of the light has to exactly match the gap.
You can make a circuit and keep the process going. This how a solar cell works or a cell in a digital
camera.
the orange arrow = shinning light.
LASER
CD, DVD, bar codes, eye surgery, welding, cutting, fusion
Charles Townes From Berkeley University exploited that energy gap to build the LASER.
A laser can use a chrystal or a gas. The electrons bounce back in the crystal , some electron-wave are
allowed (they reinforce themselves while bouncing) and create levels of energy.
Electrons are excited by an energy input (electricity) and are in an excited state. (upper band bellow).
But instead of spontaneously coming back down they stay in their excited state longer. This is called
a metastable state. When on electron comes back down, light of a given frequency is emitted and this emission
stimulates other electrons to jump back (it is not a spontaneous emission) and we get a quantum avalanche.
So all the electron emit light in a very short time. The key thing is that all the waves emitted have the same
frequency (same color), the same direction and the same phase (they reinforce each other).
light is emitted in a burst and there is an amplification effect.
This effect was predicted by Einstein. The first laser was a maser because micro wave was emitted.
note: some electrons are in the non excited state.
mirrors are used to amplify the light. one reflect 100% and the other one reflects partially:
Laser can focus energy from light in a very small area so laser are used to weld or cut with high precision/
They can be used to read bar code or CD and DVD. You can turn on and off the laser and code a message
using the ASCII language (o and 1), So information can be send in optic fibers. It was a revolution.
The light coming out of a laser is said to be coherent.
see this movie
2 pieces of silicon
LED, AC to DC
If 2 pieces of silicon are joined together and if the level of energy are changes in such way that
now they don't match anymore we get: (the crystal is doped with impurities to change the position of the bands)
Now electricity can only flow 1 way. Electricity can flow because the electrons repel each other and are pulled
by an electric field. (a voltage is applied). Electrons can flow from left to right and in the process emit light
when they jump the gap down. This is how a LED works. (light Emitting diode).
If you inverse the + and - of the circuit , electricity can't flow because electrons don't have enough energy to jump
back up. Unless you apply large current, in that case electrons can repel each other and jump the gap.
This idea is also used to transform AC to DC. If you use alternative current in the circuit, current
will flow only in 1 direction.
TRANSISTORS
transistors are the keys for computers, ipod, radio .. to work. When you have lots of them on a chip, it is called an integrated circuit.
amplifier:
- transistors can be used to amplify a weak signal : the transistor clones the weak signal and give it more power.
the signal carries information like music, talk , videos..
transistors needs a power source to work. So an ipod receives a weak signal from its antenna (radio signal) or from reading
a song in its hard drive and the transistor will copy the signal and will make it powerful for us to use it :
The more powerful signal can be transformed into sound. (the signal in a coil moves a magnet that in turn moves a membrane
that moves the air and creates sound waves. the membrane will move at the same frequencies than the signal).
switches:
- transistors can be used are switches. they can switch on and off the current. you can encode letters and numbers using 0 (off) and
1 (on) using the ASCII code, switches can also be used to do computations. This is how computers work, using millions of switches.
transistors exploits the energy gap too.
They contain 3 pieces of crystal . The middle one works like valve. A weak signal can
modify the energy levels of its conduction band. When the band is " lifted " electricity
can't flow. When the band is " lowered" electricity can flow. (see drawing)
The electricity is produced by a source of energy like a battery.
We get in the output a copy of the weak signal. the weak signal could be on and off signal or musicl.
It works like a hose of water that is turned on and off repeatedly by a valve with little effort.
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