Friday, June 24, 2011

Class X, PHYSICS, "Matter"

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.


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:


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.



" 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.



"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."


Stress = Force / Area

o = F/A (Here o represents (Rho) do not write in your examination paper)


* 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)

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.



Stress can produce a change in shape, volume or length in an object. This change in the shape of an object is called strain.



Strain = Change in Length/Length or Strain = Change in volume / volume


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


An English Physicist and Chemist Robert Hook discovered this law in 1678.


"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.

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.

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


In S.I system, Young's Modulus is measured in N/m2.



"The perpendicular force per unit area acting on a surface is called pressure."

Mathematical Expression

Pressure = Force /Area

P = F/A


* 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


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.

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.



"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


"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


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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