CHAPTER – 10
Definition of Matter
“Anything
having mass and volume is called matter.”
Kinetic
Molecular Theory of Matter
The Kinetic Molecular Theory of
Matter has the following postulates:
§
Matter is made up of very small
particles called molecules.
§
These molecules are in the same state
of motion, hence they possess kinetic energy. Their motion can be translatory,
vibratory or rotational.
§
The molecules attract each other with
a force. This force depends upon the distance between them. Force is inversely
proportional to the distance between the molecules.
§
When a substance is heated its
temperature as well as molecular motion increases. Due to this motion, kinetic
energy also increases. we can say that when the kinetic energy of the molecules
increases, then temperature of the substance rises.
Brownian Motion
In 1827, a scientist, Robert Brown
observed the motion of molecules with the help of a microscope. He observed
that the tiny particles in water are constantly moving in a zigzag path. He
called the motion, Brownian Motion.
Explanation
The cause of this tiny particle motion is the rapid motion of the molecules, which collide with the particles and push them in one direction. If some molecules come from other direction and collide with the same particles, particles change their direction. This process continues and the motion becomes zigzag.
The cause of this tiny particle motion is the rapid motion of the molecules, which collide with the particles and push them in one direction. If some molecules come from other direction and collide with the same particles, particles change their direction. This process continues and the motion becomes zigzag.
States of Matter
Matter has been classified into three
states. These states are discussed below:
1.Solid
§
According to the kinetic theory of
matter, solid has the least kinetic energy. The properties of solids are given
below:
§
The particles are very close to each
other.
§
Their shape and volume is fixed.
§
Particles in a solid vibrate to and
fro from their mean position.
§
On heating they melt and convert into
liquid.
§
Some solids also convert directly
into gas on heating.
2. Liquid
According to the kinetic theory of
matter, liquids have the following properties;
§
They have greater kinetic energy than
solids but less than that of gases.
§
The volume of liquid is fixed.
§
They move more freely than solids.
§
The attraction between molecules is
lower than solids.
§
The distance between the molecules is
greater than that of solids.
§
On heating, they convert into
vapours.
§
On cooling, they convert into solid.
3. Gas
According to the kinetic molecular
theory, gases possess the following properties.
§
Gases possess more kinetic energy.
§
Their shape and volume are not fixed.
§
The distance between their molecules
is large.
§
Their temperature is proportional to their
kinetic energy.
§
Their temperature rises with increase
in pressure.
§
On cooling, they convert into liquid
and gases.
ELASTICITY
Definition
” The tendency of a material to return to its original dimension after the deforming stress has been removed is known as elasticity.”
” The tendency of a material to return to its original dimension after the deforming stress has been removed is known as elasticity.”
If we apply a force
to a body, it is stretched. When the applied force is remove, the body returns
to its original shape. The phenomenon of turning back to its original shape is
called Elasticity.
Elastic Behaviour and Molecular Theory
The elastic
behaviour of a material can be explained by the Kinetic Theory of Matter. Since
the molecules in a solid are very close to each other, there exist strong
attracting forces between them. Thus when force is removed, the attraction
forces between the molecules pull them back again and the material is restored
to its original shape. Different material have different elasticity depending
on the nature of the material.
Elastic Limit
The maximum
resisting force of a material is called the Elastic Limit of that material.
STRESS
Definition
“When a body is made to change its length, volume or shape by the application of an external force, the opposing force per unit area is called Stress.”
“When a body is made to change its length, volume or shape by the application of an external force, the opposing force per unit area is called Stress.”
Formula
Stress = Force / Area
Stress = Force / Area
o = F/A (Here o
represents (Rho) do not write in your examination paper)
Units
S.I or MKS System – N/m2 or Pascal
(Pa)
C.G.S system – Dyne/cm2
F.P.S or B.E System – lb/ft2 and lb/in2
(Here 2 in all above systems shows square)
C.G.S system – Dyne/cm2
F.P.S or B.E System – lb/ft2 and lb/in2
(Here 2 in all above systems shows square)
Types of
Stress
Following are some
types of stress:
1. Tensile Stress:
It is a stress tending to stretch a body.
2. Bulk Stress: It
is an overall force per unit area, also known as pressure.
3. Shear Stress: It
is a stress tending to produce an angular deformation.
STRAIN
Definition
Stress can produce a change in shape, volume or length in an object. This change in the shape of an object is called strain.
Stress can produce a change in shape, volume or length in an object. This change in the shape of an object is called strain.
Formula
Mathematically,
Strain = Change in Length/Length or Strain = Change in volume / volume
Mathematically,
Strain = Change in Length/Length or Strain = Change in volume / volume
Units
Since strain is a ratio between two
similar quantities, it has no unit.
Types of
Strain
Following are some
types of strain.
1. Tensile Strain:
It is a change in length divided by original length.
2. Bulk Strain: It
is the change in volume divided by original volume.
3. Shear Strain: It
is equal to the angular displacement produced.
HOOK’S LAW
Introduction
An English Physicist and Chemist Robert Hook discovered this law in 1678.
An English Physicist and Chemist Robert Hook discovered this law in 1678.
Statement
“Strain produced is proportional to the stress exerted within the elastic limit.”
