Kinetic Theory Of Gases & Radiaton Flashcards
Define heat
Heat is defined as any spontaneous flow of thermal energy from one point to another caused by difference in temperature between the two points.
Define pressure exerted by a gas
The rate if change if momentum pre unit area of the walls of the container is termed as the pressure exerted by a gas.
What is Transmission of heat?
Heat can be transmitted from one body to another by a definite process called transmission of heat.
Three modes of heat transfer
Conduction
Convection
Radiation
Define conduction
Conduction is the process in which heat is transferred through a material medium without migration of the particles of the medium from one place to another.
write a short note on conduction
Definition
Slow process
Requires medium
Mostly in solids, also in liq and gases to some extent
Define convection
Convection is the process in which heat is transferred through a material medium due to the actual migration of particles of the medium from one place to another.
Write a short note on convection
Definition
Faster than conduction
Requires material medium
In liquids and gases
Define Radiation
Radiation is defined as the process in which heat energy flows from a body of higher temperature to a body of lower temperature in the form of electromagnetic waves.
Write a short note on radiation
Define
Fastest
Does not require material medium
Takes place at all temperatures except absolute zero
Radiant energy or radiant heat
Heat energy in the form of electromagnetic waves is called radiant energy or radiant heat.
Thermal radiation
The energy emitted by a body in the form of radiation on account of temperature is called thermal radiation.
What factors influence thermal radiation?
Nature of source
Surface area
Time
Temperature of body
Value of Boltzmann constant
1.38*10^(-3)
Avogadro’s hypothesis
Number of molecules per unit volume is the same for all gases at a fixed temperature.
Define Ideal Gas
A gas that satisfies the ideal gas equation, PV=nRT, at all temperatures and pressures is known as and ideal gas.
Assumptions of KTG
Gas contains large no of tiny molecules.
Molecules are perfectly elastic spheres of very small diameter.
All molecules of the same gas are identical in shape and size.
Actual volume occupied by gas molecules is very small as compared to the total volume occupied by the gas.
(Molecular chaos) All molecules are always in the state of random motion (all possible directions, all possible velocities)
Collisions with the walls of container and each other are completely elastic.
Between any two collisions, molecules travel in a straight path with constant velocity. (Free path)
Time taken for collision is very small as compared to the time required to cover the free path between two successive collisions.
Define Free path
The distance travelled by the molecule between two successive collisions is called the free path.
Mean free path
The average distance travelled by gas molecules between two successive collisions is called the mean free path.
Mean velocity or average velocity
The arithmetic mean of the velocities of all the molecules is known as the mean velocity or average velocity.
Mean square velocity
The average values of the square of all the velocities of the molecules is known as the mean square velocity.
Root mean square velocity
The square root of the mean square velocity is called the root mean square velocity.
Boyle’s law
Boyle’s law states that at constant temperature, the pressure exerted by a fixed mass of a gas is inversely proportional to the volume of the gas.
Crms=
Root( 3RT/M)
Relation between RMS velocity and absolute temperature
RMS velocity Iw molecules of gas is directly proportional to the square root of its absolute temperature
State Charle’s law
At fixed pressure the volume of gas is directly proportional to its absolute temperature.
State Dalton’s law
The total pressure of mixed ideal gases is the sum of the partial pressures.
What is Maxwell distribution
The molecular speed distribution gives the number of molecules dN(v) between the speeds v and v+dv, which is proportional to dv. This is called Maxwell Distribution.
Explain the degrees of freedom
If it is constrained to move along a line it required one coordinate to locate it. i.e. It has one degree of freedom.
If the molecule is constrained to move in a plane it requires two coordinates to locate it. i.e. It has two degrees of freedom.
Molecule free to move in space has three degrees of freedom.
Monatomic gas- only translational degrees of freedom
Diatomic gas- three translational degrees of freedom and two rotational degrees of freedom (since it can rotate about its centre of mass)
State law of equipartition of energy
In equilibrium, the total energy is equally distributed in all possible energy modes, with each having average energy equal to (1/2)(KBT). This is known as law of equipartition of energy.
Energy contributed by degrees of freedom
Each translational and rotational (one squared term) : (1/2)*KB*T Each vibrational (two squared terms) : KB*T
How many degrees of freedom does a polyatomic gas have?
3 translational
3 rotational
‘f ‘ vibrational
State the zeroth law of thermodynamics
The zeroth law of thermodynamics states that if two bodies P and Q are in thermal equilibrium and also P and R are in thermal equilibrium then Q and R are also in thermal equilibrium.
What is the difference between heat and work?
Heat and work are two distinct modes of energy transfer to a system that results in change in internal energy.
Heat is the energy transfer due to temperature difference between system and surroundings whereas work is the energy transfer bought about by means which do not involve any temperature difference.
Equation for first law of thermodynamics
δQ= δU + δW
energy supplied to the system = increase in energy of the system + work on the environment
Define isothermal process
A process in which the temperature of the system is kept fixed throughout is called and isothermal process.
Example of isothermal process
Expansion of gas in a metallic cylinder placed in a large reservoir of fixed temperature.
Define Adiabatic process
In an Adiabatic process, the system is insulated from the surroundings and heat absorbed or released is zero.
State the second law of thermodynamics
The second law of thermodynamics states that mechanical work can be converted completely into heat but heat cannot be completely converted into mechanical work.
How does a heat engine work?
A heat engine works in a cyclic process. Takes heat from bodies at higher temperature, converts part of it to mechanical work and remaining to a body at lower temperature.
This cycle is repeated over and over again to get useful work for some purpose.
Equation for the efficiency of a heat engine
η= W/Q1 ( efficiency= work done on environment in a cycle/heat absorbed by the system in one complete cycle ) OR W= Q1 - Q2 η= 1- Q2/Q1 Q2 => heat rejected to the environment
How does a refrigerator work?
