Presentation on theme: "Evaporation and condensation"— Presentation transcript:
1 Evaporation and condensation Individual molecules can change phase any timeEvaporation:Energy required to overcome phase cohesionHigher energy molecules near the surface can then escapeCondensation:Gas molecules near the surface lose KE to liquid molecules and merge
2 Ways to Increase Evaporation Rate Increase temperatureKinetic energy increases which increases the number of high-energy molecules that can escape from liquid stateIncrease surface area of liquidIncreases the likelihood of molecules escaping to airRemove water vapor from surface of the liquidPrevents return of vapor molecules to liquid stateReduce pressure on liquidReduces one of the forces holding molecules in liquid state
3 Capacity at present temperature Relative HumidityRatio of how much water vapor is in the air to how much water vapor could be in the air at a certain temperatureExpressed as a percentageWater vapor in airCapacity at present temperatureRelative Humidity =X 100 %
5 Heat flowThree mechanisms for heat transfer due to a temperature differenceConductionConvectionRadiationNatural flow is always from higher temperature regions to cooler ones
6 Conduction Heat flowing through matter Mechanism Hotter atoms collide with cooler ones, transferring some of their energyDirect physical contact required; cannot occur in a vacuumPoor conductors = insulators (Styrofoam, wool, air…)
7 Conduction is the flow of heat directly through a physical material.
8 Experimentally, it is found that the amount of heat Q that flows through a rod: increases proportionally to the cross-sectional area Aincreases proportionally to the temperature difference from one end to the otherincreases steadily with timedecreases with the length of the rod
9 16-6 Conduction, Convection, and Radiation Combining, we find:The constant k is called the thermal conductivity of the rod.
10 Some typical thermal conductivities: Substances with high thermal conductivities are good conductors of heat; those with low thermal conductivities are good insulators.
11 Convection Energy transfer through the bulk motion of hot material ExamplesSpace heaterGas furnace (forced)Natural convection mechanism - “hot air rises”
12 Convection is the flow of fluid due to a difference in temperatures, such as warm air rising. The fluid “carries” the heat with it as it moves.
13 RadiationRadiant energy - energy associated with electromagnetic wavesCan operate through a vacuumAll objects emit and absorb radiationTemperature determinesEmission rateIntensity of emitted lightType of radiation given offTemperature determined by balance between rates of emission and absorptionExample: Global warming
14 Electromagnetic Spectrum Transverse wavesRegenerating co-oscillation of electric and magnetic fieldsElectric, magnetic and velocity vectors mutually perpendicularForm when electric charge is accelerated by external forceFrequency depends on acceleration of chargeGreater the acceleration, higher the frequency
15 Blackbody radiation Blackbody Increasing temperature Ideal absorber/emitter of lightRadiation originates from oscillation of near-surface chargesIncreasing temperatureAmount of radiation increasesPeak in emission spectrum moves to higher frequencySpectrum of the Sun
16 All objects give off energy in the form of radiation, as electromagnetic waves – infrared, visible light, ultraviolet – which, unlike conduction and convection, can transport heat through a vacuum.Objects that are hot enough will glow – first red, then yellow, white, and blue. Objects at body temperature radiate in the infrared, and can be seen with night vision binoculars.
17 The amount of energy radiated by an object due to its temperature is proportional to its surface area and also to the fourth (!) power of its temperature.It also depends on the emissivity, which is a number between 0 and 1 that indicates how effective a radiator the object is; a perfect radiator would have an emissivity of 1.
19 ThermodynamicsThe study of heat and its relationship to mechanical and other forms of energyThermodynamic analysis includesSystemSurroundings (everything else)Internal energy (the total internal potential and kinetic energy of the object in question)Heat engines - devices converting heat into mechanical energy
20 The Zeroth Law of Thermodynamics If object A is in thermal equilibrium with object C, and object B is separately in thermal equilibrium with object C, then objects A and B will be in thermal equilibrium if they are placed in thermal contact.
21 The First Law of Thermodynamics The first law of thermodynamics is a statement of the conservation of energy.If a system’s volume is constant, and heat is added, its internal energy increases.
22 The First Law of Thermodynamics If a system does work on the external world, and no heat is added, its internal energy decreases.
23 The First Law of Thermodynamics Combining these gives the first law of thermodynamics. The change in a system’s internal energy is related to the heat Q and the work W as follows:It is vital to keep track of the signs of Q and W.
24 The First Law of Thermodynamics The internal energy of the system depends only on its temperature. The work done and the heat added, however, depend on the details of the process involved.
