thermo

Thermodynamics Overview

Definition and Scope

Thermodynamics is the study of the effects of work, heat, and energy on a system. It focuses on macroscopic (large-scale) changes and observations, rather than the atomic or molecular level.

Key Quantities in Thermodynamics

Thermodynamics can be expressed in terms of four fundamental quantities:

• Temperature (T): A measure of the average energy of motion of the particles of a substance.

• Internal Energy (U): The total energy stored in the atoms and molecules within a substance.

• Entropy (S): A measure of disorder or randomness in a system.

• Heat (Q): The energy transferred between objects at different temperatures.

Laws of Thermodynamics

The Three Laws

According to C.P. Snow, the three laws of thermodynamics can be humorously summarized as:

1. You can't win.

2. You can't break even.

3. You can't get out of the game.

First Law of Thermodynamics

The First Law of Thermodynamics is an extension of the law of conservation of energy, defined by the formula:

• Change in U = Q - W
  This law states that energy cannot be created or destroyed, only transformed from one form to another.

Second Law of Thermodynamics

The Second Law states that heat flows naturally from a hot object to a cold one, not the other way around. It also implies that no matter how efficient a system is, there will always be some waste energy, leading to an increase in entropy.

Third Law of Thermodynamics

The Third Law states that it is impossible for any system to reach absolute zero (0 Kelvin), where all atomic motion theoretically stops.

Thermodynamic Processes

Types of Processes

• Adiabatic Process: A process that transfers no heat (Q = 0). When a system expands adiabatically, work is positive and the change in internal energy (U) is negative. When a system expands adiabatically, W is positive (the system does work) so ΔU is negative. When a system compresses adiabatically, W is negative (work is done on the system) so ΔU is positive.

• Isothermal Process: A process that involves a constant temperature. To maintain thermal equilibrium, heat flow must be slow enough to allow the system to exchange heat with its surroundings. For ideal gases, if ΔT is zero, ΔU = 0, Therefore, Q = W. Any energy entering the system (Q) must leave as work (W)

• Isobaric Process: A process that involves a constant pressure. The work done in this process is represented by the formula:

• W = P(change in V)

• An isobaric process is a constant pressure process. ΔU, W, and Q are generally non-zero, but calculating the work done by an ideal gas is straightforward

• Isochoric Process: A process that involves a constant volume, where no work is done on the surroundings (W = 0).

Thermodynamic Equilibrium and Properties

Equilibrium States

Thermodynamic systems consist of the surrounding system, the system itself, and a boundary. They can be classified into closed and open systems. Equilibrium can be thermal, mechanical, phase, or chemical/material.

Properties of Systems

Thermodynamic properties are categorized into intensive and extensive variables. Intensive properties do not depend on the amount of substance (e.g., temperature, pressure), while extensive properties do (e.g., volume, mass).

Heat Engines and Efficiency

Heat Engines

A heat engine is a device that transforms heat into work. It requires a hot reservoir to supply energy and a cold reservoir to take in excess energy. The net work done by the gas during a cyclic process can be represented by the area under the PV curve in a Carnot cycle diagram.

Carnot Cycle

The Carnot cycle is a cyclical process that uses only reversible processes, including adiabatic and isothermal processes. The area enclosed by the curves in the Carnot cycle diagram represents the net work done by the engine in one cycle.

Special Topics in Thermodynamics

Real Gases vs. Ideal Gases

Real gases exhibit properties that cannot be entirely explained using the ideal gas law. This highlights the limitations of the ideal gas law in accurately describing the behavior of gases under all conditions.

Laws of Thermodynamics Summary

• 0th Law: If two objects are in thermal equilibrium with a third object, they are in thermal equilibrium with each other. This law allows us to define temperature relative to an established standard.

• 1st Law: Energy cannot be created or destroyed, only transformed.

• 2nd Law: Heat flows naturally from hot to cold, and some energy is always lost as heat, making 100% efficiency impossible.

• 3rd Law: No system can reach absolute zero.