PHY 317K 1st Edition Lecture 23Outline of Last Lecture I. QuestionsII. Work in an Ideal-Gas ProcessesIII. Heat, Temperature, and Thermal EnergyOutline of Current Lecture I. The First Law of ThermodynamicsII. Temperature Change and Specific HeatIII. Phase Change and Heat of TransformationIV. CalorimetryCurrent LectureThe First Law of Thermodynamics- Work and heat are two ways of transferring energy between a system and the environment, causing the system’s energy to change. - If the system as a whole is at rest, so that the bulk mechanical energy due to translational or rotational motion is zero, then the conservation of energy equation is- DE = W + QTemperature Change and Specific Heat- The amount of energy that raises the temperature of 1 kg of a substance by 1 K is called the specific heat of that substance. - The symbol for specific heat is c. - If W = 0, so no work is done by or on the system, then the heat needed to bring about a temperature change ∆T is Q = MCTPhase Change and Heat of Transformation- A phase change is characterized by a change in thermal energy without a change in temperature.- The amount of heat energy that causes 1 kg of substance to undergo a phase change is called the heat of transformation of that substance.- The symbol for heat of transformation is L. - The heat required for the entire system of mass M to undergo a phase change is Q = ML- Two specific heats of transformation are the heat of fusion Lf, the heat of transformation between a solid and a liquid, and the heat of vaporization Lv, the heat of transformationbetween a liquid and a gas.Calorimetry- Suppose to systems start at different temperatures T1 and T2. - Heat energy will naturally be transferred from the hotter to the colder system until they reach a common final temperature Tf.The Specific Heats of Gases- It is useful to define two different versions of the specific heat of gases, one for constant-volume (isochoric) processes and one for constant-pressure (isobaric) processes. - We will define these as molar specific heats because we usually do gas calculations usingmoles instead of mass.Conduction- For a material of cross-section area A and length L, spanning a temperature difference ∆T = TH – TC, the rate of heat transfer is where k is the thermal conductivity, which characterizes whether the material is a good conductor of heat or a poor conductorConvection- Air is a poor conductor of heat, but thermal energy is easily transferred through air, water, and other fluids because the air and water can flow. - A pan of water on the stove is heated at the bottom. - This heated water expands, becomes less dense than the water above it, and thus rises to the surface, while cooler, denser water sinks to take its place. - The same thing happens to air. This transfer of thermal energy by the motion of a fluid—the well-known idea that “heat rises”—is called
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