Lecture3

# Lecture3 - Physics 7A Lecture 3 Winter 2010...

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Physics 7A -- Lecture 3 Winter 2010 Prof. Robin D. Erbacher 343 Phy/Geo Bldg [email protected]

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Join this Class Session with your PRS clicker! (Credit begins today.) Quiz 2 is today! Lecture 1-2, DLMs 1-5. Must take it in your correct lecture time slot. Quiz 1 Rubric and solutions are posted, and grading scale is linked. Check Physics 7 Smartsite frequently for calendar & announcements. Turn off cell phones during lecture.
•E thermal = C T = mc T, Temperature is the indicator. Between phase changes, only thermal energy changes. • E bond = ± | m H|, m is the indicator. At a physical phase change, only the bond-energy system changes. H is the heat of the particular phase change. m is the amount that changed phase. In a chemical reaction, there are several bond energy changes corresponding to diff. molecular species (reactants or products). Here H is the heat of formation for a particular species. E thermal E bond

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E a E b E c Conservation of Energy The total energy of a closed physical system must remain constant. So, the change of the energies of all energy systems associated with the physical system must sum to zero. Change in closed system energy = ∆E a + ∆ E b + ∆ E c = 0
E a E b E c Conservation of Energy The change of the energies of all systems associated with an open physical system must sum to the net energy added or removed . Energy is added or removed as Heat or Work. Change in open system energy = ∆E a + ∆ E b + ∆ E c = (Energy added) - (Energy removed) = Q + W. Energy added Energy removed

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Example : Melting Ice T i = 0°C T f = room temperature Temperature Energy of substance solid liquid gas l-g coexist s-l coexist Initial T MP T BP Final
Example : Melting Ice Process 1: Ice at T=0ºC Water at T=0ºC Process 2: Water at T=0ºC Water at room temperature Temperature Energy of substance solid liquid gas l-g coexist s-l coexist Process 1 Initial T MP T BP Process 1 Final / Process 2 Initial Process 2 Final

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Example : Melting Ice Process 1: Ice at T=0ºC Water at T=0ºC Ice E thermal E bond Heat Initial phase Solid, Final phase Liquid ∆E th + ∆E bond = Q+W ∆E bond = ±| m|| H| = Q ∆T=0 ∆E th = mC p T =0
Example : Melting Ice Process 2: Water at T=0ºC Water at room temperature Ice Initial phase Liquid, Final phase Liquid E thermal E bond

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Example : Melting Ice Process 2: Water at T=0ºC Water at room temperature Ice E thermal E bond T Heat Initial phase Liquid, Final phase Liquid ∆E bond = ±| m|| H| = 0 ∆E th + ∆E bond = Q+W ∆E th = mC p T = Q
Example : Melting Ice Ice Initial phase Liquid, Final phase Solid E thermal E bond Freezing (Water at T=0°C Ice at T=0°C) Heat NOTE: Heat is released when bonds are formed!

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