Download Que44: What is the Difference between Force and Pressure

1. Give brief answers to the following questions. (2 marks each)
Que1: What are fundamental units?
Ans: The units selected for the measurement of three basic quantities (mass,
length, and time) are known as the fundamental units.
Que2: What are derived units?
Ans: The units of all other quantities which are obtained from the fundamental
units are called derived units.
Que3: Define Dimensions?
Ans: The dimensions of a physical quantity are the powers to which the
fundamental units of mass, length and time must be raised to represent a derived
unit of the quantity.
Que4: Define dimensional formulae?
Ans: An expression showing how and which of the fundamental units are required
to represent the unit of a physical quantity is called the dimensional formula.
Que5: Joule is…… unit of……
Ans: Joule is SI unit of energy.
Que 6: State Principle of Homogeneity?
Ans: It states that dimensions of all the terms on the two sides of an equation must
be same.
Que7: What are the uses of dimensional analysis?
Ans: 1) Checking the correctness of the given physical relation.
2) To derive the relationship between different physical quantities.
3) Conversion of one system of units into other.
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Que8: Define scalar quantities?
Ans: The physical quantities which possess only magnitude and no direction are
called scalar quantities. Eg: mass, length etc.
Que9: Define vector quantities?
Ans: The physical quantities which possess both magnitude as well as direction
are called vector quantities. Eg: velocity, acceleration, force, momentum etc.
Que10: Define Acceleration?
Ans: The acceleration of an object is equal to change in its velocity per unit time
i.e. the rate of change of velocity. Units- m/s2
Que11: Define linear momentum?
Ans: The product of mass of the body and its velocity is called the momentum of
the body. It is a vector quantity.
p=mv
Units: kg · m/s
Que12: Define force?
Ans: Force is an agent which changes or tends to change the state of rest or of
uniform motion in a straight line or the direction of motion of a bdy.
Que13: Define Inertia?
Ans: The property of a body of virtue of which it opposes any change in its state of
rest or of uniform motion is called inertia.
Que14: State Principle of conservation of linear momentum?
Ans: According to this principle, in the absence of any external force, the total
linear momentum of a system is constant.
Que15: Define weight?
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Ans: The weight of the body is the force by which it is attracted towards the centre
of the Earth.
W=mg
Que16: Define Impulse?
Ans: The impulse of a force is measured by the total change in momentum.
Units: Ns. Dimensions: MLT-1
Que17: Define impulsive force?
Ans: If a force acting on a body for a very short time. Such a force is called
impulsive force.
Que18: Define Angular velocity?
Ans: The angular velocity (ω) is defined as the rate of change of angular
displacement and is a vector quantity.
ω= θ/t
Units: rad/s
Que19: Define Angular acceleration?
Ans: The rate of change of angular velocity is called angular acceleration (α).
α= (ω2- ω1)/t
Units: rad/s2
Que20: Define time period?
Ans: Time taken by a body moving in a circle to complete one revolution is called
time period. It is denoted T
ω=2 π/ T
Que21: Define Centripetal force?
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Ans: The force which is required to move a body in a circular path with uniform
speed. The force acts on the body along the radius & towards the centre.
Que22: what is the relation between frequency (n) and time period (T)?
Ans: nT=1
Que23: Define centrifugal Force?
Ans: The force which is equal to centripetal force but acting radially outwards is
called centrifugal force.
Que24: What is the relation between ω, T and n?
Ans: ω=2 π n
Que25: Define Banking of roads?
Ans: The process of raising outer edge of the circular track slightly above its inner
edge is called banking of roads.
Que26: What is projectile motion?
Ans: Anybody thrown with the same initial velocity and then allowed to move
under the influence of gravity alone is called projectile.
Que27: What is rotational motion?
Ans: A body is said to possess rotational motion or rotation, if its energy particle
moves in circle, whose centre lies on the straight line is called the axis of rotation.
Que28: Define moment of inertia?
Ans: The property of the body by virtue of which it opposes the torque tending to
change its state of rest or uniform motion about a given axis, is called moment of
inertia.
Que29: Define radius of Gyration?
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Ans: It is the distance of a point from the axis of rotation where if whole of the
mass of the body was concentrated. It is denoted by K.
Que30: Define Angular Momentum?
Ans: The angular momentum of a rotating body is defined as the sum of the
moments of linear momentum of its constituent particles about the axis of rotation.
Ques31: State the principle of conservation of momentum?
Ans: When no external torque acts on a system of particles, then the total angular
momentum of the system remains always a constant..
Ques32: Define frequency?
Ans: The number of revolutions completed by a body moving in a circle is called
frequency.
ὼ=2∏n
Ques33: What is rotational motion?
Ans: The rotation of a body about fixed axis is called rotational motion. E.g.
motion of a wheel about its axis.
Ques34: Define torque?
Ans: It is measured by the product of magnitude of force and perpendicular
distance of the time of action of force from the axis of rotation.
It is denoted by τ
τ =F.r
Que35: Define force of gravity?
Ans: The force with which a body is attracted towards the centre of earth is known
as force of gravity.
Que36: Define friction?
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Ans: It is that opposing force which comes into play when a body tends to slide or
actually slides over the surface of another body.
Ques37: Define acceleration due to gravity?
Ans: The constant acceleration in the body due to the force of attraction acting on
it is called acceleration due to gravity.
Force of gravity=mass*acceleration=mg
Ques38: Define Newton’s law of gravitation?
Ans: According to this law, everybody in this universe attracts every other body
with a force which is directly proportional to the product of their masses and
inversely proportional to the square of distance between them.
F=Gm1m2 /r2
Ques39: Define Escape velocity?
Ans: The minimum velocity of a body due to which it just crosses the gravitational
field of planet or earth is called the escape velocity of the body.
Que40: Define orbital velocity?
Ans: The velocity required to put the satellite into given orbit around the earth is
called as orbital velocity. It is denoted by vo.
Que41: Define satellite?
Ans: A body which revolves around the planet is called satellite.
Satellites are of two types:
1. Natural satellite
2. Artificial satellite
Que42: what do you mean by artificial satellite?
Ans: Manmade satellite is called artificial satellite. Eg: sputnik-I, Arya Bhatta.
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Ques43: Define geostationary satellite?
Ans: A satellite which revolves around the earth with same angular speed in the
same direction as is done by earth around its axis is called geostationary satellite.
Ques44: Define work?
Ans: Work is said to be done when the force applied on a body displaces it through
certain distance in direction of applied force.
