Let Us Learn About Mechanism of Propagation of Sound
In the laboratory, the propagation of sound can be observed in following ways:
Take a tuning fork and a hard pad. Allow the tuning fork to strike the pad which makes the prongs to vibrate. When it starts vibrating, the inward and outward movements takes place in prong which causes the movement of inward and outward towards the mean position.The tuning fork is given as,
The vibaration of the tuning fork produces the compressions and refractions of the sound in the air,
When the tuning fork vibrates in air, they force the particles of the air to vibrate back and forth by a small distance. While vibrating, when the prong moves to the right side, it sends out a compression and when the prong moves to the left, it produces a rarefraction in air.
The longitudinal waves in series produce compressions and rarefractions in air from the tuning fork.These compressions and rarefractions of sound waves is formed by the vibrating particles causing vibration in the ears, the eardrum vibrates for reproduction of sound.
Thus they require air molecules for sound propagation from one place to another. In other words, material medium is needed for the sound propagation.
Sound propagates through air as a longitudinal wave. The speed of sound is determined by the properties of the air, and not by the frequency or amplitude of the sound. Sound waves, as well as most other types of waves, can be described in terms of the following basic wave phenomena.
Reflection of Sound
The reflection of sound follows the law "angle of incidence equals angle of reflection", sometimes called the law of reflection. The same behavior is observed with light and other waves, and by the bounce of a billiard ball off the bank of a table. The reflected waves can interfere with incident waves, producing patterns of constructive and destructive interference. This can lead to resonances called standing waves in rooms. It also means that the sound intensity near a hard surface is enhanced because the reflected wave adds to the incident wave, giving a pressure amplitude that is twice as great in a thin "pressure zone" near the surface. This is used in pressure zone microphones to increase sensitivity. The doubling of pressure gives a 6 decibel increase in the signal picked up by the microphone. Reflection of waves in strings and air columns are essential to the production of resonant standing waves in those systems.
The notion of sound is rather remarkable. Something happens there and we know it here, even if we are looking the other way, not paying attention, or even asleep. The fact that some sounds can produce physical and emotional effects is just short of astounding. These notes will perhaps remove some of the mystery associated with sound and hearing, but probably none of the wonder.
Sound is...
Sound is a disturbance of the atmosphere that human beings can hear. Such disturbances are produced by practically everything that moves, especially if it moves
quickly or in a rapid and repetitive manner.
Sound moves
You should be aware that the air is made up of molecules. Most of the characteristics we expect of air are a result of the fact that these particular molecules are very
light and are in extremely rapid but disorganized motion. This motion spreads the molecules out evenly, so that any part of an enclosed space has just as many molecules as any other. If a little extra volume were to be suddenly added to the enclosed space (say by moving a piston into a box), the molecules nearest the new volume would move into the recently created void, and all the others would move a little farther apart to keep the distribution even.
Because the motion of the molecules is so disorganized, this filling of the void takes more time than you might think, and the redistribution of the rest of the air molecules in the room takes even longer. If the room were ten feet across, the whole process might take 1/100 of a second or so.
If the piston were to move out suddenly, the volume of the room would be reduced and the reverse process would take place, again taking a hundredth of a second until everything was settled down. No matter how far or how quickly the piston is moved, it always takes the same time for the molecules to even out.
In other words, the disturbance caused by the piston moves at a constant rate through the air. If you could make the disturbance visible somehow, you would see it spreading spherically from the piston, like an expanding balloon. Because the process is so similar to what happens when you drop an apple into a bucket, we call the disturbance line the wavefront.
If the piston were to move in and out repetitively at a rate between 20 and 20,000 times a second, a series of evely spaced wavefronts would be produced, and we would hear a steady tone. (One wavefront is heard as a click.) The distance between wavefronts is called wavelength.
The Physics of Sound
Sound in air is the transfer of periodic movements between adjacent colliding atoms or molecules. This sonic energy typically expands away from the site of the collisions as a spherical or bubble-shaped emanation, the surface of which is in a state of radial oscillation.
The sonic bubble expands and contracts with the same periodicities as the initiating sound source. The accepted model of sound waves is incomplete because it uses the graphical representation of the mathematical law of sinusoidal energy, typically given as amplitude in the vertical axis versus time in the horizontal. While this is correct in terms of graphical depiction, it is not how the energy actually moves through space.
Sound in air does not travel as longitudinal waves as is commonly described in physics text books. Sound propagates spherically in air due to diffraction, the reactive result of atomic collisions. Reciprocal effects in air occur in the jostling of molecules initiated by a sound event, causing components of the sonic energy to move in all directions almost simultaneously. The distribution of energy within the sonic bubble is always concentrated on axis with the direction of primary propagation from the sound source.
