Explain the Laws of Thermodynamics ?
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Answer / km jyoti saini
Ist Law:-Whenever work is converted into heat or vice versa the ratio of two is constant. By this law whenever heat is converted into mechanical work or any other from of energy and vice versa the quantity of energy which disappears in one form is always equal to energy which appears in other form. It is expressed as
D8=dv+dw
Where d8 is the quantity of heat supplied or taken away dv is increase or decrease in internal energy and dw is external work dine by the system
If no external work is done, then the heat given up is spent in increasing the internal energy of the system
2nd Law:- It sates that heat can be transferred from one body to another at higher temperature unless some work is dine, It can also be expressed as follows. Heat tends to flow from higher temperature to lower temperature
Heat Engines:-It is a device by means of which heat energy is converted into mechanical energy e.g. steam engines, gas engines and oil engines are used for converting heat energy into mechanical energy
Steam Engines:-It is heat engine in which the working substance is steam. Coal is burn to produce steam and this steam after passing through engine becomes colder. The cooling is due to expansion of steam in the cylinder as well as due to radiation conduction heating of cylinder etc. by steam. This expansion property of steam is used for forcing the piston to move backwards and forwards which in turn does work. The engine consists essentially of following parts
1) Cylinder with steam parts
2) Steam chest
3) Piston with rings connected to crank by piston rod, cross heat connecting rod and crank pin
4) Slide vnlve which is connected to eccentric disc through value spindle and eccentric rod
5) Crank shaft which has fly wheel and eccentric strap
6) Bearings on which crank shaft rests
In this type of engine, steam enters the cylinder through steam parts which is uncovered by the side value. This steam pushed the piston down due to its expansion. As it expands it becomes cooler, till it becomes dead. By this time piston reaches the bottom of the cylinder. When the slide valve operis another part and steam starts entering from below and pushes the piston up. This upward motion of the piston drives out the dead time through the exhaust part. Same process occurs when piston completes its upward stroke and this it moves alternately upwards and downwards
In This type of engine steam acts on both the sides of the piston and is therefore called a reciprocating or a double acting steam engine
Power of a steam engine is calculated as
P x a x l n
H.P of Engine=--------------
33,000
Where p is average pressure on the piston
a is the area of piston
l is the distance covered in one forward or backward stroke
n is the number of strokes per minute
Efficiency of a steam engine is
Useful work done
= ---------------------------
Total energy put in
Internal Combustion Engine:-In this type of engine fuel is burnt the cylinder of the engine itself and the products of the combustion are themselves the working substance
This type of engine occupies less space as no boiler is required. It can be started at a mement’s notice and has no loss of heat
In the four stroke engine which is commonly used for motor cars, the cylinder is having two values called admission and exhaust valves. Each valve is held in closing position by spring and are operation by shaft driven by the engine itself
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