Basic Thermo Concepts Flashcards
(45 cards)
System
A system is a finite quantity of matter or a prescribed region of space enclosed by a membrane or surface to separate it from the surrounding or environment.
Boundary
The actual or hypothetical envelope enclosing the system is the boundary of the system.
Closed system
If the boundary of the system is impervious to the flow of matter, it is called a closed system.
Open system
An open system is one in which matter flows into or out of the system. Most of the engineering systems are open.
Isolated system
An isolated system is that system which exchanges neither energy nor matter with any other system or with environment.
Phase
A phase is a quantity of matter which is homogeneous throughout in chemical composition and physical structure.
Homogeneous System with examples
A system which consists of a single phase is termed as homogeneous system. Examples : Mixture of air and water vapour, water plus nitric acid and octane plus heptane.
Heterogeneous System with examples
A system which consists of two or more phases is called a heterogeneous system. Examples : Water plus steam, ice plus water and water plus oil.
Differences between macroscopic and microscopic approach
- macroscopic approach to thermodynamics is concerned with gross or overall behaviour of matter. it also known as classical thermodynamics.
- Microscopic approach to thermodynamics is concerned directly with the structure of the matter–known as statistical thermodynamics.
2. The analysis and design of macroscopic system requires simple mathematical formulae. - As the number of molecules is very large. Advanced statistical and mathematical methods are needed to explain the changes in the system.
3. The values of the properties of the system are their average values. These properties like pressure and temperature can be measured very easily. The changes in properties can be felt by our senses. - The properties like velocity, momentum, impulse, kinetic energy, force of impact etc. which describe the molecule cannot be easily measured by instruments. Our senses cannot feel them.
4. In order to describe a system only a few properties are needed. - Large number of variables are needed to describe a system
Pure substance
A pure substance is one that has a homogeneous and invariable chemical composition even though there is a change of phase.
Define thermodynamic Equilibriumo
A system is in thermodynamic equilibrium if the temperature and pressure at all points are same ; there should be no velocity gradient ; the chemical equilibrium is also necessary.
PROPERTIES OF SYSTEMS, types
A property of a system is a characteristic of the system which depends upon its state, but
not upon how the state is reached. There are two sorts of property :
1. Intensive properties. These properties do not depend on the mass of the system.
Examples : Temperature and pressure.
2. Extensive properties. These properties depend on the mass of the system. Example :
Volume. Extensive properties are often divided by mass associated with them to obtain the intensive
properties.
STATE
State is the condition of the system at an instant of time as described or measured by its
properties. Or each unique condition of a system is called a state.
Quasi-static process
Quasi means ‘almost’. This process is a succession of equilibrium states and infinite slowness is its
characteristic feature.
A Quasi-static process is a reversible process
CYCLE
with “cycle of operation” fig
Any process or series of processes whose end states are identical is termed a cycle. fig: pg 45
POINT FUNCTION
When two properties locate a point on the graph (co-ordinate axes) then those properties
are called as point function.
Examples. Pressure, temperature, volume etc.
Define path function
These are quantities which cannot be located on a graph by a point but are given by the area or so, on that graph. Examples. Heat, work etc.
temperature
The temperature is a thermal state of a body which distinguishes a hot body from a
cold body. The temperature of a body is proportional to the stored molecular energy
i.e., the average molecular kinetic energy of the molecules in a system.
It has been found that a gas will not occupy any volume at the absolute zero temperature (273.15).
Define superheated vapour
It is a vapour at a temperature higher than its vapourization (boiling point) at the absolute pressure where the temperature is measured.
Define critical point
This is the limit of distinction between a liquid and a vapour.
Define perpetual motion machine of the first kind
It is an imaginary device that produces energy without the input of energy. It violates the 1st law of thermodynamics (law of conservation of energy).
Define a weak mixture
It is a fuel-air mixture in which the fuel is proportionally less than a chemically correct mixture i.e. (1:16)
Define Refrigerator capacity
It is a measure of the effective cooling capacity of a refrigerator, expressed in Btu per hour, or in tons where one ton of capacity is equal to the heat required to melt 2,000 pounds of ice in 24 hours or 12,000 Btu per hour
Define throttling process
It is a process in which a high pressure fluid is converted to low pressure by passing it through a narrow constriction.
The process is adiabatic and irreversible
