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Kinetic Theory of Gases and Radiation

Molecular nature of matter

  • Thermodynamics deals with heat and temperature, and the inter-conversion of heat and other energy forms.

  • Thermodynamics is a macroscopic science. It deals with bulk system, and does not go into the molecular constitution of matter.

  • Thermal equilibrium: State of a system is an equilibrium state if the macroscopic variables that characterise the system do not change. Two systems at the same temperature are said to be in thermal equilibrium with each other.

  • Adiabatic wall: It is an insulating wall that does not allow flow of heat

  • Diathermic wall: It is a conducting wall that allows flow of heat. In this case, thermal equilibrium can be attained.

Zeroth Law of Thermodynamics

  • When two systems are separately in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.

  • The physical quantity which is required to be in thermal equilibrium is temperature

  • Consider three systems, X, Y and Z.

X and Y are separately in thermal equilibrium with Z.

Where,

TX = Temperature of X

TY = Temperature of Y

TZ = Temperature of Z

Hence,

The systems X and Y are also in thermal equilibrium with each other.

  • Thermodynamics deals with heat and temperature, and the inter-conversion of heat and other energy forms.

  • Thermodynamics is a macroscopic science. It deals with bulk system, and does not go into the molecular constitution of matter.

  • Thermal equilibrium: State of a system is an equilibrium state if the macroscopic variables that characterise the system do not change. Two systems at the same temperature are said to be in thermal equilibrium with each other.

  • Adiabatic wall: It is an insulating wall that does not allow flow of heat

  • Diathermic wall: It is a conducting wall that allows flow of heat. In this case, thermal equilibrium can be attained.

Zeroth Law of Thermodynamics

  • When two systems are separately in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.

  • The physical quantity which is required to be in thermal equilibrium is temperature

  • Consider three systems, X, Y and Z.

X and Y are separately in thermal equilibrium with Z.

Where,

TX = Temperature of X

TY = Temperature of Y

TZ = Temperature of Z

Hence,

The systems X and Y are also in thermal equilibrium with each other.

  • Atomic hypothesis: All things are made of atoms − little particles that move around in perpetual motion, attracting each other when they are at a little distance apart and repelling when squeezed into one another.

  • Atomic theory: Proposed by John Dalton to explain the laws of definite and multiple proportions.

  • First law states that any given compound has fixed proportion by mass of its constituents.

  • Second law states that when two elements form more than one compound, for a fixed mass of one element, the masses of the other elements are in the ratio of small integers.

  • Gay-Lussac’s law: When gases combine chemically to yield another gas, their volumes are in the ratio of small integers.

  • Avogadro’s law: Equal volumes of all gases at equal temperature and pressure have the same number of molecules.

  • Dalton’s atomic theory is referred to as the molecular theory of matter.

  • Atoms in gases are freer than those in liquid and solids.

  • Atoms are not elementary; they can be further sub divided into their constituents.

The chain of sub division does not end here.

  • Atomic hypothesis: All things are made of atoms − little particles that move around in perpetual motion, attracting each other when they are at a little distance apart and repelling when squeezed into one another.

  • Atomic theory: Proposed by John Dalton to explain the laws of definite and multiple proportions.

  • First law states that any given compound has fixed proportion by mass of its constituents.

  • Second law states that when two elements form more than one compound, for a fixed mass of one element, the masses of the other elements are in the ratio of small integers.

  • Gay-Lussac’s law: When gases combine chemically to yield another gas, their volumes are in the ratio of small integers.

  • Avogadro’s law: Equal volumes of all gases at equal temperature and pressure have the same number of molecules.

  • Dalton’s atomic theory is referred to as the molecular theory of matter.

  • Atoms in gases are freer than those in liquid and solids.

  • Atoms are not elementary; they can be further sub divided into their constituents.

The chain of sub division does not end here.

Heat

  • It is a form of energy.

  • Heat is a transfer of energy due to the temperature difference between a system and its surroundings.

  • The flow of heat between two bodies stops when their temperatures equalise; the two bodies are then in thermal equilibrium.

Internal Energy

  • Work is transfer of energy brought about by other means, such as moving the piston of a cylinder containing gas.

  • Internal energy of a system is the sum of the kinetic and potential energies of the molecular constituents of the system.

  • It includes the energy associated with the random motion of molecules of the system.

  • Internal energy (as a state variable) depends on the given state of the system, and not on the path taken to reach the state.

  • Internal energy and work are equivalent.

Difference between heat and internal energy

In thermodynamics, heat and work are not state variables; however, internal energy is a state variable.

First Law of Thermodynamics:

  • The first law of thermodynamics is based on the law of conservation of energy .
  • According to the first law of thermodynamics, the energy(Q) supplied to a system is partly spent to increase the internal energy of the system (U) and the rest is spent in doing work on the surroundings (W).
  • The equation for the first law of thermodynamics is
        Q=U+W

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Heat

  • It is a form of energy.

  • Heat is a transfer of energy due to the temperature difference between a system and its surroundings.

  • The flow of heat between two bodies stops when their temperatures equalise; the two bodies are then in thermal equilibrium.

Internal Energy

  • Work is transfer of energy brought about by other means, such as moving the piston of a cylinder containing gas.

  • Internal energy of a system is the sum of the kinetic and potential energies of the molecular constituents of the system.

  • It includes the energy associated with the random motion of molecules of the system.

  • Internal energy (as a state variable)…

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