W=F*S= force*displacement
Units: Joule (J)
Ques45: Define power?
Ans: Power is defined as the rate at which work is done by a force.
Or the work done per unit time is called power.
P=W/t
Units: watt (W)
Ques46: Define energy?
Ans: Energy of a body is defined as the capacity of the body to do the work.
Units: Joule (J)
Ques47: State the law of conservation of energy?
Ans: It states that “energy can neither be created nor be destroyed but may b9e
converted from one form to another”.
Ques48: Define potential energy?
Ans: Energy possessed by the body by virtue of its position is called potential
energy. E.g. water stored in a dam.
PE=mgh
Ques49: Define kinetic energy?
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Ans: Energy possessed by the body by virtue of its motions is called potential
energy. E.g. running water.
KE=1/2 mv2
Ques50: Define restoring force?
Ans: The force which comes its play when the deforming force has been removed
from the body and bring the body to its original position is called restoring force.
Ques51: Define plasticity?
Ans: The property of the body by virtue of which it do not regain its original
position after removing deforming force from it, is called plasticity.
Que52: Define elasticity?
Ans: The property of matter by virtue of which a body regains its original position
after removing deforming force from it is called elasticity.
Ques53: What is stress?
Ans: It is defined as the restoring force acting per unit area of the body deformed
by an external force.
Stress=restoring force/ area=F/A
Units: N/m2
Ques54: Define strain?
Ans: The ratio of change in dimension of the body to the original dimension is
called strain.
Strain= change in dimension/ original dimension
Ques55: What is longitudinal strain?
Ans: The ratio of change in length to the original length is termed as longitudinal
strain.
Longitudinal strain= change in length/ original length
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Ques56: What is volumetric strain?
Ans: The ratio of change in volume to the original volume is defined as volumetric
strain. It is produced when the applied force changes the volume of the body.
Volumetric strain= change in volume/ original volume
Ques57: What is shearing strain?
Ans: The ratio of the relative displacement of one plane to the distance from the
fixed plane is called shearing strain.
Shearing strain=relative or lateral displacement/ distance of the surface
Ques58: What is modulus of elasticity?
Ans: The ratio of stress to strain is a constant and is called modulus of elasticity or
coefficient of elasticity.
E=stress/ strain
Ques59: Define modulus of rigidity (η)?
Ans: The ratio of tangential stress to shearing strain is called modulus of rigidity.
η = tangential stress/ shear strain
units: N/m2
Ques60: Define young’s modulus of elasticity?
Ans: The ratio of normal stress to longitudinal strain is called young’s modulus of
elasticity.
Y= normal stress/ longitudinal strain
Units: N/m2
Que61: Define Bulk modulus of elasticity?
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Ans: The ratio of normal stress to volumetric strain is called bulk modulus of
elasticity. It is denoted by K.
K= Normal stress/ Volumetric strain
Que62: Define compressibility?
Ans: The reciprocal of bulk modulus of elasticity is called compressibility. It is
denoted by β.
Que63: State Hook’s law?
Ans: It states that within the elastic limit, the stress developed in a solid is directly
proportional to the strain produced in it.
Stress α strain
Que64: Define pressure?
Ans: the magnitude of force acting normally per unit area of the surface of body is
called pressure. It is denoted by p and is a scalar quantity.
P=F/A
Ques65: Define atmospheric pressure?
Ans: The pressure exerted by air is called atmospheric pressure.
P=P.g.h
Units: N/m2 or pascal(Pa)
Ques66: What is absolute pressure?
Ans: The actual pressure at a point in a fluid is called absolute pressure.
Actual pressure= pressure due to liquid+ atmospheric pressure
Ques67: Define gauge pressure?
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Ans: The difference between the absolute pressure and the atmospheric pressure at
a point in a fluid is called gauge pressure.
Que68: what is barometer?
Ans: A barometer is a device used for measuring the atmospheric pressure.
Ques69: Define surface tension?
Ans: The property of a liquid due to which its free surface at rest behaves like a
stretched membrane tending to contract so as to have minimum surface area is
called surface tension.
Surface tension(T)=force/ length of line
Units: N/m2
Ques70: What is capillarity or capillary action?
Ans: The phenomenon of raise of or fall of a liquid in a capillary tube is called
capillarity or capillary action.
Ques71: What is the effect of temperature on surface tension?
Ans: 1) Most of the liquids the surface tension decreases with increase in
temperature.
2) In case of molten copper and cadmium, it increases with increase in
temperature.
3) Surface tension of a liquid becomes zero at a particular temperature called the
critical temperature of that liquid.
Ques72: What is the effect of impurities on surface tension?
Ans: When impurities are added to the liquid, the surface tension of the liquid may
decrease or increase depending upon the degree of contamination.
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Ques73: What is stream line flow?
Ans: The flow of a fluid is said to be steady or streamlined if the velocity of every
point in the fluid remains constant in magnitude as well as direction. A streamline
may be defined as the path, straight or curved, the tangent to which at any point
gives the direction of the flow of liquid at that point.
Ques74: Define viscosity?
Ans: The property of liquids by virtue of which an internal resistance or friction
comes into play, when a liquid is in motion is called viscosity.
Que75: what is coefficient of viscosity?
Ans: It is defined as the tangential force required to maintain a unit velocity
gradient between two layers of area one unit each.
Ques76: What is the effect of temperature of viscosity?
Ans: the viscosity of liquids decreases with increase in temperature and increase
with the decrease in temperature.
Que77: Define cohesion?
Ans: cohesion is force of attraction between the molecules of same substance.
Ques78: Define heat?
Ans: Heat is a form of energy which produces in us the sensation of warmth or
hotness. Units: Joule
Ques79: Define temperature?
Ans: Temperature of a body is defined as the degree of hotness or coldness of a
body. Units: Kelvin (K)
Que80: Define thermometer?
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Ans: An instrument which is used to measure temperature is called thermometer.
Ques81: Define conduction?
Ans: The process by which heat is transferred from one part to another through a
substance in the direction of fall of temperature, without actual motion of the
molecules is called conduction.
Ques82: Define convention?
Ans: The process by which heat is transmitted through a substance from one part to
another due to the actual motion of the molecules is called convention.
Ques83: Define radiation?
Ans: The process by which heat is transferred from one place to another without
heating the intervening medium is called radiation.
Que84: Define pyrometers?
Ans: A pyrometer is an instrument used to measure the high temperature of
furnaces and sun.
Ques85: What is black body?
Ans: A perfectly black body one which absorbs completely the radiations of all
wavelengths falling on it.
