For all the circuits lumped elements in which time varying voltages / current exists , we find a relation in which the voltage is proportional to Current . The proportional quantity is in general a complex number and this is called Impedance.
It is a function of frequency of ‘w’ strictly speaking impedance is sum of its real and imaginary parts.
Z= R+ IX
Impedance is equivalent to a Resistance in series with pure imaginary Impedance - called a Reactance.
The voltage drop across the resistance is in phase with the current , while the voltage drop across the purely Reactive part is out of phase with the current.
The average energy loss in an impedance Z= R + iX depends only on real part of ‘Z’ and not on complex part.
There is no energy loss in Reactive part.
GP THOMSON EFFECT - Experimental Verification of Wave nature of Matter
G P Thomson has performed experiments with electrons accelerated from 10000 to 50000 volts.
The high energy beam of electrons is produced by a cathode ‘C’.
The experimental arrangement is as shown
below:
The electron beam is excited with potential upto a maximum of 50,000 volts. A fine beam is obtained by passing it through slit or diaphragm ‘S’.
The accelerating fine beam of electrons now falls on thin gold or Aluminum film (order of 10-6 cm thickness).
The photograph of beam from foil is recorded on photographic plate ‘P’.
After developing the plate, a symmetric pattern consisting of concentric
rings about a central spot is obtained. This pattern resembles that of X- rays.
To know that this pattern is due to electrons or due to x-rays generated
by electrons in their passage through foil, cathode rays in discharge tube are
deflected by magnetic field.
It was observed that beam shifts correspondingly showing there by that
pattern is produced by electrons and not by x-rays. (i.e X – ray pattern is not affected by
electric and magnetic fields).
As diffraction pattern can only be produced by waves and not by
particles, so Thomson concluded that electrons behave like wave.
Thus, Thomson experiment clearly demonstrated the existence of
matter waves.
BCS (Bardeen - Cooper - Schieffer) THEORY
The microscopic theory put forward by Bardeen, Cooper and Schreiffer (BCS), in 1957 provides the better quantum explanation of superconductivity and explains well all the properties exhibited by superconductors.
The basis of formulation of BCS theory are two experimental conclusions namely the isotope effect and variation of specific heat of superconductors.
For isotope effect Tc M^1/2 = constant, one can infer that transition resulting in superconducting state must involve dynamics of ion motions, lattice vibrations and Phonons.
Further we note that Tc attains a value zero when 'M' approaches infinity. This all suggests that the non zero transition temperature is a consequence of finite mass of ions which can contribute Phonons by their vibrations.
Bardeen pointed out that an electron moving through a crystal lattice has a self energy accompanied by virtual Phonons. This means that an electron moving through crystal lattice distorts the lattice and lattice in turn acts on electron by virtue of electrostatic forces between them. The oscillatory distortion of lattice is quantized in terms of Phonons and so one can interpret the interaction between lattice and electron as constant emission and re-absorption of Phonon by latter. These are called virtual Phonons.
BCS showed that basic interaction responsible for responsible for superconductivity appears to be that of a pair of electrons by means of interchange of virtual Phonons.
Suppose an electron approaches a positive ion core . It suffers attractive Coulomb interaction. Due to this attraction ion core is set in motion and consequently distorts the lattice.
Smaller the mass of positive ion core, the greater will be the distortion. Suppose towards that site another electron comes and sees this distorted lattice. Then the interaction between the two, the electron and distorted lattice occurs which in its effect lowers the energy of second electron.
Thus, we interpret that the two electrons interact via the lattice distortion or the Phonon field resulting in lowering of energy for electrons.
The lowering of electron energy implies that force between two electrons is attractive. This type of interaction is called electron-lattice-electron interaction. This interaction is strongest when two electrons have equal and opposite momenta and spins.
Since the oscillator distortion of Lattice is quantized in terms of Phonons, the above interaction can be interpreted as electron-electron interaction through Phonon as mediator.
