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PHYSICS NOTES 2020 Year 100%

PHYSICS                           CLASS  9th 
Ph: 03346718817 100% PASSED 

Q.1      What is Science?
Ans .   Science :
The knowledge gained through observation  experimentations  is called science.
Q.2     What is Physics? Write the name of its branches.
Ans .   Physics:
It is that branch of science which deals with the study of the properties of matter  ,energy and their  mutual relationship .
Branches of Physics :
1)Mechanics     2)Heat   3)Sound   4)  Light   5) Atomic physics  6)Nuclear physics   7) Plasma physics 8) Geo physics  9) Electricity and Magnetism .
Q.3      What  is Plasma?
Ans.    Plasma:
Matter assumes the state of gas ,comprising of ions and electrons at very high temperature .
Q.4      What is Plasma Physics and Nuclear Physics?
Plasma Physics:
The study of the properties of matter in this state is called Plasma Physics.
Nuclear Physics:
It deals with the properties and behavior of nuclei and the particles with in the nuclei.
Q.5      What are Physical Quantities? Give two examples.
Physical Quantities:
All measurable quantities are called Physical quantities.  e.g  Length Mass Time and Temperature.

Q.6      What are basic quantities? Write the name of the basic quantities.
Ans .    Basic Quantities:
The unit of a base quantity is called a base unit.these are the 7 basic quantities;
1)Length   2)Mass   3)Time   4)Mole   5)Temperature  6)Electric Current   7)Intensity of light
Q.7      What are Derived units? Write some examples.
Ans.     Derived Units:
Those units which are derived from basic units are called Derived Units. e.g speed ,velocity,energy
Q.8      Identify the base quantity in the following. 1)Speed  2)Area  3)Distance   4)Force
Ans      Distance is a base quantity.
Q.9      Identify the following as base or Derived  quantity . Density, force , mass, speed ,time,  
            length, volume , temperature
Ans.     Base Quantities           Mass , time , length , temperature
             Derived  Quantities     Density , force , speed , volume
Q.10     Define Scientific Notation. Give one example.
Ans.      Scientific Notation:
The method in which digits are written in some power of ten is called scientific notation.
e . g  the scientific notation of 340000  is   3.4x105 .
Q.11  Write the numbers given below in scientific notation.
  (a) 3000000000ms-1(b)6400000m (c)0.0000000016g (d)0.0000548s
Ans  =                  3x109ms-1(b)=64x105m=6.4x106m (c)16x10-10g=1.6x10-9g(d)548x10-7s=5.48x10-5s
Q.12       What are measuring instrument? Writes the name of measuring  instrument
                Which are used in Physics.
Ans         Measuring Instrument:Those which are used to measure various physical quantities.
1)Metre  Rule 2)Measuring Tape 3)Screw Gauge 4)Vernier Calipers
Q.13   What are Vernier Calipers and Screw Gauge . For what purpose they are used?
Ans     Vernier Callipers:
It is that instrument  which is used to measure the correct length of 0.1mm.The least count of vernier calipers  is 1.01mm.
 Screw Gauge:
It is that instrument  whech is used  to measure length up to 0.01mm.The least count of Screw Gauge is 0.01mm.
Q.14   Write the two names of Mass Measuring Instruments.
Ans  1)         Physical Balance   2)Lever Balance   3)Electronic Balance
Q.15               What are Significant Figures? Give Examples.
Ans      Significant Figures:
The accurately known digits and first doubtful digit in any measurement is called Significant Figures.
e.g   in  5.731cm3  volume of a cylinder has the significant figure three .
Q.16  Define Rest  and give example.
Ans     Rest: If a body does not changes its position with respect to some observer then
          it is said to be a state of Rest .e.g   a  book is lying on the table.
Q.17   Define Motion and give  example.
 Ans   Motion: If a body changes its positions with respect  to some observer then it is
                   said to be a state of Motion .   

Q.18   How many  types  of  Motion ?Write the names.
Ans    Names:   There are three  types of  motion.
         1)       Translatory  Motion      2)Rotatory  Motion       3)Vibratory Motion 
Q.19   Define  Translatory  Motion. Give one example.
Ans    It is that type of motion in which every particles of body has exactly the same motion is called
                   Translatory  motion .e.g.  motion of a gas molecule.
Q.20   How  many types of   translatory  motion? Write its Names.
 Ans:            There are three types of  translatory  motion.
                   1)Linear Motion 2)Circular Motion 3)Random  Motion .

