1 / 11

Chapter 20

Chapter 20. Heat and the First Law of Thermodynamics (cont.). Outline. Latent heat (20.3) Phase change Latent heat for different phase changes Example: Convert ice to steam Calorimetry problems involving phase change (20.3) Work and heat in thermodynamic processes (20.4) PV diagram.

more
Download Presentation

Chapter 20

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 20 Heat and the First Law of Thermodynamics (cont.) PHY 1361

  2. Outline • Latent heat (20.3) • Phase change • Latent heat for different phase changes • Example: Convert ice to steam • Calorimetry problems involving phase change (20.3) • Work and heat in thermodynamic processes (20.4) • PV diagram PHY 1361

  3. Latent heat • Phase change: The situation when the physical characteristics of the substance change from one form to another. • Common phase changes: Melting (from solids to liquid) and Boiling (from liquid to gas) • All phase changes involve a change in internal energy but no change in temperature. (Example: boiling) • Latent heat: If a quantity Q of energy transfer is required to change the phase of a mass m of a substance, the ratio L  Q/m is called the latent heat of the substance. • L depends on the nature of the phase change and the properties of the substance. • (1) Latent heat of fusion Lf (from solid to liquid); (2) latent heat of vaporization Lv (from liquid to gas; (3) latent heat of condensation (from gas to liquid); (4) latent heat of solidification (from liquid to solid). See Table 20.2. • Q = ±mL; “+” or “-” depends on whether adding or removing energy from the substance. PHY 1361

  4. Example: Convert ice to steam • Find the energy required to convert a 1.00-g cube of ice at -30.0°C to steam at 120.0°C. (See Tables 20.1 and 20.2) Figure 20.2 Experimental results PHY 1361

  5. Previous example follow-up discussions • We can describe phase changes in terms of a rearrangement of molecules when energy is added to or removed from a substance. • The latent heat of vaporization Lv for a given substance is usually somewhat higher than the latent heat of fusion Lf. • Explanation: Liquid-type bonds are slightly less strong than the solid-type bonds, while gas-type bonds are much less strong; molecules of the gas essentially are not bonded to each other. • Quick Quiz 20.3: Calculate the slopes for the A, C, and E portions of Figure 20.2. Rank the slopes from least to greatest and explain what this ordering means. PHY 1361

  6. Calorimetry problems involving phase change • Example 20.4 Cooling the steam: What mass of steam initially at 130°C is needed to warm 200 g of water in a 100-g glass container from 20.0°C to 50.0°C? What if the final state of the system is water at 100°C? Would we need more or less steam? How would the analysis above change? • See the textbook for problem-solving hints about Calorimetry-type problems PHY 1361

  7. Work and heat in thermodynamic processes • The work done on a gas in a quasi-static process that takes the gas from an initial state to a final state is the negative of the area under the curve on a PV diagram, evaluated between the initial and final states: • Quasi-static process: A process during which the system is allowed to remain essentially in thermal equilibrium at all times. • PV diagram: If the pressure P and volume V are known at each step of the process, a graph of P v.s. V is called a PV diagram. • Path: The curve on a PV diagram is called the path taken between the initial and final states. PHY 1361

  8. Example: P. 633, Problem #21 • A sample of ideal gas is expanded to twice its original volume of 1.00 m3 in a quasi-static process for which P = V2, with  = 5.00 atm/m6, as shown in the figure. • How much work is done on the expanding gas? PHY 1361

  9. The work done depends on the particular path taken between the initial and final states. PHY 1361

  10. Heat in thermodynamic processes • Energy transfer by heat, like work done, depends on the initial, final, and intermediate states of the system. PHY 1361

  11. Homework • Ch. 20, P.633, Problems: #14, 22, 26. PHY 1361

More Related