Let an electron of wave vector K1 emits a virtual phonon 'q' which is absorbed by an electron with wave vector K2. K1 is thus scattered as K1-q and K2+q.
The nature of resulting electron-electron interaction depends on relative magnitudes of electronic energy and phonon energy. If the phonon energy exceeds electronic energy, the interaction is attractive.
When such interaction occurs by phonon exchange by dominating usual repulsive interaction, two such electrons from a pair called as Cooper pair.
The energy of pair of electrons in bound state is less than energy pair in free state. The difference in energy of two states is binding energy of cooper pair.
The energy difference between free state of electron and paired state appears as energy gap at Fermi surface. The normal electron states are above the energy gap and superconducting electron states are below the energy gap at Fermi surface.
What is Meissner Effect?
The Meissner Effect
Superconductors which are resistance less materials have an additional property of exclusion of applied magnetic field on it i.e. inside a superconducting material, we always have B=0.
The property of perfect diamagnetism arises in super conductor because when a magnetic field ‘Ba’ is applied surface screening currents circulate so as to produce a flux density ‘Bi’ which every where inside the metal exactly cancels the flux density due to applied field Bi=-Ba.
The property of perfect diamagnetism arises in super conductor because when a magnetic field ‘Ba’ is applied surface screening currents circulate so as to produce a flux density ‘Bi’ which every where inside the metal exactly cancels the flux density due to applied field Bi=-Ba.
For a Super Conductor μr=0; i.e. B=μrBa=0
TThis property of exhibiting perfect diamagnetism by super conductor is known as Meissner Effect.
What is GM Counter and how does it operate?
The counter is named as GM counter based on its developers ‘Geiger’ and ‘Muller’ in 1928. They are oldest type of gas filled radiation detectors.GM counters were operated in the Geiger discharge region of gas filled ion chambers.
Construction: - The counter is usually a leak tight assembly of two electrodes electrically isolated. The counter is filled to sub atmospheric pressures of few mm of mercury. A high voltage is applied to the anode electrode.
Principle: - This counter also works on the principle of ionization caused by incoming energetic particle in the gas medium filled between anode and cathode. The electron liberated in the primary ionization event would get accelerated towards anode because of its high potential. The electron may gain sufficient energy to cause ionization of other gas molecule. This leads to a chain of ionizing events which is usually referred to as Townsend avalanche. During this process, there may be interactions in which excitation of atoms may occur due to sufficient energy of impinging electrons. Such atoms while de-exciting may emit photons which normally fall in UV or visible region. These photons which are emitted may again lead to photo electrons due to ionization of gas atoms or due to photoelectric interaction with walls of counter. Each photo electron would again cause Townsend effect. Such a series of Townsend avalanches would lead to discharge in the tube called Geiger-discharge. In such a state there is formation of dense envelope of electron-ion pairs distributed on either side of anode.
The voltage applied to anode shall be such that it is enough to trigger the avalanche mechanism and collect total charge (electrons) pertaining to single event leading to Geiger discharge.
Concept of quenching: - Practically the process would not be as simple as above. During the Geiger discharge, there is dense envelope of electrons and ions. The electrons would drift towards anode and positive ions would drift towards cathode. The positive ions which drift towards cathode having ionization potential (E) greater than the work function (W) of cathode material leads to exchange of electron from cathode and becomes neutral. The excess energy may be dissipated in two forms, one by emission of photon or an electron form cathode if excess energy is greater than the work function of the cathode material. This would again initiate another Geiger discharge. The result of this is that the tube would always be in continuous Geiger discharge and hence will not able to measure any radiation.
To overcome this problem, concept of quenching is introduced. There are two types of quenching
i) Organic quenching
ii) Halogen quenching
Organic quenching: -
This involves addition of small quantity of organic gas having complex molecule structure. This prevents the continuous Geiger discharge mechanism by charge transfer collision principle. The positive ions on their path collide with organic molecules to get neutralized. This makes only ions of organic gas reach cathode and gets neutralized. If there is any excess energy released leads to dissociation of organic molecules. Thus multiple Geiger discharges could be avoided.
