the 2nd law of thermodynamics · chapter 11: the 1st law of thermodynamics thermodynamics: the...

Chapter 11: The 1st Law of

ThermodynamicsThermodynamics: the study of processes in

which energy is transformed as heat and as

work. (“movement of heat”)

Recall: Work is done when energy is

transferred from one body to another by

mechanical means. (Ex: compressing a gas.)

Heat is a transfer of energy due to a difference

in temperature. (Ex: Heating a gas.)

Lets start with what you know…

In this video, what is energy being

transferred to the wheel from?

What needed to exist prior to the wheel

spinning?

https://www.youtube.com/watch?v=v_TsW

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https://www.youtube.com/watch?v=PsdpA

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Heat Engines Defined

A heat engine is any device that changes

thermal energy (heat) into mechanical energy

(work), such as steam engines and automobile

engines.

Any device that uses heat to do work.

Note: it is easier to produce heat (thermal nrg)

from work (rub your hands together) than it is

to produce work from thermal nrg.

Heat Engine Process

Heat is supplied to the engine at relatively high

temperatures from a place called the hot reservoir

(the hot fuel).

Part of the heat input is used to perform work via the

working substance of the engine—which is the

material w/in the engine that actually does the

work. (Ex: gas/air mixture in combustion

chamber.)

The remainder of the input heat is rejected at a

temperature lower than the input temperature to a

place called the cold reservoir (or heat sink). (Ex:

radiator or exhaust). This is waste heat!

Schematic of Heat Engine, Fig. 15.12

This schematic

representation of a heat

engine shows the input heat

(QH) that originates from

the hot reservoir, the work

(W) that the engine does,

and the heat (QC) that the

engine rejects to the cold

reservoir.

Work is only done when

heat is transferred from

high temp. to low temp.Figure 15.12

Power plants: one example of a Heat

Engine

Power plants use the heat from the burning of coal, oil, gas, or heat from nuclear fission to produce energy that does work by turning electric generators.

Waste heat is also produced, sometimes referred to “heat pollution” because it pollutes the environment.

Waste heat discharged into waterways increases the temperature of that waterway.

Waste heat discharged into air can contribute to weather changes.

Efficiency (e) of a Heat Engine

Efficiency = ratio of work the heat engine does

to the heat input at high temp.

e = (work done) / (Input heat) = W / QH

can multiply this value by 100 to get percent

efficiency

The less heat needed to do the work = greater

efficiency

An engine that is 100% efficient would have

an e value of 1 (or 100%), meaning that QH =

W (non-existent)

Heat engines must obey Conservation of

Energy Law

(1st Law of Thermodynamics), so:

QH = W + QC (1st Law)

W = QH – QC

Substitute into efficiency

equation:

e=W/QH = (QH – QC)/QH

e = 1 – (QC / QH)

Or e = 1 – (Tc / Th )

The smaller QC is, the

greater the efficiency (less

heat wasted).

Figure 15.12

Operation of a Heat Engine

Slide 11-26

Answer

Consider your body as a system. Your body is “burning” energy in food, but staying at a constant temperature. This means that, for your body,

A. Q > 0.

B. Q = 0.

C. Q < 0.

Slide 11-40

The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency?

A. 300°C 30°C

B. 250°C 30°C

C. 200°C 20°C

D. 100°C 10°C

E. 90°C 0°C

Additional Questions

Slide 11-41

Answer

The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency?

A. 300°C 30°C

B. 250°C 30°C

C. 200°C 20°C

D. 100°C 10°C

E. 90°C 0°C

Slide 11-42

4 stroke engine cycle

Internal Combustion Engine

http://www.animatedengines.com/

Refrigerators, Air Conditioners &

Heat Pumps If work is done on a

system, heat can be forced

to flow from cold to hot!

In general, this is called a

refrigeration process.

Work (W) is used to

remove heat (QC) from cold

reservoir and deposit heat

into hot reservoir (QH)

Operation of a Heat Pump

Slide 11-29

Refrigerator and Air Conditioner

Checking Understanding: Increasing Efficiency of a Heat PumpWhich of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.

A. All of the aboveB. 1 and 4C. 2 and 3

Slide 11-31

Answer

Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.