Que86: what do mean by absorbing power?
Ans: The absorbing power of a body is defined as the ratio of the amount of heat
energy absorbed to the total amount of heat energy incident on it.
Que87: Define emissive power or emissivity?
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Ans: It is defined as amount of heat energy radiated by unit surface of the body in
one second when the temperature difference between the body and the
surroundings is 10 K.
Ques88: State Stefan’s law of heat radiation?
Ans: Stefan’s law states that the total amount of heat energy radiated by a perfectly
black body per second per unit area is directly proportional to the fourth power of
its absolute temperature. This law is also known as Stefan’s fourth power law.
Ques89: State Wien’s law?
Ans: According to Wien’s law, the wavelength corresponding to the maximum
heat energy radiated is inversely proportional to the absolute temperature of the
body.
λm α 1/T
Ques90: State Kirchhoff’s law of radiation?
Ans: Kirchhoff’s law states that at a given temperature the ratio of emissive power
to the absorptive power corresponding to certain wavelength is constant for all
bodies and this constant is equal to the emissive.
Ques91: what do you mean by turbulent flow?
Ans: the flow of fluid in which velocity of all particles crossing a given point is not
same and the motion of the fluid ecomes disorderly or irregular is called turbulent
flow.
Que92: What is the relation between torque and angular momentum?
Ans: τ = dL/dt
Que93: what is the dimensional formula of gravitational constant G?
Ans: M-1 L3 T-2
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Que94: Define tensile stress?
Ans: It is defined as the restoring force acting per unit area perpendicular to the
surface of the body.
Tensile stress = F/A
Que95: Define shear stress?
Ans: It is defined as the restoring force acting per unit area tangential to the surface
of the body.
Que96: Define adhesion?
Ans: The force of attraction between molecules of different substances is called
adhesion.
Que97: Define angle of contact?
Ans: The angle between the tangent to the liquid surface at the point of contact and
the solid surface inside the liquid is called the angle of contact.
Que98: what is the relation between scales of temperature?
Ans: C/5= (F-32/9)= (K-273/5)
Que99: what is the unit of thermal conductivity (K)?
Ans: watt m-1 K-1
Que100: Define absorptivity?
Ans: Absorptivity of a surface for a given surface for a given temperature and
wavelength is the ratio of the amount of radiation absorbed by the surface in given
time to the total amount of radiation incident at the same time.
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2. 4 marks questions
Que1: convert an acceleration of 100m/s2 into km/hr2
Ans: Dimensional formula of acceleration is [M0 LT-2 ]
a=0, b =1, c = -2
The values of fundamental units in two systems are
Ist system
2nd system
M1 = 1 kg
M2 = 1 kg
L1 = 1 m
L2 = 1 km
T1 = 1 s
M2 = 1 hr
n1 = 100
n2 = ?
u1 = m/s2
u2 = km/hr2
Using the relation
n2 = n1 [M2/M1]a [L2/L1]b [T2/T1]c
=100 [1kg/1kg]0 [1m/1km]1 [1s/1hr]-2
= 100* (1/1000) * (1/3600)-2
= 12.96 * 105
n1 u1 = n2 u2
100 m/s2 = 12.96 * 105 km/hr2
Que2: Pressure (P) of a liquid filled in tank depends upon height of column (h),
density of liquid (ρ) and acceleration due to gravity (g). Derive a formula for
pressure by using the method of dimensions?
Ans: P α ha ρb gc
P = K ha ρb gc
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K is dimensionless constant
[P] = [K ha ρb gc]
[M1 L-1 T-2 ] = [M0 L0 T0]* [ L]a * [M L-3 ]b * [L T-2 ]c
[M1 L-1 T-2] = [Mb La-3b+c T-2c]
Apply principle of homogeneity of dimensions
1=b
-1 = a-3b+c
-2 = -2c
b=1
c=1
P=Khρg
Que3: Derive dimensionally an expression for centripetal force f acting on a
particle of mass m moving with a velocity v in a circle of radius r?
Ans: According to the statement
f α ma vb rc
f = K ma vb rc
where K is dimensionless constant
[M L T-2 ] = [M ]a [L T-1 ]b [L ]c
a=1
b+c=1
-b = -2 or b=2
c = 1-b = 1-2 = -1
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f = K m v2r-1
or f = Kmv2/r
Que4: What are the limitations of dimensions?
Ans: 1. It gives no information about dimensionless constants and pure numbers.
2. The method cannot be used for deriving relations involving trigonometrical and
exponential functions.
3. It cannot be used when a physical quantity depends on more than three
quantities i.e. M, L, T.
4. It cannot be used to derive the exact form of a physical relation if it consists of
more than one term.
5. This method fails to derive a relation which contains two or more variables
having the same dimensions.
6. The method does not make any difference between vector and scalar quantities.
Que5: what are the advantages and merits of S.I units?
Ans: 1. It is a coherent system of units. Therefore all the derived units are obtained
from the fundamental units without introducing numeric factor.
2. It is a relational system i.e. it uses only one unit for similar type of physical
quantities.
3. It is an absolute system i.e. there are no gravitational units used in it.
4. It is based on metric system which makes the calculations easy and quick.
5. it gives due representation to all the basic branches of physics right in the set of
base units.
6. The base units are defined on natural invariants or logically acceptable facts.
Hence their definitions are very much acceptable to the scientific and technological
world.
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Que6: state and prove principle of conservation of linear momentum?
Ans: The phenomenon of raising outer edge of the curved road above the inner
edge is to provide necessary centripetal force to the vehicles to take a safer turn
and the curved road is called Banking of Roads.
When a vehicle goes round a curved road, it requires some centripetal force. While
rounding the curve, the wheels of the vehicle have a tendency to leave the curved
path and regain the straight line path. Force of friction between wheels and the
roads opposes this tendency of the wheels. This force of friction therefore, acts
towards the centre of circular track and provides the necessary centripetal force.
Vehicle moving on level road
it is shown that a vehicle of weight ‘mg’ (acts vertically downwards) is moving on
a level curved road. R1 and R2 are the forces of normal reaction of the road on the
wheels. These are vertically upward since road is leveled. Hence,
R1 + R2 = mg
Let F1 & F2 are forces of frictions between tyre and road directed towards centre
of curved road.
∴ F1 = μ R1
And
F2 = μ R2
where μ is coefficient of friction between tyres and road.
