Properties of Matter Class 11 Notes Physics

 Page 1


Physics Class XI
158
7.1 Interatomic Forces
The forces between the atoms due to electrostatic interaction between the 
charges of the atoms are called interatomic forces.
(1) When two atoms are brought close to each other to a distance of the order 
of 10
–10
 m, attractive interatomic force is produced between two atoms.
(2) This attractive force increases continuously with decrease in r and 
becomes maximum for one value of r called critical distance, represented 
by x (as shown in the figure).
(3) When the distance between the two atoms becomes r
0
, the interatomic 
force will be zero. This distance r
0
 is called normal or equilibrium 
distance.
(4) When the distance between the two atoms further decreased,  
the interatomic force becomes repulsive in nature and increases very 
rapidly.
Page 2


Physics Class XI
158
7.1 Interatomic Forces
The forces between the atoms due to electrostatic interaction between the 
charges of the atoms are called interatomic forces.
(1) When two atoms are brought close to each other to a distance of the order 
of 10
–10
 m, attractive interatomic force is produced between two atoms.
(2) This attractive force increases continuously with decrease in r and 
becomes maximum for one value of r called critical distance, represented 
by x (as shown in the figure).
(3) When the distance between the two atoms becomes r
0
, the interatomic 
force will be zero. This distance r
0
 is called normal or equilibrium 
distance.
(4) When the distance between the two atoms further decreased,  
the interatomic force becomes repulsive in nature and increases very 
rapidly.
(5) The potential energy U is related with the interatomic force F by the 
following relation.
F = 
When the distance between the two atoms becomes r
0
, the potential energy of 
the system of two atoms becomes minimum (i.e., attains maximum negative 
value hence the two atoms at separation r
0
 will be in a state of equilibrium.
7.2 Intermolecular Forces
The forces between the molecules due to electrostatic interaction between 
the charges of the molecules are called intermolecular forces. These forces 
are also called Vander Waal forces and are quite weak as compared to inter-
atomic forces.
7.3 Solids
A solid is that state of matter in which its constituent atoms or molecules 
are held strongly at the position of minimum potential energy and it has a 
definite shape and volume.
7.4 Elastic Property of Matter
(1) Elasticity : The property of matter by virtue of which a body tends to 
regain its original shape and size after the removal of deforming force 
is called elasticity.
(2) Plasticity : The property of matter by virtue of which it does not regain 
its original shape and size after the removal of deforming force is called 
plasticity.
(3) Perfectly elastic body : If on the removal of deforming forces the body 
regain its original configuration completely it is said to be perfectly 
elastic.
A quartz fibre and phosphor is the nearest approach to the perfectly elastic 
body.
(4) Perfectly plastic body : If the body does not have any tendency to 
recover its original configuration on the removal of deforming force, it 
is said to be perfectly plastic.
Paraffin wax, wet clay are the nearest approch to the perfectly plastic 
body. Practically there is no material which is either perfectly elastic or 
perfectly plastic.
(5) Reason of elasticity : On applying the deforming forces, restoring forces 
are developed.When the deforming force is removed, these restoring 
Page 3


Physics Class XI
158
7.1 Interatomic Forces
The forces between the atoms due to electrostatic interaction between the 
charges of the atoms are called interatomic forces.
(1) When two atoms are brought close to each other to a distance of the order 
of 10
–10
 m, attractive interatomic force is produced between two atoms.
(2) This attractive force increases continuously with decrease in r and 
becomes maximum for one value of r called critical distance, represented 
by x (as shown in the figure).
(3) When the distance between the two atoms becomes r
0
, the interatomic 
force will be zero. This distance r
0
 is called normal or equilibrium 
distance.
(4) When the distance between the two atoms further decreased,  
the interatomic force becomes repulsive in nature and increases very 
rapidly.
(5) The potential energy U is related with the interatomic force F by the 
following relation.
F = 
When the distance between the two atoms becomes r
0
, the potential energy of 
the system of two atoms becomes minimum (i.e., attains maximum negative 
value hence the two atoms at separation r
0
 will be in a state of equilibrium.
7.2 Intermolecular Forces
The forces between the molecules due to electrostatic interaction between 
the charges of the molecules are called intermolecular forces. These forces 
are also called Vander Waal forces and are quite weak as compared to inter-
atomic forces.
7.3 Solids
A solid is that state of matter in which its constituent atoms or molecules 
are held strongly at the position of minimum potential energy and it has a 
definite shape and volume.