“Strain produced is proportional to the stress exerted within the elastic limit.”
Elastic Limit
The point at which
a material becomes plastic is called elastic limit on yield point.
Yield Point
The yield point is
the point at which the material begins to flow. It is also the point between
elastic region and plastic region.
Elastic Region
When the material obey’s Hook’s Law, it is said to be in Elastic Region.
When the material obey’s Hook’s Law, it is said to be in Elastic Region.
Plastic Region
When stress is applied beyond the elastic limit, the graph is no longer a straight line. In this case stress produces a permanent change in the material. The material is said to be in its Plastic Region.
When stress is applied beyond the elastic limit, the graph is no longer a straight line. In this case stress produces a permanent change in the material. The material is said to be in its Plastic Region.
Breaking
Point
The material breaks at a certain point called the Breaking Point of the material.
The material breaks at a certain point called the Breaking Point of the material.
YOUNG’S MODULUS
Definition
“The ratio of the stress on a on a body to the longitudinal strain produced is called Young’s Modulus.”
“The ratio of the stress on a on a body to the longitudinal strain produced is called Young’s Modulus.”
Mathematical Expression
According to the definition of YOung’s Modulus:
Young’s Modulus = Sress / Longitudinal Strain
According to the definition of YOung’s Modulus:
Young’s Modulus = Sress / Longitudinal Strain
Unit
In S.I system, Young’s Modulus is measured in N/m2.
In S.I system, Young’s Modulus is measured in N/m2.
PRESSURE
Definition
“The perpendicular force per unit area acting on a surface is called pressure.”
“The perpendicular force per unit area acting on a surface is called pressure.”
Mathematical Expression
Pressure = Force /Area
P = F/A
Pressure = Force /Area
P = F/A
Unit
S.I or M.K.S System – N/m2 or Pascal.
C.G.S system – Dyne/cm2.
F.P.S or B.E System – lb/ft2 and lb/in2.
S.I or M.K.S System – N/m2 or Pascal.
C.G.S system – Dyne/cm2.
F.P.S or B.E System – lb/ft2 and lb/in2.
Pressure
in Liquids
In water or other liquids, the weight
exerted on a body or the bottom of the liquid is its pressure.
PASCAL’S PRINCIPLE
Statement
When a pressure is applied to a liquid contained in a vessel, it is transmitted undiminished equally in all directions and acts perpendicularly to the walls of the container.
When a pressure is applied to a liquid contained in a vessel, it is transmitted undiminished equally in all directions and acts perpendicularly to the walls of the container.
Applications
– Hydraulic Press
Pascal’s Principle
has the application in Hydraulic press. In a hydraulic press a narrow cylinder
A is connected with a wider cylinder B and they are fitted with airtight
piston. It is filled with some incompressible liquid. Pressure can be applied
by moving the piston cylinder A in the downward direction. Piston B is used to
lift the object. The hydraulic press is provided with a rigid roof over it.
When piston B moves upward, it compresses any material placed between the rigid
roof and this piston. The hydraulic press is used for compressing soft
materials like cotton into a cotton bale and powdered materials into compact
solids.
(Diagram)
Pressure
in Gases
The kinetic theory enables us to
account for the pressure a gas exerts on the walls of its container. When a
moving molecule strikes the walls of its container, a force is exerted on the
walls during hte impact.
Atmospheric
Pressure
The atmosphere, because of its weight
exerts a pressure on the surface of the earth and on every object on the earth
including human beings. The pressure is known as Atmospheric Pressure.
Applications of Atmospheric Pressure
The fact that the
atmosphere exerts pressure has been put into use in several devices such as
siphons, pumps and syringes.
BAROMETER
Definition
“A device for measuring the atmospheric pressure is called Barometer.”
“A device for measuring the atmospheric pressure is called Barometer.”
Mercury
Barometer
In the laboratory,
the atmospheric pressure is measured by means of a mercury barometer. A mercury
barometer consists of a thick walled glass tube of 1m length, which is opened
at one end and closed from the other side. The tube is filled with mercury. The
open end is firmly covered with a thumb and then carefully inverted in a vessel
containing mercury. When the open end is completely immersed in the mercury,
the thumb is removed. Some of the mercury from the columns drops in the vessel
leaving a space. This space is called vacuum. If the mercury columns is
measured, it is found to be 760 mm. This length always remains constant even if
different diameter tubes are taken. The length of the mercury column is
referred to as the atmospheric pressure.
ARCHIMEDE’S PRINCIPLE
Statement
“When an object is immersed in a liquid, an upward thrust acts upon it, which is equal to the weight of the liquid displaced by the object.”
“When an object is immersed in a liquid, an upward thrust acts upon it, which is equal to the weight of the liquid displaced by the object.”
Mathematical Expression
Mathematically,
Archimede’s Principle may be represented by:
Apparent Weight =
Actual Weight – Weight of the liquid displaced by the object
Buoyancy
It is the tendency of an object to
float. It is equal to the up-thrust or weight of the water displaced by the
object.
Conditions for Floating Bodies
A body will float
in a liquid or a gas if it displaces liquid or gas whose weight is greater than
the weight of the body.
A body will sink if
it displaces liquid or gas whose weight is less than the weight of the body.
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