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The second law of thermodynamics

Chapter 4. The second law of thermodynamics. Direction of the process. gas. 4-1 Directions of thermodynamic processes. Irreversible processes . الاجراءات اللانعكاسية. Heat engine. A device that transforms heat partly into work or mechanical energy . second law of thermodynamics.

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The second law of thermodynamics

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  1. Chapter 4 The second law of thermodynamics

  2. Direction of the process gas 4-1 Directions of thermodynamic processes • Irreversible processes. الاجراءات اللانعكاسية

  3. Heat engine A device that transforms heat partly into work or mechanical energy

  4. second law of thermodynamics It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work, with the system ending in the same state in which it began. من المستحيل ان تحول آلة حرارية كمية الحرارة المضافة بالكامل الى شغل.

  5. 4-2 The Carnot Cycle • It consists of the following steps: • The gas expands isothermally at temperature TH, absorbing heat QH. • 2. It expands adiabatically until its temperature drops to TC. • 3. It is compressed isothermally at TC, rejecting heat . • 4- It is compressed adiabatically back to its initial state at temperature TH. • تتكون كارنو من مجموعة من الاجراءات: • الأجراء 1-2 يتمدد الغاز ايزوثيرمالى يحدث عند درجة حرارة THويتم اضافة كمية حرارة QH • الاجراء 2-3 يتمدد الغاز اديباتيكيا حتى تصل درجة حرارته الى TC • الاجراء 3-4 ينكمش الغاز ايزوثيرمالى يحدث عند TCويتم فقد كمية حرارة QC • الاجراء 4-1 ينكمش الغاز اديباتيكيا حتى يصل الى حالته الابتدائية عند درجة الحرارة TH

  6. P QH 1 2 4 3 QC V الأجراء 1-2 يتمدد الغاز ايزوثيرمالى يحدث عند درجة حرارة TH ويتم اضافة كمية حرارة QH الاجراء 4-1 ينكمش الغاز اديباتيكيا حتى يصل الى حالته الابتدائية عند درجة الحرارة TH الاجراء 2-3 يتمدد الغاز اديباتيكيا حتى تصل درجة حرارته الى TC الاجراء 3-4 ينكمش الغاز ايزوثيرمالى ويحدث عند TC ويتم فقد كمية حرارة QC

  7. Continue For Carnot cycle : W = QH - QC

  8. Example 4-1 A Carnot engine takes 2000J of heat from a reservoir at 500k, does some work, and discards some heat to a reservoir at 350k. How much work does it do, how much heat is discarded, and what is the efficiency?

  9. Solution The thermal efficiency is η = 0.3 = 30%

  10. 4-3 Entropy Entropyprovides a quantitative measure of disorder dS = dQ / T For reversible isothermal process ∆S = S2 – S1 = Q/T

  11. Example 4-2 One kilogram of ice at 0ºC is melted and converted to water at 0ºC. Compute its change in entropy, assuming that the melting is done reversibly. The heat of fusion of water is Lf = 3.34x105J/k. Solution The heat needed to melt the ice is Q = m Lf = 3.34x105 J ∆S = S2 – S1 = Q/T = 3.34x105J / 273k = 1.22x103J/k

  12. Carnot cycle on T – S diagram QH T Isothermal TH 1 2 Adiabatic TC 3 4 QC S

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