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If ‘v’ is the velocity of the vehicle while rounding the curve, the centripetal force
required is mv²/r. As this force is provided by the force of friction therefore
Hence the maximum velocity with which a vehicle can go round a level curve;
without skidding is
Que7: Derive an expression for maximum height for projectile motion?
Ans: The maximum height of projectile
Maximum height of projectile
The highest height which the object will reach is known as the peak of the object's
motion. The increase of the height will last, until
, that is,
.
Time to reach the maximum height:
20
.
From the vertical displacement of the maximum height of projectile:
Que8: A projectile is projected with a speed of 30m/s at an angle 300 to the
vertical. Calculate the maximum height it will reach and its time of flight?
Ans:
Given v= 30m/s
Angle with the vertical = 300
Angle with the horizontal = 900-300 = 600
Maximum height
H= v2sin2θ/ 2g = (30)2(sin60)2/ 2* 9.8 = 34.44 m
Time of flight
T= 2vsinθ/g = 2 * 30 * sin60/ 9.8 = 5.302 s
Que9: Find the minimum velocity with which the horizontal range is 39.2m?
Ans:
Rmax = v2 / g
Where v is the velocity of projection
39.2 = v2 / 9.8
v2 = 39.2 * 9.8
v = 19.6 m
Que10: Derive an expression for centripetal force?
Ans: Consider a particle moving in a circle or radius r with a constant speed v. Let
it move from A to B in time t.
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Triangle AOB and triangle PQS are similar
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'a' gives the magnitude of centripetal acceleration
Que11: What is the difference between mass and weight?
Ans:
S.No. Mass
1.
Mass of a body is the quantity of
matter contained in it.
2.
It is a scalar quantity.
3.
It is measured with an ordinary
balance.
4.
It does not depend upon the
distance between the body and the
centre of the earth.
5.
It is same every where.
6.
It is measured in gm and kg.
Weight
It is the force with which it is attracted
towards the centre of the earth.
It is vector quantity.
It is measured with a spring balance.
It depends upon the distance between the
body and the centre of the earth
It is different at different places.
It is measured in dyne and newton.
Que12: Derive an expression for kinetic energy of rotation?
Ans: calculate the kinetic energy of a body in linear motion with the following
equation:
where m is the mass of the object and v is the speed.
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the kinetic-energy equation gives you the following:
to sum up the kinetic energy of every bit of mass like this:
This makes the equation much simpler, because
equals the moment of inertia, I. Making this substitution takes all the dependencies
on the individual radius of each bit of mass out of the equation,
Que13: A wheel of mass 6kg and radius of gyration 40cm is rotated at 300rpm.
Find its moment of inertia and its rotational KE?
Ans:
I = MK2 = (6kg) (0.40 m)2
= 0.96kgm2
Rotational KE is ½ I ω2
Ω = 300rpm = (300 rev/min) (1min/60s) (2π rad/ 1 rev)
= 31.4 rad/s
KE of rotation= ½ (0.96 kgm2) (31.4 rad/s)
= 437 J
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Que14: State and prove law of conservation of angular momentum?
Ans:
THE LAW OF CONSERVATION OF ANGULAR MOMENTUM STATES
THAT:
"When the net external torque acting on a system about a given axis is
zero, the total angular momentum of the system about that axis remains constant."
According to the second law of motion net force acting on a body is equal to its
rate of change of linear momentum.
i.e.
Taking vector product of
on both side if above expression
.
But
is the torque acting on the body
Angular momentum is defined as:
=
x
Differentiating both sides with respect to "t"
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Which
is
the
required
equation.
This expression states that the torque acting on a particle is the time rate of
change of its angular momentum. If the net external torque on the particle is zero,
then,
OR
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Integrating both sides
Thus the angular momentum of a particle is conserved if and only if the net
external torque acting on a particle is zero.
Que15: A body of mass 10kg is moving with a velocity of 8m/s. Find the
magnitude of force to stop it in 4s?
Ans: Given Mass m= 10 kg
Initial velocity u = 8m/s
Final velocity v = 0
Time t=4s
Retardation (-a) = v-u/t = 0-8/4 = -2m/s2
a = 2m/s2
F = ma = 10 * 2= 20 N
Que16: state and prove Newton’s universal law of gravitation?
Ans: The gravitational force of attraction between any two particles is directly
proportional to the product of the masses of the particle and is inversely
proportional to the square of the distance between them.
F α m1m2
F α 1/r2
F α m1m2/r2
F = Gm1m2/r2
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Here, G is constant known as Gravitational constant. Its value depends upon the
units of quantities involved. Its value is
G = 4.67 * 10-11Nm2/kg2
Que17: Derive an expression for acceleration due to gravity?
Ans: Consider an object of mass m lying on or near the surface of the Earth. Let
Me be the mass of the Earth and Re be its radius i.e., Re is the distance between the
object and the centre of the Earth.
According to Newton's law of gravitation, the force of attraction (F) between the
Earth and the object is
A Body of Mass m Lying on the Surface of the Earth
According to Newton's second law of motion this force produces an acceleration
(g) in the object.
F = ma (a = g)
F = mg
Substituting the value of F in equation (2) we get,
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Que18: state Kepler’s laws?
Ans: First law
The orbit of every planet is an ellipse with the Sun at one of the two foci.
Second law
A line joining a planet and the Sun sweeps out equal areas during equal intervals of
time.
Third law
The square of the orbital period of a planet is directly proportional to the cube of
the semi-major axis of its orbit.
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Que19: State escape velocity? Derive an expression for escape velocity?
Ans: Escape velocity of a body is the minimum velocity with which it is to be
projected so that it escapes from the gravitational field of the earth or any other
planet.
The Law of Conservation of Energy states that the total energy of a closed system
remains constant. In this case, the closed system consists of the two objects with
the gravitational force between them and no outside energy or force affecting
either object.
Thus the total final energy—potential energy plus kinetic energy—must equal the
total initial energy:
TEi = TE∞
KEi + PEi = 0
mve2/2 − GMm/Ri = 0
mve2/2 = GMm/Ri
ve2 = 2GM/Ri
Taking the square root of each expression results in:
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ve = ±√(2GM/Ri)
Considering our gravitational convention for direction, ve is upward or away from
the other object and is thus negative:
ve = −√(2GM/Ri)
Que20: state and prove law of conservation of energy?
Ans: LAW OF CONSERVATION OF ENERGY
Energy can neither be created nor destroyed, but it is transformed from one form to
another. Alternatively, whenever energy gets transformed, the total energy remains
unchanged.
Proof – Freely falling body
It may be shown that in the absence of external frictional force the total
mechanical energy of a body remains constant.
