Atmospheric_Thermodynamics _II

# Atmospheric_Thermodynamics _II - Atmospheric Thermodynamics...

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Atmospheric Thermodynamics –II The first Law of Thermodynamics and applications Prof. Leila M. V. Carvalho Dept. Geography, UCSB

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The first Law of Thermodynamics Is the law that describes the relationships between heat, work and internal energy. It establishes the physical and mathematical framework to understand heating processes in our atmosphere, the formation of clouds, the thermodynamical modifications in parcels in movement, etc… Let’s explore these relationships and applications…
Motivation: Ex: formation of clouds Pressure q Volume expands and the parcel’s temperature decreases. q As it cools the air becomes saturated q When that begins : Lifting Condensation Level (clouds are formed) LCL: Cloud base : Releases Latent heat: The first Law of Thermodynamics provides the physical concepts to understand cloud formation: Heat

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Internal Energy u : measure of the total kinetic and potential energy of a gas Kinetic energy: depend on molecular motions -> relationship with temperature Potential energy: changes in the relative position of the molecules due to internal forces that act between molecules (small changes)
Closed System definition: Is the one in which the total amount of matter, which may be in the form of gas, liquid, solid or a mixture of these phases, is kept constant

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Suppose a closed system with one unity of mass Ø Suppose that this volume receives certain quantity of thermal energy q (joules) by ‘conduction’ and/or radiation. Ø This system may do a certain amount of external work w (also measured in Joules) . 1 2 u u w q - = - Differences will cause changes in the internal energy Where 1 is before and 2 after the change
This is the First Law of Thermodynamic s In the differential form Ø dq is the differential increment of heat added to the system, Ø dw is the differential element of work done by the system Ø du is the differential increase in internal energy of the system Changes in du depend only on the final and initial state: functions of state du dw dq = - (34)

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Visualization Volume is proportional to the distance Frictionless Every state of the substance, corresponding to a given position of the piston, can be represented in this diagram below
AREA A If the Piston moves outward with the same pressure p, then the work done by the substance in pushing the external force F through a distance dx is: F = pA Fdx dW = pA F = pdV pAdx dW = = If pressure is constant then: (35) p V dV pd V

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If the substance
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