10 Hukum Termodinamika (3)

Hukum Termodinamika II

Mesin panas adalah sebuah peralatanyang mengambil energi oleh panas danberoperasi secara siklik, sebagianenergi digunakan untuk kerja.

Sebagai contoh adalah mesin yangmenghasilkan listrik dengan membakarbatubara atau bahan bakar lain, yangmenghasilkan gas temperatur tinggiuntuk mengubah liquid menjadi steam.Steam ini menggerakkan turbin yangkemudian menggerakkan generatorpenghasil listrik.

Peralatan lain yang dimodelkan sebagaimesin panas adalah menggunakanenergi dari pembakaran bahan bakaruntuk menggerakkan pistoon yangmenghasilkan gerak pada mobil.

a heat engine schematically

The engine absorbs a quantityof energy Qh from the hotreservoir. For this discussion ofheat engines, we will useabsolute values to make all energy transferspositive and will indicate the direction oftransfer with an explicit positive or negativesign. The engine does work Weng (so thatnegative work W = - Weng is done on theengine), and then gives up a quantity of energyQc to the cold reservoir.

Because the working substance goesthrough a cycle, its initial and finalinternal energies are equal, and soΔEint = 0. Hence, from the first law ofthermodynamicsand with no change in internal energy,the net work Weng done by a heatengine is equal to the net energy Qnettransferred to it.

If the working substance is a gas, thenet work done in a cyclic process is thearea enclosed by the curverepresenting the process on a PVdiagram.

The thermal efficiency e of a heatengine is defined as the ratio of the network done by the engine during onecycle to the energy input at the highertemperature during the cycle:

It is impossible toconstruct a heat enginethat, operating in acycle, produces noeffect other than theinput of energy by heatfrom a reservoir and theperformance of anequal amount of work.

An engine transfers 2.00 x 103 J ofenergy from a hot reservoir during acycle and transfers 1.50 x 103 J asexhaust to a cold reservoir.a) Find the efficiency of the engine.b) How much work does this engine do

in one cycle?

Heat Pumps and RefrigeratorsIn a heat engine, the direction of energytransfer is from the hot reservoir to thecold reservoir, which is the naturaldirection. The role of the heat engine isto process the energy from the hotreservoir so as to do useful work.

What if we wanted to transfer energyfrom the cold reservoir to the hotreservoir?Because this is not the natural directionof energy transfer, we must put someenergy into a device in order toaccomplish this. Devices that performthis task are called heat pumps orrefrigerators.

For example, wecool homes insummer using heatpumps called airconditioners. Theair conditionertransfers energyfrom the cool roomin the home to thewarm air outside.

It is impossible toconstruct a cyclicalmachine whose soleeffect is to transferenergy continuouslyby heat from oneobject to anotherobject at a highertemperature withoutthe input of energy bywork.

The effectiveness of a heat pump isdescribed in terms of a number calledthe coefficient of performance (COP). Inthe heating mode, the COP is definedas the ratio of the energy transferred tothe hot reservoir to the work required totransfer that energy

For a heat pump operating in the coolingmode, “what you gain” is energyremoved from the cold reservoir. The mosteffective refrigerator or air conditioner is onethat removes the greatest amount of energyfrom the cold reservoir in exchange for theleast amount of work. Thus, for these deviceswe define the COP in terms of |Qc|:

A certain refrigerator has a COP of 5.00.When the refrigerator is running, its powerinput is 500 W. A sample of water of mass500 g and temperature 20.0°C is placed inthe freezer compartment. How long does ittake to freeze the water to ice at 0°C?Assume that all other parts of the refrigeratorstay at the same temperature and there is noleakage of energy from the exterior, so thatthe operation of the refrigerator results only inenergy being extracted from the water.

The Carnot EngineNo real heat engineoperating between twoenergy reservoirs canbe more efficient thana Carnot engineoperating between thesame two reservoirs.

1. Process A B is an isothermal expansionat temperature Th . The gas is placed inthermal contact with an energy reservoir attemperature Th . During the expansion, thegas absorbs energy |Qh| from the reservoirthrough the base of the cylinder and doeswork WAB in raising the piston.

2. In process B C the base of the cylinderis replaced by a thermally nonconductingwall, and the gas expands adiabatically—thatis, no energy enters or leaves the system byheat. During the expansion, the temperatureof the gas decreases from Th to Tc and thegas does work WBC in raising the piston.

3. In process C D, the gas is placed inthermal contact with an energy reservoir attemperature Tc and is compressedisothermally at temperature Tc . Duringthis time, the gas expels energy |Qc| to thereservoir, and the work done by thepiston on the gas is WCD .