Let a body of mass m falls from a point A, which is at a height h from the ground
as shown in fig.
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At A,
Kinetic energy kE = 0
Potential energy Ep = mgh
Total energy E = Ep + Ek = mgh + 0= mgh
During the fall, the body is at a position B. The body has moved a distance x from
A.
At B,
velocity v2 = u2 + 2as
applying, v2 = 0 + 2ax = 2ax
Kinetic energy Ek = 1/2 mv2 = 1/2 m x 2gx = mgx
Potential energy Ep = mg (h – x)
Total energy E = Ep + Ek = mg (h-x) + mgx = mgh – mgx + mgx= mgh
If the body reaches the position C.
At C,
Potential energy Ep = 0
Velocity of the body C is
v2 = u2 + 2as
u = 0, a = g, s = h
applying v2 = 0 + 2gh = 2gh
kinetic energy Ek =1/2 mv2=1/2 m x 2gh= mgh
Total energy at C
E = Ep + Ek
E = 0 + mgh
E = mgh
Thus sum of potential and kinetic energy of freely falling body at all points
remains same. Under the force of gravity, the mechanical energy of a body
remains constant.
Que21: State and prove parallelogram law of forces?
If two vectors acting at a point are represented in magnitude and direction by the
two adjacent sides of a parallelogram, then their resultant is represented in
magnitude and direction by the diagonal passing through the common tail of the
two vectors.
32
Let us consider two vectors and which are inclined to each other at an angle Ɵ.
Let the vectors and be represented in magnitude and direction by the two sides
OA and OB of a parallelogram OACB. The diagonal OC passing through the
common tail O, gives the magnitude and direction of the resultant .
CD is drawn perpendicular to the extended OA, from C. Let
be Ɵ
made by with
From right angled triangle OCD,
OC2 = OD2 + CD2
= (OA + AD)2 + CD2
= OA2 + AD2 + 2.OA.AD + CD2
(1)
<
From right angled ∆ CAD,
AC2 = AD2 + CD2
(2)
Substituting (2) in (1)
OC2 = OA2 + AC2 + 2OA.AD
(3)
From ∆ ACD,
CD = AC sin Ɵ
AD = AC cos Ɵ
(4)
(5)
Substituting (5) in (3) OC2 = OA2 + AC2 + 2 OA.AC cos Ɵ
33
Substituting OC = R, OA = P,
OB = AC = Q in the above equation
R2 = P2 + Q2 + 2PQ cos Ɵ
(or)
(6)
Equation (6) gives the magnitude of the resultant. From ∆ OCD,
Substituting (4) and (5) in the above equation,
(7)
Equation (7) gives the direction of the resultant.
Que22: Define Friction? What are the laws of friction?
Ans: FRICTION:
Friction is the resistance to motion of one object moving relative to another.
THE FIVE LAWS OF FRICTION
1. When an object is moving, the friction is proportional and perpendicular to
the normal force (N)
2. Friction is independent of the area of contact so long as there is an area of
contact.
3. The coefficient of static friction is slightly greater than the coefficient of
kinetic friction.
4. Within rather large limits, kinetic friction is independent of velocity.
5. Friction depends upon the nature of the surfaces in contact.
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Que23: Determine force equation from Newton’s 2nd law of motion?
Ans: Newton’s 2nd law gives the measurement of force
Consider m= mass of the body
u = initial velocity
v= final velocity
p1= m* u= initial momentum
p2= m* v = final momentum
Rate of change of momentum = mv-mu/t
According to 2nd law
Rate of change of momentum is directly proportional to applied force
F α mv-mu/t
= km (v-u)/t
= kma
K= constant of proportionality.
Que24: Explain recoil of gun?
Ans: Consider the gun and bullet in its barrel as an isolated system




In the beginning when bullet is not fired both the gun and bullet are at rest.
So
the momentum of
the
before
firing
is
zero
pi=0
Now when the bullet is fired ,it moves in the forward direction and
gun recoil back in the opposite direction
Let mb be the mass and vb of velocity of the bullet And mg and vg be the
velocity of the gun after the firing
Total momentum of the system after the firing would be
pf = mbvb +mgvg
35



since no external force are acting on the system, we can apply the law of
conservation of linear momentum to the system
Therefore
pf = pi
or mbvb +mgvg=0
or vg=-(mbvb/mg)
The negative sign in above equation shows that velocity of the recoil of gun
is opposite to the velocity of the bullet
Since mass of the gun is very large as compared to the mass of the bullet, the
velocity of the recoil is very small as compared to the velocity of the bullet
Que25: Derive a relation between linear and angular velocity?
Ans: Consider a body moving in a circle of radius r. let it starts from A to B after t,
Angle= arc/radius
Ɵ = AB/OA = S/r
Dividing both sides by t, we get
S/t = r (Ɵ/t)
S/t = Linear velocity (v)
Ɵ/t = Angular velocity (ɷ)
v= rɷ
Linear velocity = Radius * angular velocity
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Que26: Derive a relation between linear acceleration and angular acceleration?
Ans: Consider a body moving in a circle of radius r. let ɷ1 be the angular velocity
at A and ɷ2 be the angular velocity at B after time t1 then
α= ɷ2- ɷ1/t
If v1 and v2 are the linear velocities of the body at A and B then
v1 = rɷ1
ɷ1 = v1/t
v2 = rɷ2
ɷ2= v2/t
α = v2-v1/rt
a = rα
Que27: Derive an expression for radius of gyration?
Ans: It is the distance from the axis of rotation at which, if the whole mass of the
body were to be concentrated, the moment of inertia would be the same as that
with the actual distribution of mass. It is denoted by K.
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If the mass of a body is M and the radius of gyration is K, then
I = MK2 ---------- (1)
Also, I = m1r12 + m2r22 + ---------- + mnrn2
I = m (r12 + r22 + r32 + ---------- +rn2) or
But, mn = M
Comparing (1) and (2)
Hence, the radius of gyration of a body about an axis is equal to the root mean
square distance of the various particles constituting the body, from the axis of
rotation.
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Que28: Derive an expression for rotational kinetic energy?
Ans: As for any system of particles, the total kinetic energy K of a rotating rigid
body is simply the sum of the individual kinetic energies of all the particles. If the
particles have masses mi and velocities vi (where i = 1, 2, . . . , n), then
In a rigid body rotating about a given axis, all the particles move with the same
angular velocity w along circular paths. The speeds of the particles along their
paths are proportional to their radial distances:
and hence the total kinetic energy is
We will write this as
where
is the moment of inertia of the rotating body about the given axis. The units of
moment of inertia are kg · m2
Que29: Define Energy and its types?