A typical filling of organic quenched GM tubes is 90% Argon and 10% of ethyl alcohol. When organic gas gets depleted to a sufficient extent there is occurrence of multiple discharges frequently and thus the plateau length gets decreased, with slope increased.
Thus the organic quenched GM tubes are characterized by short life time and thus not suitable for operation in very high fields which leads to large count rate. To overcome this, technique of Halogen quenching is introduced.
Halogen quenching: - This involves the addition of small quantity of Halogen gas such as Chlorine or Bromine. A typical filling is about 0.1% of chlorine to Neon. The quenching action is same as that in Organic quenching process. The diatomic halogen gas molecules too gets dissociated in quenching but gets recombined to replenish the gas molecules and thus counter life gets extended.
Characteristics of GM tubes:-
The important parameters which decide the quality of functioning of Gm tubes are
i) Dead time
ii) Recovery time
iii) Plateau length &Plateau slope
i) Dead time: - As discussed above, the positive ions take considerable time to reach cathode tube compared to electrons. The reason is that the mobility of electrons is about 1000 times greater than that of electrons.
Due to the low drift velocity of positive ions, there is formation of cloud of positive ions which tend to electric field opposite to that of actual field. This reduces the electric field intensity due to anode potential and thus affects gas multiplication factor. This in turn affects the pulse heights.
In high count rates, it is more worse that there is formation of dense positive cloud which makes the electric field intensity in the vicinity of anode wire reduce by great margin thus multiplication goes down by big margin. During this phase of detector, any new ionizing event caused by incoming particle cannot be recorded. Thus the time interval during which any event caused by newly incoming particle would not get counted and called as dead time of the country.
ii) Recovery time: - After certain time, all the positive ions tend to reach cathode wall and thus the electric field begins to restore to actual value. When the electric field goes beyond a critical value there is again formation for pulses. But the process requires some time to give maximum pulse heights. Hence the total time required for GM tube to give maximum pulse height pulses is Recovery time.
iii) Plateau length & Slope: In order to decide the operating voltage of the GM tube, a graph between anode voltage (X axis) and count rate (Y axis) is plotted. After applying minimum voltage to initiate Geiger discharge, the no. of pulses shall remain same in fixed radiation field exposure. But due to formation of short pulses during recovery time there is variation in count rate. Thus one of the quality parameters deciding the operation of GM tube is that plateau slope shall be less. Usually 2-3% plateau slope is a good choice. As we go on applying voltage to the anode, the tube starts entering continuous discharge region. Thus the slope gets worsened. The region or length of voltage region during which the plateau slope remains in desired value is called as plateau length and usually the operating voltage is chosen at the midpoint of plateau length.
Comparison between Frequency Modulation(FM) & Amplitude Modulation(AM)
Frequency Modulation |
Amplitude Modulation |
|
1) The
amplitude of FM signal is constant and in depth of Modulation. |
1) Amplitude of AM signal varies depending on Modulation index. |
|
2) It
requires much wider channel (7-15 times) as compared to AM |
2) Band width, is very small, which is one of the biggest
disadvantages. |
|
3) Transmitters
are complex and hence expensive |
3) Relatively simple and cheap |
|
4) Area
of reception is small since it is limited to line of sight |
4) Area of reception is large |
|
5) Noise
can be easily minimized. Amplitude variations can be eliminated by using
limiter. |
5) More susceptible to noise, interference & low signal to noise
ratio. Difficult to eliminate effects of noise. |
|
6) Average
power in frequency modulated wave is same as contained in un-modulated wave. |
6) Average power in modulated wave is greater than carrier power.