7.4 Elastic Property of Matter
(1) Elasticity : The property of matter by virtue of which a body tends to 
regain its original shape and size after the removal of deforming force 
is called elasticity.
(2) Plasticity : The property of matter by virtue of which it does not regain 
its original shape and size after the removal of deforming force is called 
plasticity.
(3) Perfectly elastic body : If on the removal of deforming forces the body 
regain its original configuration completely it is said to be perfectly 
elastic.
A quartz fibre and phosphor is the nearest approach to the perfectly elastic 
body.
(4) Perfectly plastic body : If the body does not have any tendency to 
recover its original configuration on the removal of deforming force, it 
is said to be perfectly plastic.
Paraffin wax, wet clay are the nearest approch to the perfectly plastic 
body. Practically there is no material which is either perfectly elastic or 
perfectly plastic.
(5) Reason of elasticity : On applying the deforming forces, restoring forces 
are developed.When the deforming force is removed, these restoring 
forces bring the molecules of the solid to their respective equilibrium 
position (r = r
0
) and hence the body regains its original form.
(6) Elastic limit : The maximum deforming force upto which a body retains 
its property of elasticity is called elastic limit of the material of body.
Elastic limit is the property of a body whereas elasticity is the property 
of material of the body.
(7) Elastic fatigue : The temporary loss of elastic properties because of the 
action of repeated alternating deforming force is called elastic fatigue.
It is due to this reason :
(i) Bridges are declared unsafe after a long time of their use.
(ii) Spring balances show wrong readings after they have been used for 
a long time.
(iii) We are able to break the wire by repeated bending.
(8) Ealstic after effect : The time delay in which the substance regains its 
original condition after the removal of deforming force is called elastic 
after effect. It is negligible for perfectly elastic substance, like quartz, 
phosphor bronze and large for glass fibre.
7.5 Stress
The internal restoring force acting per unit area of cross section of the 
deformed body is called stress.
Stress = 
Unit : N/m
2
 (S.I.), dyne/cm
2
 (C.G.S.)
Stress developed in a body depends upon how the external forces are applied  
over it.
On this basis there are two types of stresses : Normal and Shear or tangential 
stress
(1) Normal stress : Here the force is applied normal to the surface.
 It is again of two types : Longitudinal and Bulk or volume stress.
(i) Longitudinal stress
(a) Deforming force is applied parallel to the length and causes increase 
in length.
Page 4


Physics Class XI
158
7.1 Interatomic Forces
The forces between the atoms due to electrostatic interaction between the 
charges of the atoms are called interatomic forces.
(1) When two atoms are brought close to each other to a distance of the order 
of 10
–10
 m, attractive interatomic force is produced between two atoms.
(2) This attractive force increases continuously with decrease in r and 
becomes maximum for one value of r called critical distance, represented 
by x (as shown in the figure).
(3) When the distance between the two atoms becomes r
0
, the interatomic 
force will be zero. This distance r
0
 is called normal or equilibrium 
distance.
(4) When the distance between the two atoms further decreased,  
the interatomic force becomes repulsive in nature and increases very 
rapidly.
(5) The potential energy U is related with the interatomic force F by the 
following relation.
F = 
When the distance between the two atoms becomes r
0
, the potential energy of 
the system of two atoms becomes minimum (i.e., attains maximum negative 
value hence the two atoms at separation r
0
 will be in a state of equilibrium.
7.2 Intermolecular Forces
The forces between the molecules due to electrostatic interaction between 
the charges of the molecules are called intermolecular forces. These forces 
are also called Vander Waal forces and are quite weak as compared to inter-
atomic forces.
7.3 Solids
A solid is that state of matter in which its constituent atoms or molecules 
are held strongly at the position of minimum potential energy and it has a 
definite shape and volume.
7.4 Elastic Property of Matter
(1) Elasticity : The property of matter by virtue of which a body tends to 
regain its original shape and size after the removal of deforming force 
is called elasticity.
(2) Plasticity : The property of matter by virtue of which it does not regain 
its original shape and size after the removal of deforming force is called 
plasticity.
(3) Perfectly elastic body : If on the removal of deforming forces the body 
regain its original configuration completely it is said to be perfectly 
elastic.
A quartz fibre and phosphor is the nearest approach to the perfectly elastic 
body.
(4) Perfectly plastic body : If the body does not have any tendency to 
recover its original configuration on the removal of deforming force, it 
is said to be perfectly plastic.
Paraffin wax, wet clay are the nearest approch to the perfectly plastic 
body. Practically there is no material which is either perfectly elastic or 
perfectly plastic.