Reverse of heat engine. Working substance extracts heat Q2 from the cold reservoir at temperature T2, some external work W is done on it and heat Q1 is released to the hot reservoir at temperature T1
Steps
1) Sudden expansion of gas ( from high to low pressure, cools it, converts it into vapor-liquid mixture )
2) Absorption by the cold fluid of heat from the region to be cooled converting it into vapor.
3) Heating up of vapor due to external work on the system.
4) Release if heat by the vapor to the surroundings bringing it to initial state, completing the cycle)
Equation for coefficient of performance of a refrigerator
α = Q2/W
( coefficient of performance of a refrigerator = heat extracted from the cold reservoir/work done on the system by the refrigerant)
Q1= W + Q2
α= Q2 / (Q1 - Q2)
Coefficient of performance of a heat pump
α= Q1/W
Define quasi-static process. Is there any real quasi-static process?
A quasi-static process is a thermodynamic process that happens infinitely slowly.
No real process is quasi-static.
When is a process reversible?
A thermodynamic process is reversible if the process can be turned back such that both system and the surroundings return to their original states, with no change anywhere else in the universe.
Define coefficient of absorption
The coefficient if absorption is defined as the ratio of the quantity of radiant heat absorbed by the body in a given time to the quantity or radiant energy incident on the body in the same time.
Coefficient of reflection
The coefficient of reflection of a body is defined as the ratio of the quantity of radiant energy reflected by a body in the given time to the quantity of radiant energy incident on the body in the same time.
Coefficient of transmission
The coefficient if transmission is defined as the ratio between the quantity of radiant energy transmitted through a body in a given time to the quantity of radiant energy incident on the body in the same time.
Relation between absorptance, reflectance, transmittance
Q= Qa + Qb + Qc
Dividing throughout by Q
1= a + r + t
What are Athermanous substances
The substances which do not transmit any incident heat radiations (i.e. are opaque to heat radiations) are called as athermanous substances.
t=0
a + r = 1
Give some examples of Athermanous substances
Water, wood, iron, copper, lampblack, water vapor
What are Diathermanous substances?
The substances which are transparent to heat radiations (through which heat radiations can pass) are called Diathermanous substances.
They are neither good absorbers nor good reflectors.
Examples of Diathermanous substances
Glass, quartz, sodium chloride, hydrogen, oxygen, dry air, rock salts
What us a perfectly black body?
A perfectly black body is a body which absorbs all the radiant energy incident on it.
a=1
r & t =0
Write a short note on Ferry’s black body.
It is an artificial black body.
Double walled hollow metallic sphere with small aperture. Inner surface coated with Lampblack, outer surface is nickel polished and has a conical projection diametrically opposite to the aperture.
Space between the two walls is evacuated (to reduce heat loss by conduction and convection).
Radiant heat entering through aperture sufferers multiple internal reflections. Each time incident on lampblack it absorbs 98% of the incident radiant heat. So after many reflections almost all the radiant heat is absorbed.
Aperture acts as perfectly black body. Effective area of black body = area of aperture
What are the observations from the experimental curves obtained by Lummer and Pringsheim?
I. Intensity of radiations emitted increase with increase in wavelength
II. For a particular wavelength, intensity of emitted radiation is maximum and then decreases with a further increase in wavelength.
III. Area under the curve is equal to the total energy emitted per unit area, per second by the black body including all the wavelengths.
What happens to the graph of energy distribution spectrum of the body when temperature is increased?
I. Energy distribution curve continues to be non uniform.
II. Peak shifts towards left ( if Temperature increases the maximum wavelength decreases)
III. At higher temperature, total energy emitted per second per unit area corresponding to all wavelengths.
State Wien’s displacement law
Wien’s displacement law states that the wavelength for which emissive power of black body is maximum is inversely proportional to the absolute temperature of the black body.
Value if Wien’s constant
b = 2.898 * 10^(-3) mK b = T * (λ max)
Define Emissive Power
Emissive power of a body at a given temperature is defined as the quantity of heat emitted per unit time per unit surface area of the body at that temperature.
Equation , dimensions and SI unit of emissive power
E = Q/(A*t)
(Emissive power= amount of energy emitted/(surface area*time for which body radiates energy))
SI unit: J/(mms) or W/(m*m)
Dimensions: [M1 L0 T-3]
What does the emissive power of a black body depend on?
- Temperature of the body
- Nature of the body
- Surface area of the body
- Nature of the surroundings
Define emissivity
Coefficient of emission of a body is the ratio of the emissive power of the body at a given temperature to the emissive power of a perfectly black body at that temperature.
Define absorptive power
Absorptive power if a body at a given temperature is defined as the radiant energy absorbed per unit area per unit time byasurface at that temperature.
What is the principle of equality of radiating and absorbing powers
All bodies when heated emit the same kind of radiations which they absorb.
State Kirchhoff’s law of radiation
Kirchhoff’s law of radiation states the the coefficient of absorption of a body is equal to the coefficient of emission at any given temperature.
e=a
State Stefan’s law of radiation
Stefan’s law if radiation states that the amount of radiant energy emitted per unit time per unit surface area of a perfectly black body is directly proportional to the fourth power of it’s absolute temperature.
Value of Stefan’s constant
σ= 5.67 * 10^8 J/(mms*(K^4))
State Newton’s law of cooling
Newton’s law of cooling states that the rate of loss of heat by the body is directly proportional to the excess of temperature of the body over surroundings provided the excess is small.
What are the limitations of Newton’s law of cooling?
It can only be used when excess of temperature of the body over the surroundings is small.
Not a radiation law since energy is also lost by conduction and convection.