This added power is provided by modulating source. |
|
7) No
restriction is placed on modulated index. |
7) Maximum modulation index is 1 otherwise over modulation would
result in distortion. |
|
8) Possible
to operate several independent transmitters on same frequency. |
8) Not possible to operate out interference. |
Advantages of Optic Fibers
1. Optical fibers have greater information carrying capacities than metallic conductors.
2. Fibers and fiber cables are very strong and flexible. So fibres are so slender that they do not break when wrapped around.
3. One of the most important advantages of fibers is that they can carry large amount of information in either digital or analog form.
4. An optical fiber is well protected from external interference and coupling with other communication channels. It is because an optic fiber made of either glass or plastic is an insulator.
5. Electromagnetic interference caused by lightning and sparking etc doesn't effect fibers.
6. As compared to Copper, corrosion due to water or chemicals is less for glass. Glass fiber themselves can withstand high temperature before deteriorating.
7. Fiber offers a degree of security and privacy. Because fibers do not radiate energy with in them, it is difficult for an intruder to detect signal being transmitted.
Classical Mechanics Conceptual Questions - Experts Answers
1) How does Earth's Rotation relate to time taken by aircraft to reach from say India to the United States and back?
How does an image change in a 3D hologram depending on angle of viewing?
A Hologram is made by taking a single coherent beam, usually from a LASER, and splitting it into two beams.
One of the beams called a reference beam, directly hits a
photographic plate where as other is reflected off an object (whose image needs
to be stored in the Hologram). The interference pattern of these two beams is
stored as Hologram.
When light is focussed on this Holographic plate, it
reflects off the plate, but after mixing with the stored pattern.
So if the original
light beam that was used is directed at the corrected angle, it cancels out the
component corresponding to reference beam and we see the object.
However, in a typical room, Light hits hologram from all
angles and is also reflected back in all angles. Hence, there will be a beam that
will fall on plate at same angle as that of reference and reflect the image of
the object. If our eye happens to be in the path of the reflected beam then we
can see the stored object.
If two interference patterns were stored simultaneously with
different reference beam, we can see two different images depending on the
angle of viewing. Because the light reaches each eye is not exactly the same,
the 3D effect or perception of depth is produced.
ALL ABOUT NUCLEAR CROSSSECTION
The probability of a Nuclear Reaction can be defined in terms of number of particles emitted or number of nuclei undergoing transmutation for a specified number of incident particles.
It is usually expressed in terms of an effective area presented by a Nucleus towards the beam of bombarding particles, such that the number of incident particles that would strike such an area, calculated upon a purely geometrical basis, is the number observed to lead to Nuclear Reaction given in question.
This effective area is called crosssection for that reaction.
Thus the probability of occurrence of a particular Nuclear Reaction is described by effective crosssection for that process.
The crosssection may also be defined as
1) The probability that an event may occur when a single nucleus is exposed to a beam of particles of total flux one particle per unit area.
2) The probability that an event may occur when a single particle is shot perpendicularly at a target consisting of one particle per unit area.
The idea of crosssection gives imaginary area associated with each nucleus, the area is so chosen that if bombarded particle passes through it the reaction takes place, otherwise it is not.
The total nuclear crosssection is effective area possessed by a nucleus for removing incident particles from a collimated beam by all possible process.
This can be written as sum of several partial crosssections which represent contributions to various distinct, independent processes which can remove particles from incident beam.
Thus,
𝛔t = 𝛔s + 𝛔r ---------------(1)
𝛔t is "Total crosssection"
𝛔s is "Scattering crosssection"
𝛔r is "Reaction Crossection"
Scattering Crossection
Scattering crosssection can be classified as
i) Inelastic scattering
ii) Elastic Scattering
Thus, we get
𝛔s = 𝛔el + 𝛔inel --------(2)
These partial crossections can still be subdivided.
In case of elastic scattering separate partial crosssections cannot be written because of possibility of interference between them.