Q.21   Difference b/w Linear and circular motion ?Give an example.
Ans:             Linear Motion:
              It is that motion in which body moves in a straight line .e.g. motion of  a freely falling bodies.
              Circular motion:
             It is that  motion in which body moves in a circular path .e.g . motion of the sum around the earth.
Q.22   Define Random motion .
Ans.      Random Motion:
It is that motion in which a body movies in an irregular manner. e.g motion of gas molecules .
Q.23   Difference b/w  Rotatory  and Vibratory Motion ?Give an example.
Ans.      Rotatory  Motion:
              It is that in which a body moves around a fixed point of axis .e.g .motion of ceiling electric fan.
              Vibratory Motion:
It is that in which a body moves to and fro about  a point  and repeats its motion again and again about the same point. e.g Motion of swing .
Q.24     What are vector and scalars? Write five example of each.
Ans.       Vectors:
               Those quantities which have magnitude and direction are called Vector .e.g. force, weight,
               acceleration  torque ,momentum etc.
               Those quantities  which have only magnitude but no direction are called Scalar. e.g. Speed,
               Time, Volume , Energy ,Work etc.
Q.25               What is position?
 Ans.    Position:
            The distance in the direction of a body from a fixed point shows its position.

Q.26    What is difference b/w  distance and displacement?
Ans.     Distance:
             When a person goes to Islamabad from Lahore through motorway .When he checks the meter,
              realize that he has covered by a distance of 400 km. It is the total distance covered by the person.
             Distance is a scalar quantity.
             The shortest distance between two points is known as displacement. Displacement  is a vector
Q.27   What is difference b/w Speed and Velocity ?
Ans:     Speed
             Distance covered by a body in unit time is called Speed of body . e.g When a body covers 5m  
            distance in 1 second then the speed of body will be 5m/s.
            If a body covers distance ‘S’ in time ‘t’ then its speed ‘V’ can written as:
Speed= distance/time
            The unit of a speed is meter per second (ms-1)  or kilometer per hour (km-1)
            Distance covered by a body in unit time in particular direction is called Velocity of that body in       
            that direction. e.g. when a body covers 5m in 1 seconds along east , then the velocity of the body
            5m/s in east .Velocity is a vector quantity. The unit of velocity is meter per second (ms-1) or
            kilometer per hour (kmh-1)
Q.28   What is difference b/w uniform speed and uniform velocity?
Ans:    Uniform speed:
            A body has  uniform speed if  it covers equal distance in equal intervals  of time however may be.

            Uniform velocity :
            A body has uniform velocity if  it covers equal displacement in equal intervals of time however  
            short the intervals may be.
Q.29   How can cheetah run with a speed ?
Ans.    Cheetah can run at  a speed  of    70 kmh-1.
Q.30    What is LIDAR gun?
Ans:                (1) A   LIDAR  gun is used as light detection.
                        (2)It is use to calculate the vehicle’s speed .
Q.31   What is acceleration ?Write its units.
Ans:     Accelration :
             The rate of change of the velocity of  a body  is called acceleration.
             The unit of the acceleration is meter per second square (ms-2).It  is denoted by ‘a’.
Q.32   Write the equation of Motion.
Ans:      Equation of  Motion:
              There are three equations of motion.
(i)           Vf  =  vi+at
( ii )       S  =Vit +1/2 at2
(ii)          2as =vf2 –vi2
Q.33   What is Gravitational acceleration ?Write its value .
Ans :     That acceleration which is produced due to earth is called gravitional acceleration.It  is denoted
              by ‘g ‘.The value of ‘g’ is10m-2.
Q.34   Difference  b/w Kinematics and Dynamics?
Ans:      Kinematics:
              It is the study of motion of bodies with out any reference to mass  or force is called  kinematics.

