The branch of physics that deals with the action of forces on matter is referred to as mechanics. All considerations of motion are addressed by mechanics, as well as the transmission of forces through the use of simple machines. In our class, the goal is a mechanical goal (placing blocks into a bin) and electronics are used to control the mechanics.
While it is not necessary to sit down and draw free body diagrams or figure out the static coefficient of friction between the LEGO tires and the game board, it is helpful to keep certain mechanical concepts in mind when constructing a robot. If a robot's tires are spinning because they do not grip the floor, then something must be done to increase the friction between the tires and the floor. One solution is to glue a rubber band around the circumference of the tire. That problem/solution did not require an in-depth study of physics. Simply considering the different possibilities can lead to more mechanically creative robots.
Describing motion involves more than just saying that an object moved three feet to the right. The magnitude and direction of the displacement are important, but so are the characteristics of the object's velocity and acceleration. To understand these concepts, we must examine the nature of force. Changes in the motion of an object are created by forces.
Click here to read more about basic mechanics and its principals.
Monday, December 27, 2010
Holography
Holography
The above image was taken through a transmission hologram. The hologram was illuminated from behind by a helium-neon laser which has been passed through a diverging lens to spread the beam over the hologram.
Holography is "lensless photography" in which an image is captured not as an image focused on film, but as an interference pattern at the film. Typically, coherent light from a laser is reflected from an object and combined at the film with light from a reference beam. This recorded interference pattern actually contains much more information that a focused image, and enables the viewer to view a true three-dimensional image which exhibits parallax. That is, the image will change its appearance if you look at it from a different angle, just as if you were looking at a real 3D object. In the case of a transmission hologram, you look through the film and see the three dimensional image suspended in midair at a point which corresponds to the position of the real object which was photographed.
These three images of the same hologram were taken by positioning the camera at three positions, moving from left to right. Note that the pawn appears on the left side of the king in the left photo, but transitions to the right of the king as you sweep your eye across the hologram. This is real parallax, which tells you that the image is truly 3-dimensional. Each perspective corresponds to looking through the hologram at a particular point.
The Holographic Image
Some of the descriptions of holograms are
Some of the descriptions of holograms are
"image formation by wavefront reconstruction.."
"lensless photography"
"freezing an image on its way to your eye, and then reconstructing it with a laser"
A consistent characteristic of the images as viewed
The images are true three-dimensional images, showing depth and parallax and continually changing in aspect with the viewing angle.
Any part of the hologram contains the whole image!
The images are scalable. They can be made with one wavelength and viewed with another, with the possibility of magnification.
"lensless photography"
"freezing an image on its way to your eye, and then reconstructing it with a laser"
A consistent characteristic of the images as viewed
The images are true three-dimensional images, showing depth and parallax and continually changing in aspect with the viewing angle.
Any part of the hologram contains the whole image!
The images are scalable. They can be made with one wavelength and viewed with another, with the possibility of magnification.
Friday, December 24, 2010
Super Conductivity
If mercury is cooled below 4.1 K, it loses all electric resistance. This discovery of superconductivity by H. Kammerlingh Onnes in 1911 was followed by the observation of other metals which exhibit zero resistivity below a certain critical temperature. The fact that the resistance is zero has been demonstrated by sustaining currents in superconducting lead rings for many years with no measurable reduction. An induced current in an ordinary metal ring would decay rapidly from the dissipation of ordinary resistance, but superconducting rings had exhibited a decay constant of over a billion years!
Meissner effect:
When a material makes the transition from the normal to superconducting state, it actively excludes magnetic fields from its interior; this is called the Meissner effect.
This constraint to zero magnetic field inside a superconductor is distinct from the perfect diamagnetism which would arise from its zero electrical resistance. Zero resistance would imply that if you tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz's law). But if the material already had a steady magnetic field through it when it was cooled trough the superconducting transition, the magnetic field would be expected to remain. If there were no change in the applied magnetic field, there would be no generated voltage (Faraday's law) to drive currents, even in a perfect conductor. Hence the active exclusion of magnetic field must be considered to be an effect distinct from just zero resistance. A mixed state Meissner effect occurs with Type II materials.
Perfect Diamagnet
If a conductor already had a steady magnetic field through it and was then cooled through the transition to a zero resistance state, becoming a perfect diamagnet, the magnetic field would be expected to stay the same.
