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# Thermodynamics - Introduction

Definition:
The study of changes in energy associated with physical and chemical reaction is called as thermodynamics. In general, it is the study of effect of work, heat and energy on a system.
The study of conservation of heat energy and other forms of energy, its conditions and necessary devices used for conservation of energy, is called physical thermodynamics. It contains thermodynamic system, components of system and environment.

Thermodynamic system is a part of the universe and may be of one, two or three dimensional. It contains many objects like radiation. These objects are the components of thermodynamic system.

Environment is a remaining part of the universe surrounding the thermodynamic system and having a direct comportment on the actions of the system.
Environment and thermodynamic system is separated by a boundary, which is called as a wall of the system. The wall of the system is responsible for the interaction taking place between system and environment.
Interaction between a system and its environment is called a thermodynamic process.

Isolated system is a system which does not interact with it surroundings and remains constant.

Thermal equilibrium:
Thermal equilibrium is a state where exchange of energy is zero or remains constant between system and environment or between two systems.

Thermal expansion:
Thermal expansion is defined as the change in dimensions of a substance due to exchange of heat. The dimension of substance increases with increase in temperature and decrease with decrease in temperature.

Linear expansion:
It is defined as the increase in the length of a body with increase in temperature. The increase in length Δl is directly proportional to original length (l) and increase in temperature ΔT.
Δl ∞ l  Δl ∞ΔT
Δl ∞ l ΔT
Δl = α l ΔT
Here, α is a constant called co-efficient of linear expansion.
Unit of α is ˚C-1 or K-1
The value of α depends on the material of the rod and its temperature.
Similarly for the isotropic substance, which shows uniform behavior in all directions, means changes in length, breath and thickness remains same.
ΔA = 2α A ΔT
ΔV = 3α V ΔT

Calculations:
Ex-1: A copper sheet is bored with a hole. Diameter of hole is 4.5 cm at 27˚C. What will be the change in diameter of the hole if the sheet is heated up to 227 ˚C. (Value of α for copper is 1.7 x 10-5 ˚C-1)

Reason:
α = 1.7 x 10-5 ˚C-1

T1 = 27˚C
T2 = 227 ˚C
l = 4.5 cm

Δl = α l ΔT
= 1.7 x 10-5 x 4.5 x (227 -27)
= 1.7 x 10-5 x 4.5 x 200
= 1.53 x 10-2 cm

Answer: The diameter of the hole increases by 1.53 x 10-2 cm
Ex-2: What is the unit of co-efficient of linear expansion?
a)  ˚C
b)  K
c)  ˚C-1 or K-1
d)  None of above
Answer: The unit of co-efficient of linear expansion is ˚C-1 or K-1
Ex-3: At thermal equilibrium, exchange of energy is ……………
a)  Infinite
b)  Lower
c)  Zero
d)  Higher
Answer: At thermal equilibrium, exchange of energy is zero.