Thermodynamics Fundamentals
🧪 Thermodynamics is a branch of physics focused on energy exchanges between a system (e.g., gas, liquid) and its external environment.

Ideal Gas Model
🎈 The ideal gas model simplifies the behavior of gases at low pressures, where interactions between entities are considered negligible.
💨 It operates on two key assumptions: entities have no interactions with each other (only with container walls) and their size is negligible compared to the distances between them.

System Description & Variables
📈 A thermodynamic system's state is described by macroscopic quantities such as mass density (ρ), temperature (T), and pressure (P), which reflect average microscopic properties.
🌡️ In thermodynamics, temperature (T) must be expressed in Kelvin (K), related to Celsius by the formula T(K) = θ(°C) + 273.15.
⚙️ The ideal gas law, PV = nRT, relates pressure (P in Pa), volume (V in m³), quantity of matter (n in mol), and temperature (T in K), with R being the ideal gas constant.
⚠️ The ideal gas model has limitations; it is only suitable for gases at low pressures, as inter-entity interactions become significant at high pressures.

Internal Energy Concept
✨ Internal energy (U) represents the microscopic energy within a system, encompassing the kinetic energy from particle agitation and potential energy from inter-particle interactions.
➡️ For systems considered at macroscopic rest (e.g., a stationary liquid), the total energy variation (ΔEtot) simplifies to solely the internal energy variation (ΔU).

Energy Transfer Mechanisms
🔄 Internal energy can be modified through two types of energy transfer: work (W), associated with macroscopic displacement, and thermal transfer (Q), linked to microscopic energy exchange due to temperature differences. Both are measured in Joules (J).
🚫 In many high school exercises, work (W) is often considered 0 J when there's no volume change (e.g., gas in a rigid container) or no observable macroscopic displacement.
🔥 Thermal transfer (Q) spontaneously moves from a hotter body to a colder body, potentially causing a temperature increase or a phase change without temperature alteration (e.g., ice melting at 0°C).
✅ By convention, W and Q are counted positively if received by the system and negatively if ceded to the external environment.

First Law of Thermodynamics
⚖️ The First Law of Thermodynamics states that for a system at macroscopic rest and not exchanging matter, the variation of internal energy (ΔU) is equal to the sum of work (W) and thermal transfer (Q): ΔU = W + Q. This law expresses the fundamental principle of energy conservation.

Incompressible Systems & Internal Energy
💧 For incompressible systems (typically liquids and solids undergoing no volume change, phase change, or chemical/nuclear reactions), the variation in internal energy (ΔU) is directly proportional to the variation in temperature (ΔT).
🔬 This relationship is expressed as ΔU = C . ΔT or ΔU = m . c . ΔT, where C is the thermal capacity (J.K⁻¹) and c is the massic thermal capacity (J.kg⁻¹.K⁻¹), with ΔT calculated as final minus initial temperature.

Key Points & Insights
💡 Master the ideal gas law (PV=nRT) and ensure all units, particularly temperature in Kelvin, are correctly applied for accurate calculations.
🎯 When analyzing systems at macroscopic rest, remember that total energy variation equals internal energy variation (ΔEtot = ΔU), simplifying problem-solving.
⚠️ Differentiate clearly between temperature (T), which describes the thermal state of a system, and heat (Q), which represents an energy transfer, especially in scenarios involving phase changes without temperature change.
➕ Accurately apply the sign convention for energy transfers (W and Q)—positive for energy received by the system, negative for energy ceded—to correctly interpret thermodynamic processes.

📸 Video summarized with SummaryTube.com on Sep 04, 2025, 16:31 UTC