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Chapter 20

Chapter 20. The First Law of Thermodynamics EXAMPLES. Chapter 20. Heat and the First Law of Thermodynamics: Examples. Example 20.1 A very Hot Frying Pan.

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Chapter 20

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  1. Chapter 20 The First Law of Thermodynamics EXAMPLES

  2. Chapter 20. Heat and the First Law of Thermodynamics: Examples

  3. Example 20.1 A very Hot Frying Pan • Accidentally an empty iron frying pan gets very hot on the stove (~300oC). What happen when you dunk it into a few inches of cool water in the bottom of the sink? Assume: Mwater ≈ Mpan • Water warms up (~10oC to 20oC) • Water does not come close to boiling. • Since: Mwater ≈ Mpan & cwater ~ 10ciron from heat Q leaving the pan enters the water, and the iron’s pan ΔT will be 10 times greater than ΔT of the water. • If you let a few drops of water fall onto the hot pan, that very small mass of water will sizzle and boil away since: Mpan ~ 100Mwater

  4. Example 20.2 Cooling a Hot Ingot • A 0.05-kg ingot of metal is heated to 200oC and then dropped into a beaker containing 0.4-kg of water initially at 20oC. If the final equilibrium temperature of the mixed system is 22.4oC, find specific heat (c) of the metal.

  5. Calorimetry Problem-Solving Strategy • Units of measure must be consistent • For example, if your value of c is in J/kg.oC, then your mass must be in kg, the temperatures in oC and energies in J • Transfers of energy are given by Q =mc ΔT only when no phase change occurs • If there is a phase change, use Q = mL • Be sure to select the correct sign for all energy transfers • Remember to use Qcold = –Qhot • The ΔT is always Tf - Ti

  6. Example 20.3 Cooling the Steam • What mass of steam initially at 130oCis needed to warm 200-g of water in a 100-g glass container from 20oC to 50.0oC • Steam loses energy in 3 stages: • 1st stage:Steam  100oC , DT = –30oC • 2nd stage: Steam  water • 3rd stage: Water  50oC , DT = –50oC

  7. Example 20.3 Cooling the Steam, final • Adding the energy transfer in the 3 stages: • Water and Glass increasing temperature: DT = 30oC • Final: Qcold = –Qhot

  8. Example 20.4 An Isothermal Expansion • A 1.0 mol of an ideal gas is kept at 0oC during an expansion from 3.0 L to 10.0 L. • (A). Find the work done on the gas during the expansion. Using equation 20.13 • (B). How much energy transfer by heat occurs with the surroundings in this process? • From the first law: ΔEint = Q + W

  9. Example 20.4 An Isothermal Expansion, final • (C). Find the work done on the gas during the expansion. • Work done in an isobaric process is: W = P (Vf – Vi) where Vf =10.0L and Vi =3.0L (reverse of part A)

  10. Example 20.5 Quick Quiz 20.5 • Two rods from different materials with the same length and diameter are connecting two regions of different temperatures. In which case is the rate of energy transfer by heat (P ) larger Series Parallel Therefore PP = 4 PS : In parallel, the rods present a larger area A and a smaller length L through which the energy can transfer.

  11. Material for the 2nd Midterm • Material from the book to Study!!! • Objective Questions: 6-12 • Conceptual Questions: 3-5 • Problems: 6-12-16-22-23-24-25-26- 30-36-49

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