TLDR;
This YouTube video by Physics Ka Manjan - Bilal Zia introduces the chapter on thermodynamics, focusing on thermal equilibrium, the zeroth law of thermodynamics, work done in thermodynamics, and internal energy. The lecture aims to build a foundation for understanding the first law of thermodynamics by explaining key concepts and their applications.
- Thermal equilibrium is achieved when two bodies in thermal contact reach the same temperature, resulting in no net heat flow.
- The zeroth law of thermodynamics defines the basis of temperature measurement.
- Work done in thermodynamics is thoroughly explained, including its calculation and significance.
- Internal energy is defined as the total energy possessed by a molecule, comprising kinetic and potential energy.
Introduction to Thermodynamics [0:01]
The video introduces a new chapter on thermodynamics, which in some textbooks may be presented as "Heat and Thermodynamics" or with heat as a separate chapter. The presenter mentions that the playlist covering heat, which includes approximately 11 lectures on topics like calorimetry, gas laws, general gas equation, and thermometry, is already complete. The new chapter will primarily cover the first and second laws of thermodynamics, including the Carnot engine, refrigerators, and entropy.
Thermal Equilibrium [2:16]
The concept of thermal equilibrium is explained using the example of a cup of tea placed in a room. Heat energy transfers from the hot tea to the cooler surroundings until both reach the same temperature. This transfer occurs due to a temperature difference (ΔT ≠ 0). Thermal equilibrium is achieved when the net heat flow between two bodies is zero because their temperatures are equal. Thermal contact is necessary for heat energy to flow between objects, influenced by factors such as time and environmental conditions.
Zeroth Law of Thermodynamics [8:51]
The zeroth law of thermodynamics states that if two systems (A and B) are separately in thermal equilibrium with a third system (C), then A and B are also in thermal equilibrium with each other. This law defines the basis of temperature measurement, ensuring that if A and C have the same temperature and B and C have the same temperature, then A and B will also have the same temperature when brought into thermal contact, with no net heat flow between them.
System and Surroundings [15:55]
A system is defined as anything under consideration or observation, while the surroundings include everything else. The video uses the example of a gas in a container with a movable piston to illustrate this concept. Systems can be open, closed, or isolated. An open system allows both mass and energy exchange with the surroundings (e.g., a room with an open door). A closed system allows energy exchange but not mass (e.g., a closed room where sound can escape). An isolated system prevents both mass and energy exchange (ideally, a thermos flask, though real-world examples are only near-isolated).
Internal Energy [24:02]
Internal energy (U) is the total energy possessed by a molecule, comprising kinetic and potential energy. Kinetic energy can be translational, vibrational, or rotational. The kinetic energy of gas molecules is directly proportional to absolute temperature (T), as described by the formula KE = 3/2 RT per mole. Therefore, internal energy is also directly proportional to temperature. Potential energy arises from intermolecular forces, but for ideal gases, it is considered zero.
Heat Capacity vs Latent Heat [31:57]
The video addresses a common confusion regarding internal energy and latent heat. While heat capacity involves a temperature change and thus a change in kinetic energy, latent heat involves a change of state at a constant temperature. During a phase change, the internal energy increases due to an increase in potential energy as molecules overcome intermolecular forces, even though the temperature remains constant.
Work Done in Thermodynamics [36:19]
The concept of work done in thermodynamics is introduced using a gas-filled container with a movable piston. When heat is added, the gas molecules move faster, exerting more force on the piston and causing it to move upwards. The work done by the gas is calculated as the force exerted by the gas multiplied by the displacement of the piston. The formula for work done is derived as W = PΔV, where P is the pressure exerted by the gas and ΔV is the change in volume.
Constant Pressure and PV Diagrams [50:10]
The formula W = PΔV is valid only when the work is done by the gas at constant pressure. A PV diagram illustrates this process, showing a horizontal line indicating constant pressure as the volume changes from V1 to V2. The area under the PV curve represents the work done in thermodynamics. For processes where pressure is not constant, the work done can still be found by calculating the area under the PV curve.
Calculating Work Done with Varying Pressure [59:27]
For processes with varying pressure, the area under the PV curve can be approximated by dividing the area into small rectangular strips and summing their areas. This concept is related to integration, where the work done is given by the integral of PDV from V1 to V2. The video emphasizes that the area under the PV diagram always gives the value of work done, regardless of the process.
Examples of Work Done Calculations [1:05:20]
Several examples are provided to illustrate the calculation of work done using PV diagrams. These examples include cases with constant pressure and varying pressure, demonstrating how to find the area under the curve to determine the work done. The video also explains how to handle different units and conversions.
Volume and Work Done [1:21:17]
If the volume of the gas remains constant during a thermodynamic process, no work is done (W = 0), as there is no displacement. This is analogous to pushing a wall that doesn't move. In a PV diagram, this is represented by a vertical line, with no area under the curve. Work done is positive when the volume increases (expansion) and negative when the volume decreases (compression).
Constant Pressure Processes and Ideal Gas Law [1:32:52]
For constant pressure processes, the work done can also be expressed as W = nRΔT, derived from the ideal gas law (PV = nRT). This formula is useful when the change in temperature is known. The video provides examples of past paper questions where this formula can be applied.
Past Paper Questions [1:38:10]
The video concludes by solving two past paper questions from the Education Testing and Evaluation Agency (ETEA) in Khyber Pakhtunkhwa. These questions involve calculating work done in constant pressure processes using both W = nRΔT and W = PΔV. The solutions demonstrate the application of the concepts discussed in the lecture.