On other hand all inelastic scattering processses are incoherent and their crosssections are additive.
𝛔inel = 𝛔1 + 𝛔2 + 𝛔3 + ......... ------(3)
Differential crosssection
The distribution in angle of emitted particles in a nuclear
reaction can be described in terms of a crosssection which is a function of
angular coordinates in problem.
The crosssection which defines a distribution of emitted
particles with respect to solid angle is called differential crosssection. It
is defined by d
Partial crosssection for a given process is
𝛔 = ∫(d
Expression of crosssection for a Nuclear Reaction
Consider a mono energetic beam of particles incident on a target shown in Fig.
Let the beam be uniform and contain ‘n’ particles per unit
volume moving with a velocity ‘V’ with respective to stationary target.
Clearly the product ‘nV’ gives number of particles crossing
a unit area perpendicular to beam per unit time. It defines flux ‘F’ of
particles in incident beam.
F = nV -------------------------------(1)
It is customary to normalize number of particles to one
particle per volume ‘V’.
n = 1/V ------------------------------(2)
The detector detects all particles scattered through an angle ‘𝛳' into solid angle dΩ.
The number of particles dN detected per unit time depends on
following factors:
i)
Flux of incident beam, F
ii) The solid angle, dΩ
iii)
Number of independent scattering centers in
target that are intercepted by the beam. Let these be N.
dN = 𝛔(𝛳)*F*dΩ*N ---------------------(3)
𝛔(𝛳) is constant of proportionality defines differential
scattering crosssection.
We can put
𝛔(𝛳) =d𝛔(𝛳) / dΩ -----------------------(4)
N = F*N*𝛔total --------------------------(5)
where,
Total Crosssection 𝛔total = ∫𝛔(𝛳) dΩ ---------------------(6)
𝛔total has dimensions of area.
Unit used to express crosssections is barn.
1 barn = 10⁻²⁸ cm².
The area 'a' intercepted by beam contains 'N' scattering centers. Total number of incident particles per unit time is given by
(Nscattered/Nincident) = (N*𝛔total)/a ------------------------(7)
𝛔total is equal to area effective in scattering for one scattering center.
What is Penetration depth in Super Conductors
In 1935, F. London and H. London described Meissner effect and zero resistivity by adding two conditions E=0(absence of Resistivity) and B=0(Meissner effect) to Maxwells Electromagnetic equations.
According to them, the applied field does not suddenly drop to zero at the surface of super conductor, but decays exponentially according to equation
B(x) = Bₐexp(-x/ƛ)
B(x) - Magnetic field at depth 'x' of material
Bₐ - Applied field; ƛ - penetration depth
Penetration depth is the length or depth from surface of metal at which the magentic field falls to 1/e of its original value.
Generally the magnetic field is likely to penetrate a superconductor to a depth of 10 - 100 nm.
Penetration depth doesn't have a fixed value but varies with temperature.
ƛ = ƛ / [1-(T/Tc)⁴]
ALPHA PARTICLE SPECTRUM? - DETAILED EXPLANATION
in 1930, S. Rosenblum, in France proved by means of spectograph that the 𝛼 particles from Thc(Bi²¹²), all of which had been thought to have same energy, actually were consisted of number of groups of particles with slightly different energies. The 𝛼 particles form a given radioactive substance were collimated with slits and after deflection thru 180⁰ with strong magnetic field, formed lines on photographic plate.
For example, ₈₄Po²¹⁴ decays to ₈₂Pb²¹⁰ byemitting 4 groups of 𝛼 particles having ranges in air 6.91cm, 7.79 cm, 9.04cm and 11.51cm. These ranges correspond to energies 7.68MeV, 8.28MeV, 9.07MeV & 10.51 MeV respectively.
Another example is decay of ₉₀Th²²⁸.
₉₀Th²²⁸ → ₈₈Ra²²⁴ + 𝛼
It comprises of five groups of 𝛼-particles with different energies.
