Lecture12_EAS1600_Fall08

# Lecture12_EAS1600_Fall08 - EAS 1600 Introduction to...

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EAS 1600 Introduction to Environmental Sciences ____________________________ Lecture 12 - Hydrologic Cycle PART 1 Today we will discuss to concepts in thermodynamics and how the relate to the movement of water through the Earth System
First Law of Thermodynamics conservation of energy Δ U=Q+W Δ U – the internal energy of a system. such as a parcel of air Q – Heat added to the system W – Work done on the system Note that heat and work have units of energy. An example of work is lifting an object in a gravitational field i.e. it takes a certain amount of energy to move the object against an opposing force.

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Work and Gas Expansion Another type of work is called PV work and deals with gas expansion and compression. Consider the piston to the right. If P is greater than mg/area the piston will move up. This means the gas in the piston is doing work. So the energy of the gas must decrease e.g. the temperature decreases. This means a gas cools as it expands. This is the reason temperature decreases with altitude.
Work and Gas Compression Consider the piston to the right. If P is less than mg/area the piston will move down. This means that work is done on the gas in the piston. So the energy of the gas must increase e.g. the temperature increase. This means a gas warms as it compresses. So as gas descends in the atmosphere it must compress (go to higher P) and increse in temperature.

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Water only comprises a few percent of the atmosphere, but still plays a very important role. Why? Because water is the only significant constituent of the atmosphere that changes phase under the conditions that are encountered in the atmosphere. The phase change is important in the weather because it results in rain, snow, etc. But the phase change is also important because it involves the conversion of energy between sensible and latent heat . Thus water can play a role in the overall energy budget of the earth/atmosphere. In this lecture we will explore this role . .. But first let’s learn a little about water and its phases. To do this we will consider a second principle: The Second Law of Thermodynamics
The three phases of water (H 2 O) Each phase change requires the absorption or release of “ latent energy or heat .” (NOTE: Latent heat is a form of energy distinctly different from sensible heat!)

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The amount of latent energy is referred to as the: heat of condensation/evaporation heat of freezing/melting
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Lecture12_EAS1600_Fall08 - EAS 1600 Introduction to...

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