Thermodynamics Notes

Overview

Thermodynamics studies heat, work, temperature and internal energy in macroscopic systems such as gases, engines and refrigerators. It begins with thermal equilibrium and the zeroth law, which gives the scientific basis of temperature measurement. The first law of thermodynamics is energy conservation applied to heat and work: heat supplied to a system changes internal energy and may do work. Different thermodynamic processes such as isothermal, adiabatic, isobaric and isochoric processes are best understood using PV diagrams. The second law explains why natural processes have a direction and why heat engines cannot convert all absorbed heat into work. Entropy measures disorder or energy dispersal. Carnot engine gives the maximum possible efficiency between two temperatures, making this chapter very important for NEET.

Overview

Thermal equilibrium is the condition in which two bodies in thermal contact do not exchange net heat. This happens when they have the same temperature. Temperature is therefore the property that decides the direction of heat flow. The zeroth law of thermodynamics states that if two systems are separately in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law makes temperature measurement possible because a thermometer acts as the third system. Thermodynamic systems may be open, closed or isolated depending on exchange of matter and energy. A state of a system is described by state variables such as pressure, volume, temperature and internal energy.

Overview

Heat, work and internal energy are central to thermodynamics. Heat is energy transferred due to temperature difference. Work is energy transferred when a system expands or compresses against external pressure. Internal energy is the total microscopic kinetic and potential energy of molecules in the system. For an ideal gas, internal energy depends only on temperature. Heat and work are path functions because their values depend on the process path, not only initial and final states. Internal energy is a state function because its change depends only on initial and final states. In the common NEET convention, heat supplied to the system is positive and work done by the system during expansion is positive.

Overview

The first law of thermodynamics is the law of conservation of energy applied to thermodynamic systems. It states that heat supplied to a system is used partly to increase its internal energy and partly to do work by the system. Mathematically, ΔQ = ΔU + ΔW under the common convention where work done by the system is positive. This law shows that heat and work are two ways of transferring energy across a system boundary. It is used in every thermodynamic process: isothermal, adiabatic, isochoric and isobaric. In numerical problems, identifying which quantity is zero or known is the main step. It is one of the most important NEET formulas.

Overview

A thermodynamic process is a path by which a system changes from one state to another. In an isothermal process, temperature remains constant; for an ideal gas, internal energy remains constant and heat supplied equals work done. In an adiabatic process, no heat is exchanged with surroundings, so work is done at the cost of internal energy. In an isochoric process, volume remains constant and work done is zero. In an isobaric process, pressure remains constant and work is PΔV. PV diagrams visually show these processes, and the area under a PV curve gives work done by gas. NEET frequently asks process identification, work comparison and first-law applications.

Overview

The first law tells us energy is conserved, but it does not tell which processes are naturally possible. The second law of thermodynamics gives the direction of natural processes. One statement says heat cannot spontaneously flow from a colder body to a hotter body without external work. Another says no heat engine can convert all absorbed heat into work in a complete cycle. Reversible processes are ideal and can be exactly retraced, while irreversible processes occur naturally with friction, heat flow through finite temperature difference, mixing and free expansion. Entropy is a state function that measures energy dispersal or disorder. For an isolated system, entropy never decreases, which explains why natural processes are irreversible.

Overview

A Carnot engine is an ideal reversible heat engine operating between two reservoirs at temperatures T1 and T2. Its cycle consists of two isothermal and two adiabatic processes. During isothermal expansion at the hot reservoir, the gas absorbs heat Q1. During adiabatic expansion, its temperature falls from T1 to T2. During isothermal compression at the cold reservoir, it rejects heat Q2. During adiabatic compression, its temperature rises back to T1. Carnot theorem states that no engine working between the same two temperatures can be more efficient than a Carnot engine. Its efficiency is η = 1 - T2/T1. A refrigerator is a reversed heat engine that uses work to transfer heat from cold to hot.