Superconductor
Remarkably, the magnetic behavior of a superconductor is distinct from perfect diamagnetism. It will actively exclude any magnetic field present when it makes the phase change to the superconducting state.
BCS Theory of Superconductivity:
The properties of Type I superconductors were modeled successfully by the efforts of John Bardeen, Leon Cooper, and Robert Schrieffer in what is commonly called the BCS theory. A key conceptual element in this theory is the pairing of electrons close to the Fermi level into Cooper pairs through interaction with the crystal lattice. This pairing results from a slight attraction between the electrons related to lattice vibrations; the coupling to the lattice is called a phonon interaction.
Pairs of electrons can behave very differently from single electrons which are fermions and must obey the Pauli exclusion principle. The pairs of electrons act more like bosons which can condense into the same energy level. The electron pairs have a slightly lower energy and leave an energy gap above them on the order of .001 eV which inhibits the kind of collision interactions which lead to ordinary resistivity. For temperatures such that the thermal energy is less than the band gap, the material exhibits zero resistivity.
Bardeen, Cooper, and Schrieffer received the Nobel Prize in 1972 for the development of the theory of superconductivity.
I want to know the BCS Theory deeply
Cooper Pairs:
The behavior of superconductors suggests that electron pairs are coupling over a range of hundreds of nanometers, three orders of magnitude larger than the lattice spacing. Called Cooper pairs, these coupled electrons can take the character of a boson and condense into the ground state.
This pair condensation is the basis for the BCS theory of superconductivity. The effective net attraction between the normally repulsive electrons produces a pair binding energy on the order of milli-electron volts, enough to keep them paired at extremely low temperatures.
Isotope Effect, Mercury
If electrical conduction in mercury were purely electronic, there should be no dependence upon the nuclear masses. This dependence of the critical temperature for superconductivity upon isotopic mass was the first direct evidence for interaction between the electrons and the lattice. This supported the BCS theory of lattice coupling of electron pairs.
It is quite remarkable that an electrical phenomenon like the transition to zero resistivity should involve a purely mechanical property of the lattice. Since a change in the critical temperature involves a change in the energy environment associated with the superconducting transition, this suggests that part of the energy is being used to move the atoms of the lattice since the energy depends upon the mass of the lattice. This indicates that lattice vibrations are a part of the superconducting process. This was an important clue in the process of developing the BCS theory because it suggested lattice coupling, and in the quantum treatment suggested that phonons were involved.
Type I Superconductors:
The thirty pure metals listed at right below are called Type I superconductors. The identifying characteristics are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and a critical magnetic field above which superconductivity ceases.
The superconductivity in Type I superconductors is modeled well by the BCS theory which relies upon electron pairs coupled by lattice vibration interactions. Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behavior correlates well with the BCS Theory.
While instructive for understanding superconductivity, the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called "soft" superconductors while the Type II are "hard", maintaining the superconducting state to higher temperatures and magnetic fields.
Type II Superconductors:
Superconductors made from alloys are called Type II superconductors. Besides being mechanically harder than Type I superconductors, they exhibit much higher critical magnetic fields. Type II superconductors such as niobium-titanium (NbTi) are used in the construction of high field superconducting magnets.
Type-II superconductors usually exist in a mixed state of normal and superconducting regions. This is sometimes called a vortex state, because vortices of superconducting currents surround filaments or cores of normal material.
Meissner effect:
When a material makes the transition from the normal to superconducting state, it actively excludes magnetic fields from its interior; this is called the Meissner effect.
This constraint to zero magnetic field inside a superconductor is distinct from the perfect diamagnetism which would arise from its zero electrical resistance. Zero resistance would imply that if you tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz's law). But if the material already had a steady magnetic field through it when it was cooled trough the superconducting transition, the magnetic field would be expected to remain. If there were no change in the applied magnetic field, there would be no generated voltage (Faraday's law) to drive currents, even in a perfect conductor. Hence the active exclusion of magnetic field must be considered to be an effect distinct from just zero resistance. A mixed state Meissner effect occurs with Type II materials.
Perfect Diamagnet
If a conductor already had a steady magnetic field through it and was then cooled through the transition to a zero resistance state, becoming a perfect diamagnet, the magnetic field would be expected to stay the same.
Remarkably, the magnetic behavior of a superconductor is distinct from perfect diamagnetism. It will actively exclude any magnetic field present when it makes the phase change to the superconducting state.