𝛼-particle which is emitted by transformation of excited state of parent nucleus to ground state of daughter nucleus will have maximum energy.
In the above example, out of 5, four groups of 𝛼-particles leave the daughter nuclei in excited state. The fifth group of 5.42MeV 𝛼-rays take one to ground state of daughter nuclei. We can note from figure that excited states of nuclei reach ground state by emitting 𝜸-rays shown by vertical wavy lines.
Thus the fine structure of 𝛼-spectrum tells us about energy levels in daughter nuclei. We emphasize that existence of these different 𝛼-energy groups and 𝜸-rays proves the existence of nuclear energy levels.
Long Range 𝛼-particles:
If the parent 𝛼-emitter emits 𝛼-particles when it is in an excited state, then we get long range 𝛼-particles. This is because the energy of excitation becomes available to 𝛼-particles as they reach the ground state of daughter nuclei.
The following figure illustrates the emission of long range 𝛼-particles from ₈₄Po²¹⁴.
𝛼ₒ - normal 𝛼-group corresponding to transition between ground states of ²¹²Po and ²⁰⁸Pb.
Thus, nuclear spectroscopic studies of long range 𝛼-emitters provide information about nuclear energy levels of parent.
How to set trip point in overload relay of starter?
Well, most of the engineers, who use the motors find sometimes get quizzed that how they should find right trip point on the overload relay in the starter for the motor.
For this, one must find the current rating provided on the motor. This is usually the safe current which the winding of the motor can work with out failure. Usually the winding of motor can withstand high current also based on the time. If the motor curve is available it is easy to find out the required trip current based on the trip time.
or else, one has to consider the 1.5 times of the rated current of motor. When we set the trip point on the relay at the required current, the next step is to ensure the trip time. Trip time could be find out by using equivalent load.
The final step to ensure that you have used the proper relay is to perform rotor lock test
of the motor.
What is rotor lock test?
The name itself gives and idea that it is locking of the shaft or rotor. One has to lock the rotor and switch on the motor.
This will lead to high current passage through winding. The relay should get tripped within prescribed time by manufacturer
to stop damage to the winding. This is the worse condition of failure and thus, suitable use of relay and trip set point, will always protect the motor.
Any queries are welcome.
Polar Dielectric in Uniform Electric Field
i) There are permanent dipoles present in Polar Dielectric which are randomly aligned in such a way that there exists permanent dipole moment Pp.
ii) When a dipole is present in an uniform electric field the dipole tries to align itself in the direction of electric field.
iii) Because of this all dipoles in polar dielectric are partially aligned in the direction of the field. This partial alignment is responsible for the induced dipole moment Pi.
Therefore the electric dipole moment is increasing.
P = Pp + Pi
iv) The electric dipole moment of a polar dielectric increases
a) by increasing the applied electric field
b) by decreasing the temperature
Thermodynamics - important points to be noted for competitive exams
➔ Tephigram is the name of temperature entropy diagram
➔ PV graph in a adiabatic change is called Isentropic.
➔ Entropy of a system is an index of “unavailable energy”.
➔ When gas is expanded, work is done by the gas on surroundings.
➔ The size of “Kelvin degree” is equal to “Centigrade”.
➔ The efficiency of a carnot engine increases by raising the temperature of the source.
➔ Work done per cycle is given by the area enclosed in the indicator diagram.
➔ Conversion of heat energy into electrical energy can be made by “Thermocouple”.
➔ f(P,V,T) =0 exists for an equilibrium state and is called equation of state.
➔ The area of cycle of T-S diagram gives the “available thermal energy for useful work” in a reversible process.
➔ Uses of TS diagram:
a) used in meteorology b) check efficiency of heat engine c) useful in predicting defects of performance of engine d) to obtain work value of fuel used.
➔ Change in entropy of universe due to free expansion is
∆S = nR log e(Vf/Vi)
➔ Loss of available of energy = To.dS where ‘To’ is lowest available temperature in system.