Ans: Energy is the ability to do work. Its units are Joules (J).
39
Potential Energy = the energy that an object has as the result of its position or state.
Some examples of potential energy include: chemical, elastic, gravitational.
PE= mgh
Kinetic Energy = the energy that appears in the form of an object’s motion.
KE = 1/2mv2.
Some examples of kinetic energy include: sound, electrical, light.
Que30: Define stress and its types?
Ans: Elastic bodies regain their original shape due to internal restoring forces. This
internal restoring force, acting per unit area of a deformed body, is called a stress.
Different types of stress : Stress is of two different types mainly (i) Normal Stress
(ii) Shearing or Tangential Stress .
Normal Stress: If the stress is normal to the surface, it is called normal stress.
Stress is always normal in the case of a change in length or a wire or in the case of
change in volume of a body
Longitudinal Stress: When a normal stress change the length of a body then it is
called longitudinal stress which is given by
Longitudinal S tress = Deforming Force / Area of cross section =
The longitudinal stress can be further divided into two types. When a wire or a rod
is stretched at the two ends by equal and opposite forces, the stress is called tensile
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stress. When a rod is pushed at the two ends by equal and opposite forces, it will be
under compression. The stress in such a case is called compressive stress. The
pillars of a building experience compressive stress.
Volume Stress (or) Bulk Stress: When a normal stress changes the volume of a
body then it is called volume stress. When a solid body is immersed in a fluid, the
force at any point is normal to the surface of the body and the magnitude of the
force on any small area is proportional to the area i.e., the body is under the action
of a pressure P.
Bulk Stress =
= Pressure
Shearing Stress : When the Stress is tangential to the surface due to the application
of forces parallel to the surface, then the stress is called tangential or shearing
stress. It changes the shape of the body.
Shearing Stress = Force / Surface Area = F / A
Que31: Define strain. What are its types?
Ans: Normal stress on a body causes change in length or volume and tangential
stress produces change in shape of the body. The ratio of change produced in the
dimensions of a body by a system of forces or couples, in equilibrium, to its
original dimensions is called strain.
Strain is of three types depending upon the change produced in a body and the
stress applied. The three types of strain are (i) Longitudinal strain (ii) Volume
strain and (iii) Shearing strain
Longitudinal Strain: It is the ratio of the change in length of a body to the original
length of the body. If L is the original length of a wire or a rod and the final length
of the wire or the rod is L + e under the action of a normal stress, the change in
length is e.
Longitudinal Strain = Change in length / Original length = e / L
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If the length increases due to tensile stress, the corresponding strain is called
tensile strain. If the length decreases due to compressive stress, the strain is called
compressive strain.
Volume Strain: It is the ratio of the change in volume of a body to its original
volume.
If V is the original volume of a body and v + v is the volume of the body under
the action of a normal stress, the change in volume is v.
Volume Strain = Change in volume / Original volume =
.
Shearing Strain: If is the angle through which a face originally perpendicular to the
fixed face is turned. (Or) It is the ratio of the displacement of a layer to its distance
from the fixed layer.
As strain is a ratio, it has no units and dimensions.
Que32: Define satellites? What are the different types of satellites?
Ans: Satellites: A small body revolving around a much larger body is called its
satellites. For example the moon revolves around the Earth, so moon is a satellite
of the earth.
Types of satellites:
1. Natural Satellites:
A natural satellite is any celestial body in space that orbits around a larger body.
Moons are called natural satellites because they orbit planets.
2. Artificial Satellites:
Satellites that are made by people and launched into orbit using rockets are called
artificial satellites. There are thousands of artificial satellites orbiting the Earth.
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Que33: Define geostationary satellites? What are the essential conditions for
geostationary satellites?
Ans: A satellite which revolves around the Earth in the same direction with the
same angular speed as that of earth is called geostationary satellite.
Essential conditions for geostationary satellite:
1. Its period of revolution around the earth should be the same as that of the
earth about its own axis i.e. exactly 24 hours.
2. Its direction of rotation should be same as that of earth i.e. from west to east.
3. It should revolve in an orbit concentric and coplanar with the equatorial
plane.
Que34: A pump is required to lift 240kg of water per minute from a well 10m deep
and eject it with a speed of 100m/s
a) How much work is done per minute in lifting the water?
b) How much work is done in giving it KE?
c) What is the power of the engine needed for this?
Ans: a) m = 240kg
H=10m
Work done in lifting water per minute = mgh = 240 * 9.8 * 10 J
= 23520J
b) KE=1/2 mv2 = ½ * 240 * 102 = 12000J
c) Total work done in one minute = 23520+ 12000
= 35520
Power = work/ time = 35520/60 = 592 watts
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Que35: Define modulus of Elasticity? Explain young’s modulus of elasticity?
Ans: Modulus of Elasticity:
The applied force on a body produces a change in configuration of the body either
in length, volume and shape.
Young’s Modulus of Elasticity:
It is defined as the ratio of normal stress to the longitudinal strain within elastic
limit.
F is the normal force applied at the end of its wire, A is area of cross section
(= pr2), Dl is the extension produced due to normal force, L is the original length of
the wire.
Que36: Explain Bulk Modulus of Elasticity?
Ans: It is defined as the ratio of normal stress to the volumetric strain within the
elastic limit. Thus,
Consider a spherical solid body of volume V and surface area a, when a force F is
applied normally, the volume decreases by
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Que37: Explain Modulus of Rigidity?
Ans: It is defined as the ratio of tangential stress to the shearing strain within the
elastic limit.
Considering a solid metal cube whose lower face is fixed and its upper face is
subjected to a tangential force F. The body suffers a change in its shape but not in
its volume. If qis angle through which upper layer is sheared then,
(Modulus of Rigidity)
Que38: What is the effect of temperature and impurities on surface tension?
Ans: 1) Most of the liquids the surface tension decreases with increase in
temperature.
2) In case of molten copper and cadmium, it increases with increase in
temperature.
3) Surface tension of a liquid becomes zero at a particular temperature called the
critical temperature of that liquid.
When impurities are added to the liquid, the surface tension of the liquid may
decrease or increase depending upon the degree of contamination
45
Que39: Explain the relationship between stress and strain?
