Evaporation and condensation

• Individual molecules can change phase any time

• Evaporation:– Energy required to

overcome phase cohesion

– Higher energy molecules near the surface can then escape

• Condensation: – Gas molecules near the

surface lose KE to liquid molecules and merge

Ways to Increase Evaporation Rate• Increase temperature

– Kinetic energy increases which increases the number of high-energy molecules that can escape from liquid state

• Increase surface area of liquid

– Increases the likelihood of molecules escaping to air

• Remove water vapor from surface of the liquid

– Prevents return of vapor molecules to liquid state

• Reduce pressure on liquid

– Reduces one of the forces holding molecules in liquid state

Relative Humidity

• Ratio of how much water vapor is in the air to how much water vapor could be in the air at a certain temperature

• Expressed as a percentage

Water vapor in air

Capacity at present temperatureRelative Humidity = X 100 %

Heat Transfer

Heat flow

Three mechanisms for heat transfer due to a temperature difference

1. Conduction2. Convection3. Radiation

Natural flow is always from higher temperature regions to cooler ones

Conduction

• Heat flowing through matter

• Mechanism– Hotter atoms collide

with cooler ones, transferring some of their energy

– Direct physical contact required; cannot occur in a vacuum

• Poor conductors = insulators (Styrofoam, wool, air…)

Conduction is the flow of heat directly through a physical material.

Experimentally, it is found that the amount of heat Q that flows through a rod:

• increases proportionally to the cross-sectional area A

• increases proportionally to the temperature difference from one end to the other

• increases steadily with time

• decreases with the length of the rod

16-6 Conduction, Convection, and Radiation

Combining, we find:

The constant k is called the thermal conductivity of the rod.

Some typical thermal conductivities:

Substances with high thermal conductivities are good conductors of heat; those with low thermal conductivities are good insulators.

Convection

• Energy transfer through the bulk motion of hot material

• Examples– Space heater– Gas furnace

(forced)

• Natural convection mechanism - “hot air rises”

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.

Radiation

• Radiant energy - energy associated with electromagnetic waves

• Can operate through a vacuum • All objects emit and absorb radiation• Temperature determines

– Emission rate– Intensity of emitted light– Type of radiation given off

• Temperature determined by balance between rates of emission and absorption– Example: Global warming

Electromagnetic Spectrum

• Transverse waves• Regenerating co-oscillation of

electric and magnetic fields• Electric, magnetic and velocity

vectors mutually perpendicular

• Form when electric charge is accelerated by external force

• Frequency depends on acceleration of charge– Greater the acceleration, higher

the frequency

Blackbody radiation

Blackbody– Ideal absorber/emitter of

light– Radiation originates from

oscillation of near-surface charges

Increasing temperature– Amount of radiation

increases– Peak in emission

spectrum moves to higher frequency

Spectrum of the Sun

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.

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.

Thermodynamics

Thermodynamics • The study of heat and

its relationship to mechanical and other forms of energy

• Thermodynamic analysis includes – System– Surroundings

(everything else)– Internal energy (the

total internal potential and kinetic energy of the object in question)

Heat engines - devices converting heat into mechanical energy

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.

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.

The First Law of Thermodynamics

If a system does work on the external world, and no heat is added, its internal energy decreases.

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.

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.

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.

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.

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 cold

reservoir (the inside of the refrigerator) and exhausts to the kitchen.

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.

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.

Entropy

For 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.

Second law: Entropy• Real process =

irreversible process • Measure of disorder =

entropy Second law, in these

terms: • The total entropy of the

Universe continually increases

• Natural processes degrade coherent, useful energy– Available energy of the

Universe diminishing – Eventually: “heat

death” of the Universe • Direction of natural

processes– Toward more disorder– Spilled milk will never

“unspill” back into the glass!

18-8 Entropy

A 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.

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.

18-8 Entropy

As 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.

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.

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.

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.

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.

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 A

B) Substance B

C) Both require the same amount of heat.

D) Answer depends on the density of each substance.

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.

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.

Q4. The transfer of energy from molecule to molecule is called

A) convection.

B) radiation.

C) conduction.

D) equilibrium.