Atoms that have the same number of protons but different numbers of neutrons are called isotopes. The element hydrogen, for example, has three commonly known isotopes: protium, deuterium and tritium.
Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.
Atoms of a specified element have the similar atomic number that is they include the same number of protons. However, they may include dissimilar numbers of neutrons. They correspond to the similar element and are chemically identical; they have dissimilar mass number (iso means same, tope means same place; they occupy the same place in the periodic table).
Isotopes:
Therefore atoms of the similar element having similar number of protons (atomic number) but unlike number of neutrons (dissimilar mass numbers), are known as isotopes. Isotopes may be logically happening or synthetically made.
Radio isotopes:
The term radio-isotope is the shortened form of radioactive isotope. If the isotope of an element is radioactive, then the isotope is called radio-isotope, referred to as radio-nuclide. Atleast one radio-isotope of every element is available. Over a thousand of them can be made artificially, mostly in nuclear reactors using slow neutrons as bombarding particles.
Examples for radio- isotopes:
Cobalt -60 an isotope of cobalt is radioactive; it is usually referred to as radio-cobalt. Radio-iodine(I181), radio-iron (Fe59), radio-sodium(Na24), Radio- phosphorous(P30), radio-cobalt(Co60), radio-sulphur(S35) and radio- carbon(C14) are some of the radio- isotopes.
Uses of Isotopes:
Radio isotopes find applications in various fields; some are listed below.
a) Radio- phosphorous is used in agriculture to determine the kind of phosphates required for a given soil and crop.
b) Radio- iodine is used in the treatment of overactive thyroid glands and radio-cobalt in the treatment of cancer. Radio- sodium is used to study the action of medicines.
c) Radio- cobalt or radio-iridium is used in industry to check machine parts.
d) Radio- carbon is used to estimate the age of fossils and archaeological specimens.
Characteristics of isotopes
All isotopes of an element have the same number of valence electrons thus have identical chemical properties.
The physical properties of the isotopes are different due to the difference in the number of neutrons in their nuclei. The densities, melting points and boiling points etc., are slightly different.
Types of Isotopes
The isotopes are called as the atoms of the similar elements contain the similar atomic number and the similar chemical properties but various only in mass number.
The atomic masses of each isotope of the same element are different because the number of neutrons present in their nucleus is different.
The types of isotopes are declared in two types.
Radioactive isotopes
Non-radioactive isotopes
The first type is radioactive isotopes. These isotopes are unstable and impulsively disintegrate and give out alpha, beta and gamma rays.
The second type is non-radioactive isotopes. This type of isotopes is stable.
Application of the isotopes
An isotope of uranium (U-235) is used in nuclear reactors to produce nuclear energy.
An isotope of cobalt is used to treat patients with cancer.
An isotope of iodine is used in the treatment of goiter.
14C6 is used in estimating the age of old and dead objects in archaeology.
Electrons are much smaller than neutrons and protons. The mass of a single neutron or proton is more than 1,800 times greater than the mass of an electron. An electron has a mass of 9.11 x 10-28 grams.
Electrons have a negative electrical charge, with a magnitude which is sometimes called the elementary charge or fundamental charge. Thus an electron is said to have a charge of -1. Protons have a charge of the same strength but opposite polarity, +1. The fundamental charge has a strength of 1.602 x 10-19 coulomb.
The electron (also called negatron, commonly represented as e−) is a subatomic particle. In an atom the electrons surround the nucleus of protons and neutrons in an electron configuration. The word electron is a transliteration of the Greek word ηλεκτρον, which means electrum, an alloy of silver and gold.
Electrons have an electrical charge and when they move, they generate an electric current. Because the electrons of an atom determine the way in which it interacts with other atoms, they play a fundamental part in chemistry.
An electron is defined as a subatomic particle which carries one unit of electrical charge (1.602 x 10-19 C) and has a mass (9.1 x10 -28g).
The mass of an electron is almost negligible, being 1/1837th the mass of an atom of hydrogen. The charge of an electron is referred to as unit negative charge and is the smallest known electrical charge.
The discharge tube experiments showed that irrespective of
The gas used
The nature of the material of the cathode, all electrons were found to have the same mass and same charge and therefore the same e/m ratios. Thus electrons of all cathode rays are the same and only electrons (no gaseous atoms) make up the fundamental common particles fo the rays.
It has been found that all electrons emitted from all sources and by all methods have the same mass and same charge. The electron in the atom is considered the universal constituent of all matter.
