Page 20 - Hsslive-Plus-One-Physics-Class-Notes-Part-III (1)

Heat, Thermodynamics & Kinetic Theory {19} Vinodkumar M, St. Aloysius H.S.S, Elthuruth, Thrissur.
tional to longitudinal strain.
i.e, lateral strain  longitudinal strain

Lateral Strain
Or   ( Poission s ' Ratio ) .
Longitudin al Strain
Therefore, the ratio of lateral strain to the longitudinal strain is called Poission’s ratio.
 d
If d is the change in diamter and d is the original diameter, then lateral strain =
d

d
    d  d  
Longitudinal strain = . Therefore,   d    . Note: Poission’s ratio has no unit or dimension.


Elastic potential energy in a stretched wire
To stretch a wire, work has to be done against interatomic forces. This workdone will be stored in the
wire as elastic potential energy.
Consider a wire of length L and area of crosssection A, subjected to a deforming force F so that it elongates
F L Y A 
by  . Then Young’s modulus, Y  .  Force , F  .
A  L
Y A  d
Now let the wire be further elongated by a small length, d . Then workdone, W = F x d =
L
Therefore, the total workdone in increasing the length from 0 to ,

2
  2
Y A  Y A Y A  1    1
W   dw   d    d   i.e, W  Y   A  = x Young’s modulus x
0 L L 0 2 L 2  L  2
Strain x volume of
2
wire.
1 1
i.e, W = stress/strain x strain x volume of wire. = x stress x strain x volume of wire.
2
2 2
This workdone is stored as elastic potential energy.

1
Thus Potential energy per unit volume, U = stress x strain.
2
1
W    . Where  is compressive stress and  longitudinal strain.
2

v e d R 
Critical velocity. We know Reynold’s number, R e  . Hence, v  . This velocity is called
e
  d
critical velocity. Thus critical velocity is the maximum velocity of a fluid in a tube so that the flow
remains streamlined.
Blackbody radiation
A body which absorbs all the radiations falling on it is called a black body.
The energy of a blackbody radiation varies with wavelength . If  is the wavelength for which energy is
m
maximum,. then this wavelength decreases with increase in temp T.
-3
i.e, T = constant. This is called Wien’s displacement law. The value of constant = 2.9 x 10 mK.
m
This law is used to find the surface temperature of celestial bodies like moon, sun and stars.
If a blackbody is a prefect readiator, then energy emitted per unit time H = A T .; where A = area, and
4
T = absolute temperature of the body. This relation is called Stefan - Boltzmann law.  is called Stefan -
-8
Boltzman constant = 5.6 x 10 W/m /K .
4
2
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