              Dynamics :
It is the study of motion of body under the action of force.
Q.35   What is force ?Write its unit .
Ans       Force:
             Force is the angency which changes  or tends to change the state of  rest or uniform motion of a             
             Unit of force:
       In S.I  the unit of fore is unit Newton.
Q.36     What is Inertia ?
Ans:      Inertia :
              It is that property  of a body  due to which  it resists against any change in its state of rest or of    
              uniform motion.
Q.37   What is momentum ?Write its unit.
Ans:      Momentum
              The product of mass and velocity is called momentum.If momentum is denoted by  ‘P’ ,mass
            denoted by “m”  and velocity denoted by “v” then we can write        P = mv
             In S.I  unit of the momentum is Kilogram meter per second (kgms-1).                                                                
   It can also be written as Newton Second (N.S).
Q.38   State the First Law Of Motion.
Ans       First  Law of Motion:
          This law states that everybody continues in its state of rest or if uniform motion in a straight line  unless it is complete to change that state by an external source impressed upon it.

Q.39   State the second law of motion . Write its formula.
Ans.    Second  Law of  Motion :
This law states that when a force  acts upon a body an acceleration is produced in the direction of
a force  and the magnitude of the acceleration is directly proportional to the force and is inversely
proportional to the mass of the body .The formula of motion is given as                  F = ma
             Where F = Force , m = mass and a=acceleration
Q.40   State the Third Law of Motion. Give one example.
Ans       Third Law of Motion:
              This law states that action and reaction are equal in magnitude but opposite in direction .
              e.g  Motion of a man on earth is a good example of third law of motion.

Q.41   Difference between Mass and Weight.
i)The quantity of matter in body is called mass.
i)The force of attraction between body and earth is
called weight.
ii)It is a scalar quantity.
ii)It is a vector quantity.
iii)It is denoted by “m” .
iii)It is denoted by ‘w’ .
iv)The unit of the mass is kilogram.
iv)The unit of the weight is Newton.
v)Mass cannot be zero.
v)Weight can be zero.
vi)Mass is measured by ordinary balance .
vi)Weight is measured by spring balance.
vii)m =  F/a .
vii)   w = mg

Q.42  What is Tension. Write its unit.
Ans    Tension:
           Tension is a force , which is equal to the weight of  body opposite in direction. Tension and weight
           Of the body all ways acting in opposite  direction. The unit of  tension  is Newton.
Q.43  What is the Law of  Conservation of  Momentum.
Ans.    Law of  Conservation of  Momentum:
           This law states that when two or more bodies collies with one another the total momentum
           Of  the system remains same provided no external force acts upon them.
Q.44   What is Friction?
Ans:     Friction:
             The force which resists the motion of one surface on another surface on another is called friction.
Q.45     Write the method to reduce the Friction.
Ans:      following method are used to reduce the friction.
We can reduce the friction by polishing the parts of  the machine.
We can reduce the friction by using the ball bearings.
We can reduce the friction by making the pointed shape of the front of the high speed vehicles aeroplans and ships.
We can reduce the friction by  use of oil or grease.
Q.46   Explain the Rolling friction and Sliding Friction.
Ans.     Rolling friction and Sliding Friction:
When a body moves with the help of wheels the friction in this case is called rolling friction.
The opposite friction of  moving body from one surface of  the body to surface on the body is called sliding friction.
Q.47   What are disadvantages of friction.
Ans.      Disadvantages of friction:
Energy is wasted due to friction.
Due to the friction moving parts of machines become where and tear.

Q.48   What is Circular Motion? Give one example.
Ans.      Circular Motion:
              When a body moves in a circle then its motion is called circular motion e.g motion of a bicycle in
              a Circle.   
Q.49   What is Centripetal Force? Write its Formula.
Ans.     Centripetal Force:
A force which acts on an object moving in a circle and is directed towards the center of the circle    is called centripetal force.
Formula of centripetal force is given as
Fc  =  mv2/r
Q.50   What is Centrifugal Force? Write its Formula.
Ans.     Centrifugal Force:
It is that force which is equal in magnitude to the centripetal force but opposite in direction is
called centrifugal force.
Formula of centripetal force is given as
Fr  =  mv2/r