Ans: Stress - Strain Relationship in a wire
AO = Elastic Range
P = Yield point
OD| = Breaking stress or tensile stress
E = Breaking point
OO1= Permanent set
When the stress-strain relationship in a wire is studied, one finds that stress is
directly proportional to the strain upto the point A (see the graph). The point 'A' is
called the elastic limit and AO is called the elastic range. The Hooke's law is valid
up till A. Beyond A, if the stress is removed, graph between stress and strain does
not follow AO. BO| is followed when stress is zero, strain is not zero or a
permanent deformation sets in the material. Therefore, OO| represents the
permanent set. Notice that beyond 'A', the stress - strain graph is a curve and that
for a small stress, large strain is produced in the material. The material beyond A
46
and upto 'P' is partly elastic and partly plastic in behaviour. Beyond 'P', the
behaviour of the wire is very erratic. There is a large increase in the strain but a
very small change in the stress.
At this stage, the wire flows down upto the point C. The point 'P', when the wire
yields to the applied stress and begins to flow, is called the yield point. The region
PC is called the plastic region. Materials used to make sheets or wires must have a
longer plastic region and must be ductile.
Beyond C, the graph has a hump at D. Even if the wire is loaded by a little amount,
the wire becomes thin at weak portions of the wire and tends to break at E. The
stress corresponding to the breaking point is called the breaking stress. Brittle
substances generally have a small plastic region and the breaking stress lies closer
to the elastic limit.
Que40: Explain barometer?
Ans: A barometer is a device used for measuring the atmospheric pressure.
It is called a simple barometer. In this, a glass tube open at one end and having a
length of about one meter is filled with mercury. The open end is temporarily
closed by a thumb. The tube is inverted in a dish of mercury. The thumb is then
removed. The movement this is done, the mercury level drops to a height as shown
and finally stays there. The upper part of the tube contains vacuum as the mercury
goes down and no air is allowed in. The pressure at the upper end of the mercury
column inside the tube is p= zero.
The pressure is due to the atmosphere above.
47
P=hdg
H= height of mercury column, d is the density of mercury and g is the acceleration
due to gravity
Que41: Explain rise of a liquid in capillary tube?
Ans: Consider a liquid of surface tension T in a capillary tube of radius r. Let Ɵ be
the angle of contact between the liquid and the glass.
Surface tension T acts along the tangents of the spherical meniscus of the liquids at
points A and B. the reaction R=T acts just opposite to the surface tension.
Resolve R = T into two components
i) TcosƟ which acts at every point of the meniscus in the upward direction and
is responsible for the rise of liquid in capillary tube.
ii) TsinƟ acting in the horizontal direction cancel each other when taken all
over the meniscus.
The total upward force acting on the circular meniscus of radius r is given by
F=TcosƟ * 2πr
48
Now weight of liquid column in tube is
W=mg
Where m is the mass of liquid and g is acceleration due to gravity
But, m = volume * density = V* d
W= V*d*g
Now total volume of liquid in tube, V= volume of cylinder of length h and radius r.
V=π r2 h
W= π r2 h d g
In equilibrium, total upward force= total downward force
F=W
TcosƟ. 2πr = π r2 hdg
h= 2TcosƟ/rdg
In case of pure water Ɵ= 00 cosƟ = 1
Then T= hrdg/2
Que42: what are the different types of fluid motion?
Ans: Fluid flow can be described in terms of two main types--streamline flow and
turbulent flow. Streamline flow, also known as laminar flow, is illustrated brloe - flow through a pipe and flow around an airplane wing.
49
In streamline flow, the motion of a particle after it passes a particular point is the
same as the motion of the particle that preceded it at that point. The path that a
particle takes is called a stream line. Every particle that passes any particular point
will follow the stream line that goes through that point. A bundle of stream lines,
like the ones here, is known as a stream tube. Fluid never crosses the surface of a
stream tube.
Turbulent flow is illustrated below -- flow through a small constriction in a pipe
and flow around an airplane wing which is inclined at a steep angle. In turbulent
flow, the motion of a particle after it passes a particular point may be quite
different from the motion of the particle that preceded it at that point. Turbulent
flow is characterized by randomness or irreproducibility of the motion of
individual particles. It usually occurs in fluids moving at high speeds. As you
might expect, friction is far greater in turbulent flow. We will concentrate most of
our attention on streamline flow.
50
Que43: What's the Difference Between Gauge and Absolute Pressure
Absolute Pressure
The absolute pressure - pa - is measured relative to the absolute zero pressure - the
pressure that would occur at absolute vacuum. All calculation involving the gas
laws requires pressure (and temperature) to be in absolute units.
Gauge Pressure
A gauge is often used to measure the pressure difference between a system and the
surrounding atmosphere. This pressure is often called the gauge pressure.
Therefore Gauge pressure is measured from atmospheric and absolute is measured
from 0 (as all absolute scales are measured from). They both use the same scale for
measuring.
Que44: What is the Difference between Force and Pressure
Ans: Force
A force is defined as a push or a pull that makes an object change its state of
motion. When a soccer player hits the ball with his legs, he applies force upon it
which decides that the ball, which was static comes into a state of motion and
51
remains in motion till it is stopped by friction and force of gravity. A force can
cause a moving body to stop, make it move faster, or even change its direction.
Force is a vector quantity which means it has a magnitude as well as direction.
Force is dependent upon the mass of the body which accelerates upon application
of force and the three are related as per following equation (Newton’s second law
of motion)
Force= Mass x Acceleration
Pressure
Pressure is a physical quantity that is a force spread over some area. For
convenience you can think of pressure as force per unit area. If you know the
amount of force being applied on a body, divide it with the area of contact and you
get the pressure being applied on the body. This means that same force, when
applied on a smaller area will produce greater results than when applied to a larger
surface area. Pressure is a scalar quantity and has no direction and has only
magnitude. Units of pressure are Pascal (P) or Newton per square meter.
Que45: what are the different types of temperature scales?
Ans: There are three temperature scales in use today, Fahrenheit, Celsius and
Kelvin.
Fahrenheit temperature scale is a scale based on 32 for the freezing point of water
and 212 for the boiling point of water, the interval between the two being divided
into 180 parts. The 18th-century German physicist Daniel Gabriel Fahrenheit
originally took as the zero of his scale the temperature of an equal ice-salt mixture
and selected the values of 30 and 90 for the freezing point of water and normal
body temperature, respectively; these later were revised to 32 and 96, but the final
scale required an adjustment to 98.6 for the latter value.
Celsius temperature scale also called centigrade temperature scale, is the scale
based on 0 for the freezing point of water and 100 for the boiling point of water.