25 The Second Law of Thermodynamics We observe that heat always flows spontaneously from a warmer object to a cooler one, although the opposite would not violate the conservation of energy. This direction of heat flow is one of the ways of expressing the second law of thermodynamics:When objects of different temperatures are brought into thermal contact, the spontaneous flow of heat that results is always from the high temperature object to the low temperature object. Spontaneous heat flow never proceeds in the reverse direction.
26 Refrigerators, Air Conditioners, and Heat Pumps While heat will flow spontaneously only from a higher temperature to a lower one, it can be made to flow the other way if work is done on the system. Refrigerators, air conditioners, and heat pumps all use work to transfer heat from a cold object to a hot object.
27 Refrigerators, Air Conditioners, and Heat Pumps If we compare the heat engine and the refrigerator, we see that the refrigerator is basically a heat engine running backwards – it uses work to extract heat from the coldreservoir (the inside of the refrigerator) and exhausts to the kitchen.
28 Refrigerators, Air Conditioners, and Heat Pumps An air conditioner is essentially identical to a refrigerator; the cold reservoir is the interior of the house or other space being cooled, and the hot reservoir is outdoors. Exhausting an air conditioner within the house will result in the house becoming warmer, just as keeping the refrigerator door open will result in the kitchen becoming warmer.
29 Refrigerators, Air Conditioners, and Heat Pumps Finally, a heat pump is the same as an air conditioner, except with the reservoirs reversed. Heat is removed from the cold reservoir outside, and exhausted into the house, keeping it warm. Note that the work the pump does actually contributes to the desired result (a warmer house) in this case.
30 EntropyFor this definition to be valid, the heat transfer must be reversible.In a reversible heat engine, it can be shown that the entropy does not change.
31 Second law: Entropy Real process = irreversible process Measure of disorder = entropySecond law, in these terms:The total entropy of the Universe continually increasesNatural processes degrade coherent, useful energyAvailable energy of the Universe diminishingEventually: “heat death” of the UniverseDirection of natural processesToward more disorderSpilled milk will never “unspill” back into the glass!
32 18-8 EntropyA real engine will operate at a lower efficiency than a reversible engine; this means that less heat is converted to work. Therefore,Any irreversible process results in an increase of entropy.
33 Entropy To generalize: The total entropy of the universe increases whenever an irreversible process occurs.The total entropy of the universe is unchanged whenever a reversible process occurs.Since all real processes are irreversible, the entropy of the universe continually increases. If entropy decreases in a system due to work being done on it, a greater increase in entropy occurs outside the system.
34 18-8 EntropyAs the total entropy of the universe increases, its ability to do work decreases. The excess heat exhausted during an irreversible process cannot be recovered; doing that would require a decrease in entropy, which is not possible.
35 18-9 Order, Disorder, and Entropy Entropy can be thought of as the increase in disorder in the universe. In this diagram, the end state is less ordered than the initial state – the separation between low and high temperature areas has been lost.
36 18-9 Order, Disorder, and Entropy If we look at the ultimate fate of the universe in light of the continual increase in entropy, we might envision a future in which the entire universe would have come to the same temperature. At this point, it would no longer be possible to do any work, nor would any type of life be possible. This is referred to as the “heat death” of the universe.
37 18-9 Order, Disorder, and Entropy So if entropy is continually increasing, how is life possible? How is it that species can evolve into ever more complex forms? Doesn’t this violate the second law of thermodynamics?No – life and increasing complexity can exist because they use energy to drive their functioning. The overall entropy of the universe is still increasing. When a living entity stops using energy, it dies, and its entropy can increase rather quickly.
38 The Third Law of Thermodynamics Absolute zero is a temperature that an object can get arbitrarily close to, but never attain. Temperatures as low as 2.0 x 10-8 K have been achieved in the laboratory, but absolute zero will remain ever elusive – there is simply nowhere to “put” that last little bit of energy.This is the third law of thermodynamics:It is impossible to lower the temperature of an object to absolute zero in a finite number of steps.
39 Q1. Substance A has a higher specific heat than substance B Q1. Substance A has a higher specific heat than substance B. Which requires the most energy to heat equal masses of A and B to the same temperature?A) Substance AB) Substance BC) Both require the same amount of heat.D) Answer depends on the density of each substance.
40 Q2. Anytime a temperature difference occurs, you can expect A) cold to move to where it is warmer.B) energy movement from higher temperature regions.C) no energy movement unless it is warm enough, at least above the freezing temperature.D) energy movement flowing slowly from cold to warmer regions.
41 Q3. As a solid goes through a phase change to a liquid, heat is absorbed and the temperature A) increases.B) decreases.C) remains the same.D) fluctuates.
42 Q4. The transfer of energy from molecule to molecule is called A) convection.B) radiation.C) conduction.D) equilibrium.
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