Charge and Mass of Electron
The charge to mass ratio is found by measuring the deflection of a ray under the simultaneous influence of electrical and magnetic fields, applied perpendicularly to each other as well as to the direction of the flow of light. This is illustrated in the figure below:
A high voltage charge accelerates cathode ray electrons between cathode and anode. After the anode, a circular disc selects a straight beam and directs it past the electric and magnetic fields, which are perpendicular to each other as well as to the direction of the motion of the light beam. The beam is deflected according to the relative strengths of the electric and magnetic fields and the ratio of e/m controls the deflection. By measuring the deflection and the field strengths of the two fields the e/m ratio can be calculated
The proton was discovered by Ernest Rutherford in 1918 since the proton is in the atomic nucleus, it is a nucleon. Since it has a spin of -1/2, it is a fermion. Since it is composed of three quarks, it is a triquark baryon, a type of hadron. Since the atom is electrically neutral there had to be positively charged particles present in the atom to neutralize the negative charge of the electrons.
Production of Anode Rays
Goldstein experimentally proved the existence of positive charge in matter.
In his experiments, a perforated cathode was used in a discharge tube along with air at very low pressure of about 0.001 mm of mercury. When a high voltage of about 10,000 volts was applied to this cathode in the discharge tube, a faint red glow was observed behind the perforated cathode.
The rays were formed at the anode and when these rays struck the walls of the discharge tube behind the anode they produce a faint red light. Since the rays were formed at the positive electrode or anode, they were known as anode rays or positive rays.
Formation of Positive Rays
When high electrical voltage is applied to a gas, its ato
ms break up into negatively charged particles (electrons) and positively charged particles. These positively charged particles formed by the removal of electrons from the gas atoms are called positive rays.
Effect of Low Pressure in the Discharge Tube
When the gas atoms in the discharge tube are at atmospheric pressure they collide with the electrons preventing them from reaching the anode. As no electrons reach the anode no current flows through the discharge tube. When the gas pressure is very low there are few gas atoms in the discharge tube. As such there is no hindrance to the movement of electrons the gas conducts electricity.
Characteristics of a Proton
Mass of a proton
A proton can be thought of as a hydrogen atom that has lost its electron. Since the mass of an electron is small, the mass of a proton is equal to the mass of a hydrogen atom. As the mass of hydrogen atom is for all practical purposes 1 a.m.u., the relative mass of a proton is 1 a.m.u. The absolute mass of a proton is 1.6 x 10-24 gram.
Compared to an electron, a proton is very dense. It is about 1837 times denser than an electron. It means that almost all the mass of the atom is in the nucleus.
Charge of a Proton
The proton has an equal and opposite to the charge of an electron. So, the absolute charge of a proton is 1.6 x 10-19 coulomb of positive charge. This being the smallest positive charge carried by any particle, it is taken as 1 unit positive charge. The relative charge of a proton is +1 (plus one).
Effect of low pressure in the discharge tube
When the gas atoms in the discharge tube are at atmospheric pressure they collide with the electrons preventing them from reaching the anode. As no electrons reach the anode no current flows through the discharge tube. When the gas pressure is very low there are few gas atoms in the discharge tube. As such there is no hindrance to the movement of electrons and the gas conducts electricity.
When wave energy like sound or radio waves travels from two objects, the wavelength can seem to be changed if one or both of them are moving. This is calledthe Doppler effect.
The Doppler Effect causes the received frequency of a source (how it is perceived when it gets to its destination) to differ from the sent frequency if there is motion that is increasing or decreasing the distance between the source and the receiver. This effect is readily observable as variation in the pitch of sound between a moving source and a stationary observer. Imagine the sound a race car makes as it rushes by, whining high pitched and then suddenly lower. Vrrrm-VROOM. The high pitched whine is caused by the sound waves being compacted as the car approaches you, the lower pitched VROOM comes after it passes you and is speeding away. The waves are spread out.
When the distance between the source and receiver of electromagnetic waves remains constant, the frequency waves is the same in both places. When the distance between the source and receiver of electromagnetic waves is increasing, the frequency of the received wave forms is lower than the frequency of the source wave form. When the distance is decreasing, the frequency of the received wave form will be higher than the source wave form.
Besides sound and radio waves, the Doppler Effect also affects the light emitted by other bodies in space. If a body in space is "blue shifted," its light waves are compacted and it is coming towards us. If it is "red shifted" the light waves are spread apart, and it is traveling away from us. All other stars we have detected are "red shifted," which is one piece of evidence for the theory that the universe is constantly expanding, perhaps from a "big bang."
The Doppler Effect is something which occurs when something which emits sound or light moves relative to an observer. The object, observer, or both can move, causing an apparent change in the frequency of the wavelengths being emitted by the object. The Doppler Effect explains why a rude driver's car horn appears to change in frequency as he or she zooms by while leaning on it, and an understanding of the Doppler Effect can help scientists make a variety of observations about the world around them.