BCS Theory of Superconductivity:
The properties of Type I superconductors were modeled successfully by the efforts of John Bardeen, Leon Cooper, and Robert Schrieffer in what is commonly called the BCS theory. A key conceptual element in this theory is the pairing of electrons close to the Fermi level into Cooper pairs through interaction with the crystal lattice. This pairing results from a slight attraction between the electrons related to lattice vibrations; the coupling to the lattice is called a phonon interaction.
Pairs of electrons can behave very differently from single electrons which are fermions and must obey the Pauli exclusion principle. The pairs of electrons act more like bosons which can condense into the same energy level. The electron pairs have a slightly lower energy and leave an energy gap above them on the order of .001 eV which inhibits the kind of collision interactions which lead to ordinary resistivity. For temperatures such that the thermal energy is less than the band gap, the material exhibits zero resistivity.
Bardeen, Cooper, and Schrieffer received the Nobel Prize in 1972 for the development of the theory of superconductivity.
I want to know the BCS Theory deeply
Cooper Pairs:
The behavior of superconductors suggests that electron pairs are coupling over a range of hundreds of nanometers, three orders of magnitude larger than the lattice spacing. Called Cooper pairs, these coupled electrons can take the character of a boson and condense into the ground state.
This pair condensation is the basis for the BCS theory of superconductivity. The effective net attraction between the normally repulsive electrons produces a pair binding energy on the order of milli-electron volts, enough to keep them paired at extremely low temperatures.
Isotope Effect, Mercury
If electrical conduction in mercury were purely electronic, there should be no dependence upon the nuclear masses. This dependence of the critical temperature for superconductivity upon isotopic mass was the first direct evidence for interaction between the electrons and the lattice. This supported the BCS theory of lattice coupling of electron pairs.
It is quite remarkable that an electrical phenomenon like the transition to zero resistivity should involve a purely mechanical property of the lattice. Since a change in the critical temperature involves a change in the energy environment associated with the superconducting transition, this suggests that part of the energy is being used to move the atoms of the lattice since the energy depends upon the mass of the lattice. This indicates that lattice vibrations are a part of the superconducting process. This was an important clue in the process of developing the BCS theory because it suggested lattice coupling, and in the quantum treatment suggested that phonons were involved.
Type I Superconductors:
The thirty pure metals listed at right below are called Type I superconductors. The identifying characteristics are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and a critical magnetic field above which superconductivity ceases.
The superconductivity in Type I superconductors is modeled well by the BCS theory which relies upon electron pairs coupled by lattice vibration interactions. Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behavior correlates well with the BCS Theory.
While instructive for understanding superconductivity, the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called "soft" superconductors while the Type II are "hard", maintaining the superconducting state to higher temperatures and magnetic fields.
Type II Superconductors:
Superconductors made from alloys are called Type II superconductors. Besides being mechanically harder than Type I superconductors, they exhibit much higher critical magnetic fields. Type II superconductors such as niobium-titanium (NbTi) are used in the construction of high field superconducting magnets.
Type-II superconductors usually exist in a mixed state of normal and superconducting regions. This is sometimes called a vortex state, because vortices of superconducting currents surround filaments or cores of normal material.
Thursday, December 23, 2010
Refrigeration Cycle
- The compressor
- The condenser
- The expansion device
- The evaporator
The copper refrigerant tube (a tube that connects these air conditioner parts)
We’ll be discussing the refrigeration cycle from split-central air conditioner units perspective; to make it easier.
Remember: refrigeration is a process that removes heat from an area that is not wanted and transfers that heat to an area that meaningless.
Ok, lets get started.
In this refrigeration diagram, the four major components split into two sections: Indoor and Outdoor. In indoor units, we have the AC parts number 1 and 2. In outdoor units, we have the AC parts number 3 and 4.
These four majors’ components are divided into two difference pressure: high pressure and low pressure.
The high pressure side is the condenser units (outdoor) and the low pressure side is the air conditioning evaporator (indoor). The divided point between high and low pressure cut through the compressor and the expansion valve.
Refrigeration cycle is a process that removes heat from indoor evaporator to outdoor condenser units. How does it do that?
AC parts # 1, Air conditioner evaporator.
The air conditioning evaporator is a heat exchanger that absorbs heat into the air conditioner system. The evaporator does not exactly absorb heat! It’s the cooled refrigerant fed from the bottom of the evaporator coils absorb the heat.