4. In the final process D A, the base of thecylinder is replaced by a nonconducting wall,and the gas is compressed adiabatically. Thetemperature of the gas increases to Th , andthe work done by the piston on the gas isWDA.

A heat engine takes in 360 J of energyfrom a hot reservoir and performs 25.0J of work in each cycle. Find (a) theefficiency of the engine and (b) theenergy expelled to the cold reservoir ineach cycle.

A particular heat engine has a usefulpower output of 5.00 kW and anefficiency of 25.0%. The engine expels8 000 J of exhaust energy in eachcycle. Find (a) the energy taken induring each cycle and (b) the timeinterval for each cycle.

Suppose a heat engine is connected to twoenergy reservoirs, one a pool of moltenaluminum (660°C) and the other a block ofsolid mercury ( - 38.9°C). The engine runs byfreezing 1.00 g of aluminum and melting 15.0g of mercury during each cycle. The heat offusion of aluminum is 3.97 x 105 J/kg; theheat of fusion of mercury is 1.18 x 104 J/kg.What is the efficiency of this engine?

Next meeting

Carnot Engine

Gasoline and Diesel Engines

1. During the intake stroke O : A, the piston movesdownward, and a gaseous mixture of air and fuel isdrawn into the cylinder at atmospheric pressure.In this process, the volume increases from V2 to V1 .This is the energy input part of the cycle—energyenters the system (the interior of the cylinder) aspotential energy stored in the fuel.

2. During the compression stroke A : B, the pistonmoves upward, the air–fuel mixture is compressedadiabatically from volume V1 to volume V2 , and thetemperature increases from TA to TB . The workdone on the gas is positive, and its value is equal tothe negative of the area under the curve AB

3. In process B : C, combustion occurs when the spark plugfires. This is not one of the strokes of the cycle because itoccurs in a very short period of time while the piston is at itshighest position. The combustion represents a rapidtransformation from potential energy stored in chemical bondsin the fuel to internal energy associated with molecular motion,which is related to temperature. During this time, the pressureand temperature in the cylinder increase rapidly, with thetemperature rising from TB to TC . The volume, however,remains approximately constant because of the short timeinterval. As a result, approximately no work is done on or by thegas.

4. In the power stroke C : D, the gas expandsadiabatically from V2 to V1. This expansion causesthe temperature to drop from TC to TD. Work is doneby the gas in pushing the piston downward, and thevalue of this work is equal to the areaunder the curve CD.

5. In the process D : A, an exhaust valve is openedas the piston reaches the bottom of its travel, and thepressure suddenly drops for a short time interval.During this interval, the piston is almost stationaryand the volume is approximately constant. Energy isexpelled from the interior of the cylinder andcontinues to be expelled during the next process.

6. In the final process, the exhaust stroke A : O, thepiston moves upward while the exhaust valveremains open. Residual gases are exhausted atatmospheric pressure, and the volume decreasesfrom V1 to V2. The cycle then repeats.

A refrigerator has a coefficient ofperformance equal to 5.00. Therefrigerator takes in 120 J of energyfrom a cold reservoir in each cycle. Find(a) the work required in each cycle and(b) the energy expelled to the hotreservoir.

A Carnot engine has a power output of150 kW. The engine operates betweentwo reservoirs at 20.0°C and 500°C.(a) How much energy does it take in perhour? (b) How much energy is lost perhour in its exhaust?

One of the most efficient heat engines everbuilt is a steam turbine in the Ohio valley,operating between 430°C and 1 870°C onenergy from West Virginia coal to produceelectricity for the Midwest.(a) What is its maximum theoretical efficiency(b) The actual efficiency of the engine is42.0%. How much useful power does theengine deliver if it takes in 1.40 x 105 J ofenergy each second from its hotreservoir?

• A heat engine operating between200°C and 80.0°C achieves 20.0% ofthe maximum possible efficiency.What energy input will enable theengine to perform 10.0 kJ of work?

• The exhaust temperature of a Carnotheat engine is 300°C. What is theintake temperature if the efficiency ofthe engine is 30.0%?

A Carnot heat engine uses a steam boiler at 100°Cas the high-temperature reservoir. The low-temperature reservoir is the outside environment at20.0°C. Energy is exhausted to the low-temperaturereservoir at the rate of 15.4 W.(a) Determine the useful power output of the heatengine.(b) How much steam will it cause to condense in thehigh temperature reservoir in 1.00 hλv = 2,26 x 106 J/kg

An electric power plant that would make useof the temperature gradient in the ocean hasbeen proposed. The system is to operatebetween 20.0°C (surface water temperature)and 5.00°C (water temperature at a depth ofabout 1 km). (a) What is the maximumefficiency of such a system? (b) If the usefulpower output of the plant is 75.0 MW, howmuch energy is taken in from the warmreservoir per hour?