Thermodynamics - Physics Notes | Study General Science(Prelims) by IRS Divey Sethi - UPSC

Definition

Thermodynamics is the study of energy

Laws of Thermodynamics

First Law of Thermodynamics

• Energy can be changed from one form to another, but it cannot be created or destroyed.

First Law of Thermodynamics 

The Second Law of Thermodynamics 

• It states that "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state."This is also commonly referred to as entropy.
• One of the important fields of thermodynamics is heat transfer, which relates to transfer of heat between two media.
• There are three modes of heat transfer:
  • conduction,
  • convection and
  • radiation.
• The concept of heat transfer is used in wide range of devices like heat exchangers, evaporators, condensers, radiators, coolers, heaters, etc.

Second Law of Thermodynamics 

1. Conduction Heat Transfer :

• In this process heat transferred without bodily movement of the particles of medium.

2. Convection Heat Transfer :

• In this process heat is transferred by the bodily movement of particles of the medium due to difference in densities of different parts of the medium.
• All liquids and gases are heated by convection.

3. Radiation Heat Transfer :

• Quickest way of transmission of heat is known as radiation.
• In this mode of heat transmission heat is transferred from one place to another without effecting the intervening medium.
• Heat comes to us from the sun by radiation.

Perfectly Black Body

• Perfectly black body is that which neither reflects nor transmits the radiations falling on it, inspite of it the black body absorbs all radiations falling on it and hence it appears black.
• It is clear that, when heated, the black body will radiate all the energy absorbed by it.

Memorable Points

• All metals are good conductors of heat. Silver is the best conductor.
• Good conductors of heat are good conductors of electricity.
• Mica is an exception which although being a good conductor of heat and a bad conductor of electricity.
• Cooking utensils are provided with wooden or ebonite handles, since wood or ebonite is a bad conductor of heat.
• In winter, wooden chair appears hotter than the metal one because metal is a good conductor of heat while wood is a bad conductor of heat.
• In a room, ventilators are provided at ceiling, to escape the hot air by convection.
• Principle of chimney used in the kitchen or a factory is based on the convection.
• Land and sea breezes are due to the convection.
• Temperature of the upper of the flame is more then the temperature on the sides, because the currents of air carry the heat upwards.
• Highly polished surfaces are bad absorbers and bad emitters but they are good reflectors.
• If a thermous flask containing tea is shaken vigorously, temperature of the tea rises because of increase in internal energy.
• If the door of refrigerator is kept open, it will not cool the room, it will increases the temperature of the room because heat rejected by the refrigerator to the room will be more than the heat taken by the refrigerator from the room.
• If heat rejection portion of the refrigerator is outside the closed room, the opening of refrigerator door will cool the room gradually.

Latent Heat

• It is defined as the amount of heat absorbed or given out by a body during the change of state. It is of two types :

(1) Latent Heat of Fusion

• The latent heat of fusion of a substance is quantity of heat required to change unit mass of solid substance from solid state to liquid state.

(2) Latent Heat of Vaporization

• It is the quantity of heat required to change unit mass of liquid substance from liquid to vapour state while temperature remaining constant.
• The latent heat of vaporization of water is 536 cals
• Latent heat of vaporization is maximum at 0°C and decreases on increasing temperature.
• Latent heat of vaporization of water decreases on increasing pressure or we may say that the latent heat of vaporization decreases with increase in boiling point.
• Ice of 0°C feels colder than water at 0°C.
• Hot water (100°C) burns are less serve than steam burns (100°C), because steam has high latent heat.
• The melting point of solids which contract on melting decreases with increase in pressure e.g., ice and cast iron. These solids float on their corresponding liquid.
• The melting point of solids which expand on melting, increase with increase in pressure e.g., wax, glass, gold etc.,
• Boiling point increases on increasing pressure.
• Cooling a liquid below freezing point without turning it to a solid is called super-cooling. Water can be super – cooled to temperature as low as -12°C.
• Heating a liquid above its boiling point without converting it vapour state is called super – heating. Water can be heated up to 137°C without boiling.

Temperature

• Temperature is the property of a body which determines whether it is in thermal equilibrium with the neighbouring with the neighbouring body or not.

Mercury thermometer

• they generally contain two types of scale: Celsius scale (from 0^oC to 100^oC) and Fahrenheit scale(from 32^oF 212^oF).
• They are related as-

C   = F-32

100   180

• In mercury thermometer, mercury is placed in a small spherical bulb attached to a long, thin glass capillary tube sealed at the top mercury is used because it undergoes uniform thermal expansion

Kelvin or absolute scale

• on this scale the freezing point of water is 273.15k (or 273k) and boiling point of water is 372K.
  • K = 273 + ^oC
  • ^OC = K – 273
  • If C = - 273^o then K = 0.
  • This temperature is called absolute zero.

Thermal expansion

• The expansion of substances on heating is called thermal expansion.
• This expansion may be in length, area and volume. They are respectively called liner expansion, superficial expansion and volume expansion.
• Greensand all the liquids expand In volume when they are heated but water decreases in volume when heated from 0^oC  to 4^oC and then after 4^oC, it increases in volume on further heating.
• The volume of a fixed mass of water is minimum at 4^oC, i.e., its density is maximum at ^oC (1.000 X 10³ kg/ m³) with further rise in temperature its density decreases.

Anomalous expansion of water

• Greensand all the liquids expand In volume when they are heated but water decreases in volume when heated from 0^oC  to 4^oC and then after 4^oC, it increases in volume on further heating.
• The volume of a fixed mass of water is minimum at 4^oC, i.e., its density is maximum at ^oC (1.000 X 10³ kg/ m³) with further rise in temperature its density decreases.