A. All of the aboveB. 1 and 4C. 2 and 3

Slide 11-32

The Theoretical Maximum Efficiency of aHeat Engine

Slide 11-27

Note: This equation does not depend on the type of fuel used in the heat engine nor is that important.

Slide 11-23

Closed system: ΔU = Q + W (Only nrg

exchanged)

Open system: ΔU = Q + W (but need to take

into account the change in internal nrg due to the

increase/decrease in the amount of matter)

Isolated system: ΔU = Q + W = 0 (since no nrg

in any form can leave or enter the system… i.e.,

ΔU=0.)

Energy is conserved in this process:

QH = W + QC

This process is reversible.

The inside of a refrigerator is the cold

reservoir, the outside is the hot reservoir. The

warm air you feel blowing out of your frig is

equal to the heat removed from inside your frig

plus the work done in removing it. QH = W +

QC

The same holds for air conditioner:

house=inside of frig, etc.

Refrigeration Process:

So, is it possible to cool your kitchen by leaving your

frig door open (or placing an air conditioning unit on

the floor in the middle of the room)?

NO!

Heat is removed from inside frig and exhausted into

kitchen, plus the work to remove this heat is also

exhausted… result?

Your kitchen gets warmer when you open the frig

door!

Heat is removed from room by A/C unit, but exhausted

back into room, in addition to the work to remove this

heat is also exhausted! Doesn’t cool the room; heats it

up and wastes electricity!!

Reading Quiz

3. A refrigerator is an example of a

A. reversible process.B. heat pump.C. cold reservoir.D. heat engine.E. hot reservoir.

Slide 11-10

Answer

3. A refrigerator is an example of a

A. reversible process.B. heat pump.C. cold reservoir.D. heat engine.E. hot reservoir.

Slide 11-11

Checking Understanding: Increasing Efficiency of a Heat PumpWhich of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.

A. All of the aboveB. 1 and 4C. 2 and 3

Slide 11-31

Answer

Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.

A. All of the aboveB. 1 and 4C. 2 and 3

Slide 11-32

Refrigeration Cycle… how does a

refrigerator or A/C work anyway?1. Low pressure gas enters compressor where it becomes

high pressure, hot gas.

2. The gas goes into condenser coil (behind or under the frig) where heat* is removed (by coil) and cold (high pressure) fluid comes out (gas liquid: releasing heat in process). *Part of this heat is from what was extracted from inside frig.

3. The high pressure fluid goes through an expansion valve which allows the pressure (& temp.) to drop.

4. The low pressure (cold) fluid goes into evaporator coils in frig which turns into a gas as energy is absorbed from inside frig.

5. Back to the compressor… step 1.

Refrigeration Cycle

Heat Pumps

The air conditioner and heat pump do closely

related jobs. The air conditioner refrigerates

the room and heats up the outdoors, while the

heat pump refrigerates the outdoors and heats

up the inside room.

A heat pump can serve in a dual capacity—

equipped w/a switch that converts them from

A/C units in summer to heaters in winter…

cool!

Heat PumpsA heat pump takes heat

from QC and with the

addition of work, places it

in QH.

QH = QC + W Heat is removed from inside frig (or

outside of house where it’s cold) and

exhausted into kitchen (or inside your

house), plus the work to remove this

heat is also exhausted… result?

Your kitchen (inside of house) gets

warmer

(Remember… opening the frig door

causes the kitchen to heat up!)

What’s more efficient, a heat pump or

conventional electric heating system? A heat pump is very efficient

b/c if 1000J is used to do work of moving QC from outside to inside, you get heat totaling QH=QC+W, or QH=QC+1000J inside!

Your heat pumps 1000J + the Qc taken from the outdoors!

Versus, if you have an electric heater and 1000J of electric nrg is being delivered, then all you get is 1000J of heat, QH.

conventional electric heating system

Does it take more or less work the greater the

temperature difference when heating your

house with a heat pump?

More… it takes more work to pump heat

“uphill” from cold reservoir (outside) to hot

reservoir (inside). Remember, heat naturally

flows from hot to cold, so going from cold to

hot is an “uphill climb” for the heat. http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cf

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