➔ In order to maintain a body in an isothermal condition, heat has to be either supplied or withdrawn.
➔ When a gas expands adiabatically, the temperature decreases.
➔ When a gas is compressed, the temperature increases because work is done on the gas.
➔ The work done in an adiabatic change in a particular gas depends upon only change in temperature.
➔ In an adiabatic compression, the decrease in volume is associated with increase in temperature & increase in pressure.
➔ For an isothermal expansion of a perfect gas, the value of dP/P is equal to -dV/V.
➔ Change in entropy of universe due to free expansion is
∆S = nR log e(Vf/Vi)
➔ Loss of available of energy = To.dS where ‘To’ is lowest available temperature in system.
➔ In order to maintain a body in an isothermal condition, heat has to be either supplied or withdrawn.
➔ When a gas expands adiabatically, the temperature decreases.
➔ When a gas is compressed, the temperature increases because work is done on the gas.
➔ The work done in an adiabatic change in a particular gas depends upon only change in temperature.
➔ In an adiabatic compression, the decrease in volume is associated with increase in temperature & increase in pressure.
➔ For an isothermal expansion of a perfect gas, the value of dP/P is equal to -dV/V.
➔For an adiabatic expansion of a perfect gas, the value dP/P is equal to -𝛾dV/V.
➔ A reversible process is always “quasi-static”, but every quasi-static process need not be a reversible process.
➔ For reversible cycle: ∆P = ∆V = ∆T = ∆U = ∆H
➔ dW = PdV is only applicable to reversible process.
➔ In case of “irreversible processes”, dW is not equal to PdV;
➔ For reversible cycle: ∆P = ∆V = ∆T = ∆U = ∆H
➔ dW = PdV is only applicable to reversible process.
➔ In case of “irreversible processes”, dW is not equal to PdV;
➔For free expansion, dW=0
➔For free expansion, dV=0, the work may be zero (in case of PV work)
➔For free expansion, dV=0, the work may be zero (in case of PV work)
➔ Work and heat are path functions.
➔ Work is not a thermodynamic property as it is not a state function and it is not a exact differential.
➔ Both thermodynamic and temperature scales use a single reference temperature i.e triple point of water.
➔ dW = PdV is only applicable to reversible process.
➔ Work is not a thermodynamic property as it is not a state function and it is not a exact differential.
➔ Both thermodynamic and temperature scales use a single reference temperature i.e triple point of water.
➔ dW = PdV is only applicable to reversible process.
➔ In case of “irreversible processes”, dW is not equal to PdV;
Heat Transfer due to conduction
In this mechanism, heat transfer is due to vibration amplitudes of molecules & atoms present in solids.
Consider a cubicle of solid. Let us maintain one face of cube at high temperature (TH) and other opposite face at low temperature(Tc).
Due to temperature difference an amount of heat energy (Q) passes from hot face to cold face in time 't'.
Conduction rate Pcond (amount of energy transferred for uni time) is
Pcond = Q/t = K*A*(TH-Tc)/d;
where 'K' is coefficient of thermal conductivity, a constant for given material.
'd' is thickness of slab
'A' Area of slab
't' is time of conduction
Therefore, Q= K*A(TH-Tc)*t/d
Note: i) 'K' depend on nature of material of which slab is made
ii) A good thermal conductor has 'K' greater value.
Thermal Resistance to conduction(R-value):
This explains resisting of thermal conductivity. The R-value(thermal resistance) of a slab of thickness 'd' is defined as R=d/k. Thus material having less value of 'K' will have higher R-value and thus acts as a good thermal insulator.
Note:
i) 'R' is properly assigned to specified thickness of slab but not to material of slab.
ii) In steady state, conducting rates thru any no. of materials must be equal.