The liquid refrigerant usually flows from the bottom of the evaporator coils and boils as it moves to the top of the evaporator coils.
The reason it’s fed from the bottom is to ensure the liquid refrigerant boils before it leave the evaporator coils.
If a refrigerant was to fed from the top, the liquid refrigerant would easily drop to the bottom of the coils before it absorbs enough heat and boil.
If evaporator was too feed liquid refrigerant into air conditioner compressor; it will shorts the air conditioner compressor life.
This how air conditioning evaporator boils liquid refrigerant to vapor.
A (40°F) refrigerant flows through the evaporator, it absorbs 75°F indoor heat, causing the liquid refrigerant in the evaporator to boils.
Side Notes* the temperature of the refrigerant will always try to equalize. The 75°F heat will flow to 40°F refrigerant and it will increases the 40°F temperature and boils it.
After the liquid refrigerant travel across the evaporator coils, the entire liquid refrigerant should have boils. At this point it’s known as saturated vapor point.
The air conditioner evaporator has three important tasks:
Its absorb heat
Boils all the refrigerant to vapor aka saturated vapor
Superheat
Air conditioner parts # 2, Air conditioner compressors.
The air conditioning compressor is known as the heart of the air conditioner units. It’s one of the divided points between high and low side.
As you can see in the refrigeration cycle diagram; the compressor has a refrigerant inlet line and refrigerant outlet line.
The compressor inlet lines are known as:
Suction pressure
Back pressure
Low side pressure
The compressor outlet lines are known as:
High side pressure
Discharge pressure
Head pressure
The compressor absorbs vapor refrigerant from the suction line and compresses that heat to high superheat vapor.
As the refrigerant flows across the compressor, it also removes heat of compression, motor winding heat, mechanical friction, and other heat absorbs in the suction line.
The air conditioner units compressor produce the pressure different, it’s the air conditioner compressors that cause the refrigerant to flow in a cycle.
The compressor is a VAPOR pump!
Air conditioner parts # 3, Air conditioner condenser .
In this refrigeration cycle diagram, the air conditioner condenser is air cooled condenser. It functions the same way as the evaporator but it does the opposite.
The condenser units are located outdoor with the compressor. It purposes is to reject both sensible and latent heat of vapor absorb by the air conditioner units.
The condenser receives high pressure and high temperature superheats vapor from the compressor and rejects that heat to the low temperature air. After rejected all the vapor heat, it turns back to liquid refrigerant.
The condenser has three important steps:
Its remove sensible heat or (de-superheat)
Remove latent heat or (condense)
Remove more sensible heat or (subcooled)
Air conditioner parts # 4, Air conditioner expansion valve or Thermostatic Expansion Valve (TXV’s) (TEV’s).
or Thermostatic Expansion Valve (TXV’s) (TEV’s).
All expansion device or metering device has similar function (to some extent); it’s responsible for providing the correct amount of refrigerant to the evaporator.
This is done by creating a restriction within the thermostatic expansion valve. The restriction causes the pressure and temperature of the refrigerant entering the Evaporator to reduce.
The refrigeration cycle diagram above has a thermostatic expansion valve. This expansion device has
Remote Bulb
Capillary Tube
TXV Body
Thermostatic expansion valve has other components besides these three. However, they are not important right now.
How does TXV provides the correct amount of refrigerant?
TXV provides the correct amount of air conditioner refrigerant to the evaporator by using a remote sensing bulb as a regulator. The remote sensing bulb and capillary tube has a refrigerant inside.
As you can see in the refrigeration cycle diagram above, the remote sensing bulb is tie with the suction line. The temperature from the suction line transfer heat to the sensing bulb through conduction.
Sensing bulb responds to the temperature of the suction line and as a result, it decreases or increases the temperature and pressure inside the sensing bulb due to suction line temperatures. The sensing bulb also has a diaphragm on the other end. This diaphragm is with the TXV body.
The diaphragm is the device that pushes or releases the needle from the valve seat. There is so much to it, but I hope this explains how expansion device work.
Remember* Refrigeration cycle diagram will always have the same basic components (compressor, condenser, expansion device, evaporator, and refrigerant tube.
These components may be in difference shape, capacity and size, but it does the same thing.
If you understand how the refrigeration cycle works, you understand how any air conditioner works. Since all air conditioners have the same basic five components and basic refrigeration cycle.
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