Q.51   What are the parallel forces Define its types.
Ans.     parallel forces:
If a number of forces acting on a body in such a way that their points of action are different but
lines of action are parallel to each other then these force are called parallel forces.
Types of  parallel forces:
There are two types of parallel forces.
Like parallel force
Un- Like parallel force
Like parallel force:
If the direction of the parallel forces is the same then these type of the forces are called Like
parallel forces.
Un- Like parallel force.
If the direction of the parallel forces are opposite to one another then these types of the forces are called Un-Like parallel forces
Q.52   What is resolution of force?
Ans.      Resolution  of   force:
Division of its components are called resolution of a force . Normally a force can be resolved into   
two components. First component along  x-axis is called Horizantal component and the second 
component along y-axis is called the vertical component of a force.
Q.53   What is Torque or Moment of a force. Write its unites.
Ans.     Torque or Moment of a force:
The turning effect of the force is called torque or Moment of a force.            
Unit of the Torque or momentum of a force  :
In SI unit of  the torque or moment of force is Newton-meter (N.m).
Q.54   What factors depend on the torque .Write the formula of the torque.
Ans:    Factors depend on torque:
            There are two factors, which depend on the torque.
Magnitude of the force.(F)
          Moment arm ®
These two factors are directly proportional to the producing torque. if  troque is denoted by we write.
T =F x r

Q.55   What is the centre of gravity. Give two examples.
Ans.      Centre of Gravity
It is that point in a body where the whole weight of the body acts is called centre of gravity of
that body . e.g
Centre of gravity of uniform rod is centre of the rod
Centre of gravity of  the sphere is centre of the sphere.
Q.56   What is Couple?
Ans.      Couple:
When two equal, opposite and parallel forces act at two different points of the same body, they
form a couple. e.g opening nut of the tyre mechanics applies a couple
Q.57   What is Equilibrium?
Ans.      Equilibrium:
If no net force is acting on the body then the body is said to be in equilibrium  e.g. the book
Lying on the table.
Q.58   State the conditions of the equilibrium?
Ans.     Conditions of the equilibrium:
There are two conditions of equilibrium
i)       First conditions of the equilibrium:
There are two conditions states that if sum of all the forces acting on the body is equal
to zero then the body will be in equilibrium mathematically, we can write as:
F  =  0

ii)      Second Conditions of the Equilibrium:
The second conditions states that if algebraic sum of all the torques acting on a body equal to zero
then the body will be equilibrium.
Mathematically, We can write as:
=  0
For a complete equilibrium then it should be:
F  =  0  and    =  0
Q.59   How many states of the Equilibrium. Define each with example.
Ans.     States of Equilibrium:
There are three states of equilibrium.
Stable equilibrium
Un-stable equilibrium
Neutral equilibrium
  Stable Equilibrium:
It is that state of the equilibrium in which the body comes back its original position when
Set free after slightly lifting from one side. On lifting, the centre of the gravity of the body is raised up as compared to the initial position. e.g. A wooden rectangular block.
Un-stable Equilibrium:
it is that state of the equilibrium which if the position of the body is disturb it dose not come back its original position of the body is disturb it dose not come back to its original position. In such a case the centre of gravity is lowered than its original e.g. Free falling a wooden rectangular block.
Neutral Equilibrium:
It is that type of the equilibrium in which after disturbance, the body again comes to rest position of its        centre of gravity dose not change. e.g. A ball lying on the table.
Q.60   What are Rigid Body and axis of Rotation.
Ans.  Rigid Body:
              A Rigid Body is the one that is not deformed by force or forces acting on it.
              Axis of Rotation:
            Consider a rigid body rotating about a line The particles of  the body move in circles with lying on
                    this line. This line is called the axis of the body.
Q.61      State Principle of Moments.
Ans.      Principle of Moments:
              According  to this to principle,  “a body  is balanced if the sum of clockwise moments acting
              on the body is equal to the sum of anticlockwise moments acting on it”.
Q.62  Describe the types of  Torque or Moment of Force.
Ans.  Types of  Torque Moment or of Force:
          There are two types of  torque of  moment of force.
          1)Clockwise moment.              2)Anti clockwise moment
           Clockwise moment.
          That torque which is produced in the clockwise direction is called clockwise moment.
           Anti clockwise moment.
           That torque which is produced in the Anti clockwise direction is called Anti clockwise moment.
Q.63   What is Centre of  Mass.
Ans.    Centre of Mass
            Centre of Mass of a system is such a point where an applied force causes the system to move with
            Out rotation.
Q.64   What is force of Gravitation?
Ans.    Force of Gravitation:
In the universe there is a force due to which every body of the universe attracts every other body       which is called force of gravitation.

