Thermodynamics (GA Physics) Notes for SSC CGL, CHSL, IB & Railways

Thermodynamics

The branch of physical science that deals with the relations between heat and other forms of energy (such as mechanical, electrical, or chemical energy), and, by extension, of the relationships between all forms of energy.

First Law of Thermodynamics

The total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.

• Its a version of the law of conservation of energy, adapted for thermodynamic systems.
```   ΔQ = ΔU + ΔW

ΔQ - Heat energy given to the system
ΔU - Increase in the internal energy of the system
ΔW - Work done by the system
```

Second Law of Thermodynamics

The total entropy of an ideal system remain constant where the system is in a steady state (equilibrium), or is undergoing a reversible process.

Zeroth Law of Thermodynamics

If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

Isothermal Process

If changes are taking place in a system in such a way that temperature of the system remains constant throughout the change.

If the changes are taking place in a system in such a way that there is no exchange of heat energy between the system and surrounding.

• e.g., If CO2 gas is suddenly expanded, it is changed into dry ice.

Entropy

The degree of disorder or uncertainty in a system.

Kelvin’s statement

Whole of the heat can never be converted into work.

Clausius Statement

Heat by itself can not flow from a cooler body to a hotter body.

• Refrigerator is based on Clausius Statement.

Heat

In thermodynamics, heat is energy transferred from one system to another as a result of thermal interactions.

• SI Unit: Joule (J)
• 1 cal = 4.2 J

Heat Engine (Internal Combustion Heat Engine, External Combustion Heat Engine)

A device which is used to convert heat into mechanical work.

Specific Heat (s)

The amount of heat required to raise the temperature of unit mass (m) of a substance through 1° C, is called its specific heat.

• SI Unit: J kg-1 K°C
• Specific Heat of water: 4200 J kg-1 K°C
• Since specific Heat of water is very high as compared to most substances, therefore, water is used as coolant in radiator in vehicle and hot water is used for the fermentation.

Molar Heat

The amount of heat required to raise the temperature of 1 mole of a gas by 1°C is called molar specific heat.

Latent Heat (L)

The heat energy absorbed or released at constant temperature per unit mass for change of state, is called latent heat.

• SI Unit: J kg-1

Heat Transfer

Heat is transferred by the actual movement of particles of medium. Heat is transferred without actual movement of particles. Heat is transferred directly without any need of medium.
Conduction takes place in solids. Convection takes place in Liquids and Gases. Heat from Sun to Earth’s atmosphere is an example of Radiation.

Temperature

Temperature is the measure of hotness or coldness of a body.

• SI Unit: Kelvin (K)
• Freezing point of water: 0° C, Boiling Point of water: 100° C
• 0° C = 273.15° K = 32° F

Thermometers (Clinical, Electronic, Others) (Invented by Galileo Galilei)

The instrument which is used to measure temperature of a body is called as thermometers.

• Clinical Thermometers (Range: 96° F to 110° F) are used to measure human body temperatures
• Electronic Thermometers (Range: -40° F to 450° F) have basic components such as Thermisters or Thermoresistors.
• Other Thermometers include constant volume gas thermometer, platinum resistance thermometer, etc.
• Mercury is used in thermometers as it is liquid at room temperature and it has high coefficient of expansion.

Thermal Expansion

The tendency of matter to change in shape, area and volume in response to the change in temperature.

Practical Applications of Thermal Expansion

• Some gap is given in Telephone wires to allow the wires contract in winter
• In summer the pendulum clock runs slower, because the length of pendulum increases.
• Iron Rails of the Railway Tracks are provided with some gap, so that the rails can easily expand in Summer and thus will not bend.

White colour is a bad absorbers and good reflectors of heat radiations while black colour is good absorbers and bad reflectors of heat. Therefore, clothes of light colours are more suitable in summer and clothes of dark colours are more suitable in winter.

Black Body

A body that absorbs all the radiation incident on it is called perfectly black body.

• Absorptive Power (a): Ratio of heat absorbed (radiation) to total incident radiation for a body. It has no unit.

Humidity

The amount of water vapour (moisture) in the atmosphere, is called as humidity.

• Absolute Humidity: The amount of water vapour present in the unit volume of atmosphere.
• Relative Humidity is measured by hygrometer.