Invented in 1742 by the Swedish astronomer Anders Celsius, it is sometimes called
52
the centigrade scale because of the 100-degree interval between the defined points.
The following formula can be used to convert a temperature from its representation
on the Fahrenheit ( F) scale to the Celsius (C) value: C = 5/9(F - 32). The Celsius
scale is in general use wherever metric units have become accepted, and it is used
in scientific work everywhere.
Kelvin temperature scale is the base unit of thermodynamic temperature
measurement in the International System (SI) of measurement. It is defined as 1/
273.16 of the triple point (equilibrium among the solid, liquid, and gaseous phases)
of pure water. The kelvin (symbol K without the degree sign []) is also the
fundamental unit of the Kelvin scale, an absolute temperature scale named for the
British physicist William Thomson, Baron Kelvin. Such a scale has as its zero
point absolute zero, the theoretical temperature at which the molecules of a
substance have the lowest energy. Many physical laws and formulas can be
expressed more simply when an absolute temperature scale is used; accordingly,
the Kelvin scale has been adopted as the international standard for scientific
temperature measurement. The Kelvin scale is related to the Celsius scale. The
difference between the freezing and boiling points of water is 100 degrees in each,
so that the kelvin has the same magnitude as the degree Celsius.
Que46: The boiling point of water is 100°C. What temperature does water boil at
in the Fahrenheit scale?
Ans:
53
The boiling point of water is 212°F.
Que47: Explain Bi-metallic thermometers?
Ans: These thermometers are based on the principle that the solids show a thermal
expansion.
Principle: A bimetallic thermometer is based upon the principle that the linear
expansion upon the same increase in temperature occurs differently for different
materials.
Theory: A bimetallic strip is the main working part of this thermometer. Consider
two thin identical strips of different metals. E.g. copper and iron. These strips are
riveted together at several points, while in cold state. This makes sure that the two
strips are in a tight contact and will not slip over each other. When this formation is
heated iron and copper show different linear expansions. Therefore, the bimetallic
strip bends.
The bending of a bimetallic strip is used to turn a pointer attached to it. When the
pointer moves on a dial one can read the temperature.
Construction and working:
1. In this model a long bimetallic strip is cut to make spiral. This spiral acts
like a hair string. One end of spiral is fixed to an adjustment block and other
end, which is at the centre, is permanently fixed.
2. When the temperature increases the inner and outer layers show different
extension.
3. For a thermometer to be used in room or in a car, a small block of a metal is
sufficient to heat up the spiral. But in case of a furnace it is not wise to heat
up the entire instrument.
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Que48: Explain pyrometers?
Ans: Pyrometer, an instrument for measuring temperature. Although the term
pyrometer is generally considered to apply to instruments that measure high
temperatures only, some pyrometers are designed to measure low temperatures.
Two common types of pyrometers are the optical pyrometer and the radiation
pyrometer.
A heated object gives off electromagnetic radiation. If the object is sufficiently hot,
it will give off visible light, ranging from dull red to blue-white. Even if the object
is not hot enough to glow, however, it gives off infrared radiation.
An optical pyrometer determines the temperature of a very hot object by the color
of the visible light it gives off. The color of the light can be determined by
comparing it with the color of an electrically heated metal wire. In one type of
pyrometer, the temperature of the wire is varied by varying the strength of the
current until the operator of the instrument determines that the color of the wire
matches the color of the object. A dial, operated by the current that heats the wire,
indicates the temperature.
A radiation pyrometer determines the temperature of an object from the radiation
(infrared and, if present, visible light) given off by the object. The radiation is
directed at a heat-sensitive element such as a thermocouple, a device that produces
an electric current when part of it is heated. The hotter the object, the more current
is generated by the thermocouple. The current operates a dial that indicates
temperature.
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Que49: Define heat? What are the different modes of heat?
Ans: Heat is a form of energy which transfers between bodies which are kept
under thermal interactions. When a temperature difference occurs between two
bodies or a body with its surroundings, heat transfer occurs. In this article, we are
going to deal with the different modes of heat transfer. Heat transfer occurs
basically in three modes:
1.
Conduction
2.
Convection and
3.
Radiation
CONDUCTION:
Conduction is the mode of heat transfer occurs from one part of a substance to
another part of within the substance itself or with another substance which is
56
placed in physical contact. In conduction, there are no noticeable movement of
molecules.
CONVECTION:
Conductive heat transfer occurs within a fluid itself and it is carried out by transfer
of one fraction of the fluid to the remaining portion. Hence unlike conduction,
transfer of molecules occurs during convection. Since movement of particles
constitutes convection, it is the macro form of heat transfer. Also convection is
only possible in fluids where the particles can moved easily and they rate of
convective heat transfer depends on the rate of flow to a great extend. Convection
can be of two types:
1.
Natural convection: In this type of convection, the movement of particles
which constitutes convection occurs by the variation in densities of the fluids. As
we already know, as temperature increases, the density decreases and this variation
in density will force the fluid to move through the volume. This cause convection
to occur.
2.
Forced Convection: The difference between natural convection and forced
convection is that in forced convection, a work is done to make movement in the
fluid. This is done using a pump or blower.
RADIATION
Radiation is the third mode of heat transfer. This mode of heat transfer didn’t
require any medium to occur. Every matter having a temperature above absolute
zero will emit energy in the form of electromagnetic waves and called radiation. It
is the same way the energy of the Sun reaches us.
Que50: what are the properties of thermal radiations?
Ans: 1. They do not require a medium for their propagation.
2. Media are transparent or opaque depending upon the wavelength of incident ave.
3. They travel with a velocity 3*108m/s in vacuum.
57
4. Their velocity obeys the relation C=vλ
wavelength
where
v=
frequency
and
λ=
5. They undergo reflection, interference, diffraction and polarization.
6. Thermal radiations travel in straight line.
7. They obey inverse square law.
8. Thermal radiations do not heat the intervening medium.
Que51: Explain Stefan’s and Wien’s laws?
Ans: Boltzmann Law are illustrated in the following equations.
The Wien Law gives the wavelength of the peak of the radiation distribution, while
the Stefan-Boltzmann Law gives the total energy being emitted at all wavelengths
by the blackbody (which is the area under the Planck Law curve). Thus, the Wien
Law explains the shift of the peak to shorter wavelengths as the temperature
increases, while the Stefan-Boltzmann Law explains the growth in the height of the
curve as the temperature increases. Notice that this growth is very abrupt, since it
varies as the fourth power of the temperature.
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