Q.65   State the Newton’s law of Gravitation?
Ans.    Newton’s law of gravitation:
This law states that every object in the universe attracts every other object with a force, which is Directly  proportional  to the product of their masses and inversely proportional to the square of  thedistance  between  their  centre.
Q.66   Write the values of G, M & R?
Ans.       G =  6.67x10-11Nm2Kg-2                       M = 6x1024Kg                                   R = 6.4x106m
Q.67     What are artificial satellites  ?Also write the name of natural satellite of our earth.
Ans.      Artificial  satellite:
              It is that object which is used for communication and space  research  revolve around  the earth
              under the force of gravity which provides the necessary centripetal force .The orbital speed of the                     
             satellite  is 7.9  kms-1. Moon is a natural satellite of our earth.
Q.68   What is height of the Geo satellite from the surface of the earth? What is its velocity with                                                                         respect to earth.
Ans.   The height of the Geo Stationary satellite is about 42.300 km from the surface of the earth
           Its  velocity with respect to earth is zero.
Q.69   What is Global positioning system (GPS).What is its function.
Ans.    Global Positioning System (GPS):
            It is a satellite of navigation system. It helps us to fine the exact position of an any where of the
            land on the sea or in the air . GPS consist of 24 earth satellite These satellites revolve around the
            earth twice a day with a speed of 3.87 km s-1.
Q.70   What is a work Write its unit.
  Ans    Work :
              The product of force and distance covered  in direction of the force is equal to the work  done. In
              S.I  unit of work is Joule .If work is denoted by W, force by F and distance by S then we can   
              write                                              W=F.S

Q.71   What is Energy .Write its unit.
Ans.      Energy:
             The ability of a body to do work is called energy is joule.
Q.72   What is Kinetic Energy? Write its formula.
Ans.      Kinetic Energy:
It is that energy which is produced due to motion is called kinetic energy.
Its formula is given as:-                         K.E   =          1\2 mv2
Q.73   What is Potential Energy ? Write its formula.
Ans       Potential Energy:
Energy of a body due to its height from the surface of the earth is called Potential Energy.
Its formula is given as :-     P.E  =  mgh
Q.74   What are the forms of Energy?
Ans     Forms of Energy:
         1)Mechanical Energy    2)Heat Energy     3)Electrical Energy     4)Sound Energy
         5)Light Energy   6)Chemical Energy      7)Nuclear Energy
Q.75   What is Mass - Energy  Equation ?Write its formula .
Ans      Mass-Energy Equation:
              Einstein’s  gave the mass energy equation . Its formula is given below :
                                                E = mc2

Q.76   What is Efficiency .Write its formula .
Ans       Efficiency:
              The ratio of the useful work done by a machine to the work done on the machine is called
              efficiency of the machine. The formula of  efficiency is given as :
               Efficiency  =   Output

                Usually Efficiency is calculated in percentage .Therefore we can write 
                Percentage Efficiency = Output   x  100    
Q.77       What  is Power ? Write its formula and unit .
Ans         Power :
               The rate of doing work with respect the time is called Power.
                                     Power = Work
                In the S.I  unit of Power is watt  .
Q.78       What is Density ? Write its formula.
Ans         Density:
                Density of a substance is defined as its mass per unit volume .
                Density  =  mass of a substance         ,        d =     m
                                   volume of a substance                          v

Q.79        What is Pressure .Write its formula and unit .
Ans.         Pressure:
              The force exerted perpendicularly on unit area of an object is called Pressure.
                 Pressure = Force       ,         P  =       F  
                                     Area                               A                               
                 In the S.I  unit of Pressure is Newton Per Square meter (Nm-2) . It is also called Pascal.
Q.80        What is Stress and Strain? Write its units .
Ans.        Stress:
            Stress is the force that acts on the unit area of an object and thus changes its shape or size.
               In the S.I  unit of stress is Newton Per Square meter (Nm-2)
               It is defined as the change in the shape of an object caused due to stress. Strain has no unit.
 Q.81    Define the Hooke’s Law?
Ans.      Hooke’s  Law:
              It states that within the elastic limit of an object , the strain produced in an object is directly
              proportional to the applied stress.
Q.82     State the Pascal’s  Law.
Ans.      Pascal’s  Law:
              This law states that if pressure is exerted on a liquid , the liquid distributes it equally in
               all directions.
Q.83      State the Archimedes Principle.
Ans.       Archimedes Principle:
               It states that when an object immersed in a liquid, the liquid exerts a buoyant force on the
object  which is equal to the weight of the liquid displaced by the object .