(5) Reason of elasticity : On applying the deforming forces, restoring forces 
are developed.When the deforming force is removed, these restoring 
forces bring the molecules of the solid to their respective equilibrium 
position (r = r
0
) and hence the body regains its original form.
(6) Elastic limit : The maximum deforming force upto which a body retains 
its property of elasticity is called elastic limit of the material of body.
Elastic limit is the property of a body whereas elasticity is the property 
of material of the body.
(7) Elastic fatigue : The temporary loss of elastic properties because of the 
action of repeated alternating deforming force is called elastic fatigue.
It is due to this reason :
(i) Bridges are declared unsafe after a long time of their use.
(ii) Spring balances show wrong readings after they have been used for 
a long time.
(iii) We are able to break the wire by repeated bending.
(8) Ealstic after effect : The time delay in which the substance regains its 
original condition after the removal of deforming force is called elastic 
after effect. It is negligible for perfectly elastic substance, like quartz, 
phosphor bronze and large for glass fibre.
7.5 Stress
The internal restoring force acting per unit area of cross section of the 
deformed body is called stress.
Stress = 
Unit : N/m
2
 (S.I.), dyne/cm
2
 (C.G.S.)
Stress developed in a body depends upon how the external forces are applied  
over it.
On this basis there are two types of stresses : Normal and Shear or tangential 
stress
(1) Normal stress : Here the force is applied normal to the surface.
 It is again of two types : Longitudinal and Bulk or volume stress.
(i) Longitudinal stress
(a) Deforming force is applied parallel to the length and causes increase 
in length.
(b) Area taken for calculation of stress is area of cross section.
(c) Longitudinal stress produced due to increase in length of a body 
under a deforming force is called tensile stress.
(d) Longitudinal stress produced due to decrease in length of a body 
under a deforming force is called compressional stress.
(ii) Bulk or V olume stress
(a) It occurs in solids, liquids or gases.
(b) Deforming force is applied normal to surface at all points.
(c) It is equal to change in pressure because change in pressure is 
responsible for change in volume.
(2) Shear or tangential stress : It comes in picture when successive layers 
of solid move on each other i.e., when there is a relative displacement 
between various layers of solid.
(i) Here deforming force is applied tangential to one of the faces.
(ii) Area for calculation is the area of the face on which force is applied.
(iii) It produces change in shape, volume remaining the same.
7.6 Strain
The ratio of change in configuration to the original configuration is called 
strain. It has no dimensions and units. Strain are of three types :
(1) Linear strain : Linear strain = 
Linear strain in the direction of deforming force is called longitudinal 
strain and in a direction perpendicular to force is called lateral strain.
(2) Volumetric strain : V olumetric strain = 
(3) Shearing strain : It is defined as angle in radians through which a plane 
perpendicular to the fixed surface of the cubical body gets turned under 
the effect of tangential force.
Page 5


Physics Class XI
158
7.1 Interatomic Forces
The forces between the atoms due to electrostatic interaction between the 
charges of the atoms are called interatomic forces.
(1) When two atoms are brought close to each other to a distance of the order 
of 10
–10
 m, attractive interatomic force is produced between two atoms.
(2) This attractive force increases continuously with decrease in r and 
becomes maximum for one value of r called critical distance, represented 
by x (as shown in the figure).
(3) When the distance between the two atoms becomes r
0
, the interatomic 
force will be zero. This distance r
0
 is called normal or equilibrium 
distance.
(4) When the distance between the two atoms further decreased,  
the interatomic force becomes repulsive in nature and increases very 
rapidly.
(5) The potential energy U is related with the interatomic force F by the 
following relation.
F = 
When the distance between the two atoms becomes r
0
, the potential energy of 
the system of two atoms becomes minimum (i.e., attains maximum negative 
value hence the two atoms at separation r
0
 will be in a state of equilibrium.
7.2 Intermolecular Forces
The forces between the molecules due to electrostatic interaction between 
the charges of the molecules are called intermolecular forces. These forces 
are also called Vander Waal forces and are quite weak as compared to inter-
atomic forces.
7.3 Solids
A solid is that state of matter in which its constituent atoms or molecules 
are held strongly at the position of minimum potential energy and it has a 
definite shape and volume.
7.4 Elastic Property of Matter
(1) Elasticity : The property of matter by virtue of which a body tends to 
regain its original shape and size after the removal of deforming force 
is called elasticity.
