Thermodynamics, Heat Transfer & Respiratory Physics – Unit II (Part I)

Vocabulary flashcards summarizing key thermodynamic, phase change, humidity, and respiratory surface-tension concepts from Unit II Part I lecture notes.

First Law of Thermodynamics

Energy within a system is conserved; the total energy equals heat added minus work done by the system.

Second Law of Thermodynamics

In any energy exchange, entropy increases in a closed system as it moves toward the lowest possible energy state.

Third Law of Thermodynamics

At absolute zero (0 K) all molecular motion ceases and entropy reaches its minimum value.

Triple Point of Water

The unique temperature-pressure point at which water exists simultaneously as solid, liquid, and gas.

Heat Transfer

Movement of thermal energy from a warmer object or region to a cooler one by conduction, convection, or radiation.

Conduction

Direct transfer of heat through molecular contact, predominant in solids; metals conduct efficiently due to high thermal conductivity.

Thermal Conductivity

A material’s ability to transfer heat via conduction; higher values mean faster heat flow.

Convection

Heat transfer by bulk movement and mixing of fluid molecules at different temperatures (liquids or gases).

Radiation (Heat)

Transfer of energy by electromagnetic waves without direct contact; occurs even across a vacuum.

Melting

Phase change from solid to liquid when heat input breaks intermolecular bonds.

Melting Point

Temperature at which a solid becomes a liquid; for ice, 0 °C (32 °F).

Latent Heat of Fusion

Extra heat (cal g⁻¹) required to convert a solid to a liquid at its melting point without temperature change.

Freezing

Phase change from liquid to solid as heat is removed; releases the same energy required for melting.

Sublimation

Direct transition from solid to vapor without passing through a liquid phase (e.g., dry ice).

Pascal’s Principle

In a confined fluid, pressure at a given depth is the same in all directions, independent of container shape.

Buoyancy (Archimedes’ Principle)

An upward force exerted by a fluid equal to the weight of the displaced fluid (ρ × g × V).

Vaporization

General term for converting a liquid into a gas; includes boiling and evaporation.

Boiling Point

Temperature at which a liquid’s vapor pressure equals ambient pressure, allowing bulk vapor formation.

Latent Heat of Vaporization

Heat required to convert a liquid to vapor at its boiling point without temperature change.

Evaporation

Surface vaporization of a liquid below its boiling point; removes heat from the remaining liquid/air.

Molecular Water

Invisible, individual H₂O molecules in the gaseous phase; distinct from visible mist or fog.

Water Vapor Pressure

Partial pressure exerted by molecular water in a gas mixture; rises with temperature.

Absolute Humidity (AH)

Actual mass of water vapor present in a given volume of gas, expressed in mg L⁻¹.

Relative Humidity (RH)

Ratio (percentage) of actual water vapor content to the maximum possible at the same temperature.

Dew Point

Temperature at which air becomes fully saturated (RH = 100 %) and condensation begins.

Percent Body Humidity (%BH)

Ratio of actual water vapor in a gas to the saturated capacity at body temperature (43.8 mg L⁻¹ at 37 °C).

Humidity Deficit (HD)

Amount of water vapor the body must add to inspired gas: HD = 43.8 mg L⁻¹ – actual content.

Temperature Effect on Vaporization

Higher temperature increases kinetic energy and vaporization rate; lower temperature does the opposite.

Pressure Effect on Vaporization

High ambient pressure opposes molecule escape, decreasing vaporization; low pressure accelerates it (e.g., high altitudes).

Surface Area Effect on Vaporization

Greater exposed liquid surface increases evaporation rate; spreading water thinly accelerates drying.

Surface Tension

Cohesive force at a gas-liquid interface that pulls surface molecules inward, minimizing surface area.

Law of Laplace

For spheres, distending pressure P = 2T ⁄ r; pressure is directly proportional to surface tension (T) and inversely to radius (r).

Pulmonary Surfactant

Phospholipid produced by type II alveolar cells that lowers alveolar surface tension and stabilizes lung units.

Alveolar Stability

Balance between surface tension and surfactant action that prevents small alveoli from collapsing or over-distending.

Elastic Recoil (Lung)

Tendency of lung tissue and alveoli to return to resting size due to elastic fibers and surface tension forces.

Infant Respiratory Distress Syndrome (IRDS)

Condition of premature infants lacking surfactant, leading to high surface tension, alveolar collapse, and severe breathing effort.

Condensation

Change of water vapor back to liquid when gas cools below its dew point, releasing heat to surroundings.

Aerosol

Suspension of fine solid or liquid particles in a gas, maintained partly by buoyant forces.

Humidifier

Device that adds molecular water to a gas stream to maintain or raise humidity level for patient care.

Heat of Vaporization–Cooling Effect

Vaporization absorbs heat from the environment, producing local cooling (e.g., sweating).

Influence of Altitude on Boiling

Lower atmospheric pressure at high altitude reduces boiling point, so water boils at temperatures < 100 °C.

Water Vapor Capacity

Maximum amount of water vapor a gas can hold at a given temperature; rises exponentially with temperature.

Factor: Surface Area (Gas Exchange)

Increasing alveolar or water surface area enhances diffusion and evaporation processes.

Molecular Kinetic Energy

Energy associated with motion of molecules; increases with temperature, affecting phase changes and vapor pressure.