Q.84     State the Principle of Flotation:
Ans.      Principle of  Flotation:
              According to this Principle ‘ A floating object displaces a fluid having weight equal to
              the weight  of the object.
Q.85     What is Young’s  Modulus? Write its  Equation.
Ans        Young’s  Modulus:
               Tensile stress divided by strain is called Young’s Modulus .It is denoted by  “Y”.
                Its formula can written as:     Y = F x L
                                                                       A x L
Q.86       What is Elasticity.
Ans         Elasticity:
               The property of a body to restore its original size and shape as the deforming force
               ceases to act is called elasticity.
Q.87   What is Heat? Write its unit?
Ans        Heat:
               It is a form of energy which is  transferred  from  one body  to  the other due  to the
               difference of temperature between them. The unit of Heat is ‘Joule’.
Q.88       What is Temperature?
Ans         Temperature:
                It is the degree of hotness and coldness of a body.It can be measured in three different scales.
Q.89        For what scales are used to measure the Temperature?
Ans          Scales to measure the temperature:
                 There are three scales are used to measure the temperature.
                  i)Fahrenheit Scale     ii)Celsius Scale     iii)Kelvin Scale

Q.90       What is Specific Heat? Write its unit. Also write the specific heat of water.
Ans         Specific  Heat:
               The quantity of heat is required to change temperature of 1kg  mass of a substance by one
                Kelvin is called Specific Heat.
                The unit of specific heat is Joule per Kilogram Per Kelvin (Jkg-1K-1).
                 The specific heat of water is 4200 Jkg-1K-1.
Q.91         What is Latent Heat of Fusion .Give one example.
Ans.          Latent Heat of  Fusion:
                  The quantity of heat required to change 1kg of a solid substance to liquid state without
                  changing  the temperature  is called  latent  heat of  fusion. e.g.  Latent  Heat of  Fusion
                   of Ice is 336000 Jkg-1.
Q.92            What is Latent Heat of  Vaporization. Give one example.
Ans.                    Latent Heat of  Vaporization:
                     The amount of  heat  needed to change 1kg of a liquid to vapor or gaseous  state at a
                      fixed  temperature is called Latent  heat of Vaporization. e.g.  Heat of  Vaporization 
                       of water 2,60,000 Jkg-1.
Q.93              What is the Evaporation?
Ans.               Evaporation:
                       It is the changing of a liquid into vapors (gaseous state) from the surface of the liquid
                       without  heating it.
Q.94              What factors depend upon the rate of  Evaporation?
Ans.                Factors Affecting on Evaporation:
                       1)     Temperature       2)    Surface Area       3)    Nature of  Liquid         4)    Wind

Q.95                How many methods are used for the transmission of  heat. Also define them.
Ans.               Methods for the Transmission of heat:
                        There are three methods for the transmission of heat.
                        1)Conduction    2)Convection      3)Radiation     
1)         Conduction:
                              It is the process in which heat is transmitted from one body to another by the interaction of
                        molecules and electrons.

2)        Convection:
                         It  is  that  process  of   transfer of  heat in which   transmission of heat is due to the actual
                          movement of the molecules of the substance from one place to the other.
3)         Radiation:
                          It is that process of transfer of heat energy reaches in the form of waves from one place to
                          the other without effecting the medium on its way.
Q.96  What is the importance of Conduction in our daily life?
Ans.   Importance of  conduction:
          Normally metals are the good conductor of heat and non metals like wood or plastic are the
          bad conductor of heat. Therefore  the cooking  utensils are  made  of  metals  and  wood or
          plastic  is  used for spoon  handles. Wood or plastic being  insulators  don’t  allow the heat
          transfer and metallic vessels are  preferred for freezing  purpose. Air is a good insulator of
          heat. Those object which can hold air in them behave as insulators. e.g  fur, wood dust etc.

Q.97  What is the importance of the Convection in daily life ?
Ans:    Importance of Convection in daily life:
Geysers  in our houses work on convection  principal. Water  in the boiler is heated by  convection  and reaches the upper portion of the tank .Cold water from the water tank replaces the hot water .
Convection  may take place in any liquid  or gas. Ventilation of air in houses is only possible by convection .
              To speed up natural convection ,fans pumps are used which is known as the forced convection.
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