(2) Plasticity : The property of matter by virtue of which it does not regain 
its original shape and size after the removal of deforming force is called 
plasticity.
(3) Perfectly elastic body : If on the removal of deforming forces the body 
regain its original configuration completely it is said to be perfectly 
elastic.
A quartz fibre and phosphor is the nearest approach to the perfectly elastic 
body.
(4) Perfectly plastic body : If the body does not have any tendency to 
recover its original configuration on the removal of deforming force, it 
is said to be perfectly plastic.
Paraffin wax, wet clay are the nearest approch to the perfectly plastic 
body. Practically there is no material which is either perfectly elastic or 
perfectly plastic.
(5) Reason of elasticity : On applying the deforming forces, restoring forces 
are developed.When the deforming force is removed, these restoring 
forces bring the molecules of the solid to their respective equilibrium 
position (r = r
0
) and hence the body regains its original form.
(6) Elastic limit : The maximum deforming force upto which a body retains 
its property of elasticity is called elastic limit of the material of body.
Elastic limit is the property of a body whereas elasticity is the property 
of material of the body.
(7) Elastic fatigue : The temporary loss of elastic properties because of the 
action of repeated alternating deforming force is called elastic fatigue.
It is due to this reason :
(i) Bridges are declared unsafe after a long time of their use.
(ii) Spring balances show wrong readings after they have been used for 
a long time.
(iii) We are able to break the wire by repeated bending.
(8) Ealstic after effect : The time delay in which the substance regains its 
original condition after the removal of deforming force is called elastic 
after effect. It is negligible for perfectly elastic substance, like quartz, 
phosphor bronze and large for glass fibre.
7.5 Stress
The internal restoring force acting per unit area of cross section of the 
deformed body is called stress.
Stress = 
Unit : N/m
2
 (S.I.), dyne/cm
2
 (C.G.S.)
Stress developed in a body depends upon how the external forces are applied  
over it.
On this basis there are two types of stresses : Normal and Shear or tangential 
stress
(1) Normal stress : Here the force is applied normal to the surface.
 It is again of two types : Longitudinal and Bulk or volume stress.
(i) Longitudinal stress
(a) Deforming force is applied parallel to the length and causes increase 
in length.
(b) Area taken for calculation of stress is area of cross section.
(c) Longitudinal stress produced due to increase in length of a body 
under a deforming force is called tensile stress.
(d) Longitudinal stress produced due to decrease in length of a body 
under a deforming force is called compressional stress.
(ii) Bulk or V olume stress
(a) It occurs in solids, liquids or gases.
(b) Deforming force is applied normal to surface at all points.
(c) It is equal to change in pressure because change in pressure is 
responsible for change in volume.
(2) Shear or tangential stress : It comes in picture when successive layers 
of solid move on each other i.e., when there is a relative displacement 
between various layers of solid.
(i) Here deforming force is applied tangential to one of the faces.
(ii) Area for calculation is the area of the face on which force is applied.
(iii) It produces change in shape, volume remaining the same.
7.6 Strain
The ratio of change in configuration to the original configuration is called 
strain. It has no dimensions and units. Strain are of three types :
(1) Linear strain : Linear strain = 
Linear strain in the direction of deforming force is called longitudinal 
strain and in a direction perpendicular to force is called lateral strain.
(2) Volumetric strain : V olumetric strain = 
(3) Shearing strain : It is defined as angle in radians through which a plane 
perpendicular to the fixed surface of the cubical body gets turned under 
the effect of tangential force.
162
f = 
• When a beam is bent both compression strain as well as an extension
strain is produced.
7.7 Stress-strain Curve
(1) When the strain is small (region OP) stress is proportional to strain. This 
is the region where the so called Hooke’s law is obeyed. The point P is 
called limit of proportionality and slope of line OP gives the Young’s 
modulus Y of the material of the wire. Y = tan ?.
(2) Point E known as elastic limit or yield-point.
(3) Between EA, the strain increases much more.
(4) The region EABC represents the plastic behaviour of the material of  
 wire.
(5) Stress-strain curve for dif ferent materials, are shown in following figure.
Brittle material Ductile material Elastomers
The plastic region between 
E and C is small for brittle 
material and it will break 
soon after the elastic limit 
is crossed.
The material on this type 
have a good plastic range 
and such materials can 
be easily changed into 
different shapes and can 
be drawn into thin wires.
For ealstomers the strain 
produced is much larger 
than the stress applied. 
Such materials have no 
plastic range and the 
breaking point lies very 
close to elastic limit. 
Example : rubber.