These are the simplest and most popular type of Heat-pipe and are used in most applications to transfer heat energy from one point to another. They can also be used as heat spreaders to isothermalise components where a uniform temperature is desired. Although Heat-pipes are predominantly used in cooling applications, they can also be used very effectively in heating applications, this negates having multiple electrical heating elements, and simplifies cabling, design and installation.
Annular Heat-pipes
These are Heat-pipes with an axial concentric hole through the middle. These may be used in a variety of situations where high heat transfer is required together with mechanical access, examples include; as a flow channel for gas or liquid, for cabling or thermocouple access, as a push rod mechanism location sleeve.Larger diameter Heat-pipe rollers have also been produced similarly with open bore for use as rotary thermal spreaders and having internal air cooling.
Thermosyphon- gravity assisted wickless heat pipe. Gravity is used to force the condensate back into the evaporator. Therefore, condenser must be above the evaporator in a gravity field.
Leading edge-placed in the leading edge of hypersonic vehicles to cool high heat fluxes near the wing leading edge.
Rotating and revolving-condensate returned to the evaporator through centrifugal force. No capillary wicks required. Used to cool turbine components and armatures for electric motors.
Cryogenic- low temperature heat pipe. Used to cool optical instruments in space
Flat Plate- much like traditional cylindrical heat pipes but are rectangular. Used to cool and flatten temperatures of semiconductor or transistor packages assembled in arrays on the top of the heat pipe
Micro heat pipes- small heat pipes that are noncircular and use angled corners as liquid arteries. Characterized by the equation: rc /rh³1 where rc is the capillary radius, and rh is the hydraulic radius of the flow channel. Employed in cooling semiconductors (improve thermal control), laser diodes,photovoltaic cells, medical devices.
Variable conductance- allows variable heat fluxes into the evaporator while evaporator temperature remains constant by pushing a non- condensable gas into the condenser when heat fluxes are low and moving the gas out of the condenser when heat fluxes are high, thereby, increasing condenser surface area. They come in various forms like excess-liquid or gas-loaded form. The gas-loaded form is shown below. Used in electronics cooling
Capillary pumped loop heat pipe- for systems where the heat fluxes are very high or where the heat from the heat source needs to be moved far away. In the loop heat pipe, the vapor travels around in a loop where it condenses and returns to the evaporator. Used in electronics cooling.
Diode Heat-pipes Diode Heat-pipes have a high thermal conduction in one direction and a low thermal condution in the opposite direction. They are typically used in aerospace applications where it is possible for heat sink temperatures to exceed the Heat-pipe evaporator temperature due to solar radiation. Usually an array of diode Heat-pipes are used together to dissipate heat to a number of alternative heat sinks so that heat sinking facilities are constantly maintained.
Variable Conductance Heat-pipes VCHP Heat-pipes provide a means for automatic temperature control. By regulation of the Heat-pipe thermal conductivity it is possible to maintain the evaporator section at near constant temperature with around only 5 °C fluctuation. VCHP’s are specifically pre-set to operated over a specific temperature range of varying ambient condidions and variable heat input loading. Where very precise control is required they can be also integrated with electronic control achieving temperature stabilization of better than 1°C with minimal electrical power comsuption for the controlling circuit.
Flexible Tubular Heat-pipes A limited degree of flexibility is possible with most standard range Heat-pipes, as a function of their length and diameter. Often this may be sufficient to accommodate small dimensional tolerance differences encountered during assembly and installation with other components. Truly flexible Heat-pipes incorporate an intermediate convoluted bellows section providing excellent flexibility and anti-vibration characteristics.
High Performance Heat-pipe Heat Sinks Integral Heat-pipe heat sinks built with cooling fin assemblies provide one of the most effective means of providing efficient cooling for power electronics components. The forced air cooled assembly shown opposite achives an oustanding thermal performance. Performance: Rth = 0.09°C/W.
i.e with 100 W heat dissipation the input heat block temperature is only 9°C higher than the ambient cooling air temperature!
Baffle Heat-pipes Baffled Heat-pipes are used in water cooled applications i.e. core cooling applications used commonly in plastic injection moulding tools. The Heat-pipes are installed into manifolds and transfer heat from the heat source to the cooling water. The baffle plates direct water along the Heat-pipe cooling length to give an increased cooling area contact.
Bent Tubular Heat-pipes Common application requirements require Heat-pipes to be bent to fit a route in a particular installation, possibly involving complex 3-D architecture. By using special internal wick structures, CRS Engineering is able to produce Heat-pipes which can be formed to shape by customers upon installation. Where exact bending is required, or where tight bending radii are involved, it is essential that CRS Engineering carry out this process during manufacture.