Therefore, Pcond = A*(TH-Tc) / Σ(d/K)
iii) Heat is transferred from molecule to molecule by conduction. In this case molecules do not bodily move but simply vibrate.
Consider a cubicle of solid. Let us maintain one face of cube at high temperature (TH) and other opposite face at low temperature(Tc).
Due to temperature difference an amount of heat energy (Q) passes from hot face to cold face in time 't'.
Conduction rate Pcond (amount of energy transferred for uni time) is
Pcond = Q/t = K*A*(TH-Tc)/d;
where 'K' is coefficient of thermal conductivity, a constant for given material.
'd' is thickness of slab
'A' Area of slab
't' is time of conduction
Therefore, Q= K*A(TH-Tc)*t/d
Note: i) 'K' depend on nature of material of which slab is made
ii) A good thermal conductor has 'K' greater value.
Thermal Resistance to conduction(R-value):
This explains resisting of thermal conductivity. The R-value(thermal resistance) of a slab of thickness 'd' is defined as R=d/k. Thus material having less value of 'K' will have higher R-value and thus acts as a good thermal insulator.
Note:
i) 'R' is properly assigned to specified thickness of slab but not to material of slab.
ii) In steady state, conducting rates thru any no. of materials must be equal.
Therefore, Pcond = A*(TH-Tc) / Σ(d/K)
iii) Heat is transferred from molecule to molecule by conduction. In this case molecules do not bodily move but simply vibrate.
X-Rays discovery
Wilhelm Roentgen was professor of physics at university of Wurzburg, Germany when he discovered X-rays in 1985. The discovery was entirely serendipitous; Roentgen was merely studying a beam of electrons in a highly evacuated glass vessel. When the electrons, moving at great speed slammed into glass wall, they produced a very high penetrating radiation - a wholly unexpected occurrence. Roentgen first noticed the radiation when it caused a paper coated with Barium Platino-cyanide to glow. The chemical compound was a standard detector of UV light which causes the chemical to fluorescence i.e. to emit visible light after it has absorbed UV light. But Roentgen's evacuated vessel was tightly covered with black cardboard and so no UV light could emerge from it. The glow must be some other kind of radiation.
When he announced the discovery of the new radiation, Roentgen wrote:
"I posesss, for instance, photographs of ............the shadow of bones of hand, the shadow of a covered wire enclosed in a box.........."
Earlier in the paper, he noted that "the darker shadow of bones is seen with in the slightly dark shadow image of hand itself.
The new radiation quickly became a diagnostic tool in hospitals all over the world. Roentgen could not determine what the rays are made of and thus rays are named as X-rays.
When he announced the discovery of the new radiation, Roentgen wrote:
"I posesss, for instance, photographs of ............the shadow of bones of hand, the shadow of a covered wire enclosed in a box.........."
Earlier in the paper, he noted that "the darker shadow of bones is seen with in the slightly dark shadow image of hand itself.
The new radiation quickly became a diagnostic tool in hospitals all over the world. Roentgen could not determine what the rays are made of and thus rays are named as X-rays.
PHYSICS DICTIONARY
Zeeman
Effect
In a magnetic
field, the energy of a particular atomic state depends on value ‘m’, the
magnetic quantum number. A state of total quantum number ‘n’ breaks up into
several sub-states when the atom in the magnetic field and the energies are
slightly more or less than energy of state in the absence of magnetic field.
This phenomenon leads to splitting of individual spectral lines when atoms
radiate in magnetic field. The spacing of lines depends on magnitude of
fields.
Zero
Point Energy
Energy possessed by atoms or molecules even
at absolute zero.
Zeroeth
Law of Thermodynamics
This law was
first enunciated by R H Fowler in 1831. According to this law, when two systems
A and B are in thermodynamic equilibrium with another system C, then A & B
will also be in thermal equilibrium.
Zone
Plate
The optical
device which verifies rectilinear propagation of light approximately by wave
theory.
Subscribe to:
Posts (Atom)