soal-soal fisika

 

 

4-28. Sketch a Carnol cycle for an ideal gas on a (a) u-v diagram, (b) u·T diagram, (c)

u-h diagram, (d) p. T diagram.

ANSWER:

 

u – v diagram

 

 

 

 

 

 

 

 

 

u·T diagram

 

 

 

 

 

 

u-h diagram

 

 

 

 

 

 

                                                                                                    

 

 

p. T diagram.

 

 

 

 

 

 

 

 

 

 

 

 

 

4-30. A Carnot engine is operated between two heat reservoirs at temperatures of 400 K

and 300 K. (a) If the engine reoeives 1200 Cal from the reservoir at 400 K in each cycle,

how many Calories does it reject to the reservoir at 300 K? (b) If the engine is operated as

a refrigerator (i.e., in reverse) and receives 1200 Cal from the reservoir at300 K, how many

Calories does it deliver 10 the reservoir at 400 K? (c) How much work is done by the engine

in each case?

ANSWER:

a) given the heat that received by the engine at the host side is Q_H = 5024,16 Kj and determine the that discarded at the cold reseivor Qc. By knowing  T_H, T_C and Q_H  we get

Q_C = -Q_H

Given values for TH, TC, and QH into Eg. (1) to get Q_C

Q_C = -5024,16 Kj

                                                      = -3748, 12 Kj

                                                      = 900 Cal

 

(b) we know that  T_H = 400 K , T_C = 300 K and Q_H =  5024, 6 Kj.

Q_H = Q_C

                                                Q_H = 5024,16 Kj

                                                       = 6698,879998 Kj

                                                       = 1600 Cal

(c) in part (a) we calculate Q_C and by knowing Q_C we can use the Eq. to find W by,

                                                W = | Q_H – Q_C |                                                ………(2)

We have Q_H and Q_C,  so if we plug these values in Eq. (2)  we get

                                                W = | Q_H | - | Q_C |

                                                W = 1200 Cal – 900 Cal

                                                     = 300 Cal

4. 31. (a) Show that for Camel engines operating between the same high temperature reservoirs and different low temperature reservoirs, the engine operating over rhe largest

temperature difference has the greatest efficiency. (b) Is the more effective way to increase

the efficiency of a Carnol engine to increase the temperature of the hotter reservoir,

keeping the temperature of the colder reservoir constant, or vice versa? (c) Repeal pans

(a) and (b) to find the optimum coefficient of performance for a Carnot refrigerator.

 

ANSWER:

(a)        T_a1        = T_b1    = T

T_a2        = ¾ T

T_b2       = ½ T

Carnor machine b memiliki perbedaan suhu yang lebih besar dari Carnot machine a

µ_a         = ( 1 – T2/ T1 ) x 100%  

            = ( 1 – ¾ T / T ) x 100%

            = ¼  x 100 %

= 25%

µ_b         = ( 1 – T2/ T1 ) x 100%  

            = ( 1 – ½ T / T ) x 100%

= ½ x 100%

= 50%

(b) by lowering the high temperature

 

 

 

 

 

 

(c) koefisien kinerja (kp)

Kpa      =                                                                         

=

=

= 3

Kpb     =

=

 

=

= 1

Kpb > Kpa (Semakin besar perbedaan suhu semakin kecil Kp)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4-34. Calculate the efficiency and the coefficient of performance of the cycles shown in

(a) Problem 3-26, and (b) Problem 3- 27.

ANSWER :

µ and Kp from problem (a) 3.26 (b) 3.27

 

 

 

 

 

 

Proses a-b-c   

 Q₁       = 80J

  W       = 30J

Proses a-d-b

ΔW      = W_acb – W_adb

= 30J – 10J = 20J

Q₂        = Q1 – W

= 80J – 20 J = 60J

µ =  x 100%

   =   x 100%

   = 25%

K_p = 

      =  60J / 20J

      = 3

 

 

 

µ and Kp from problem (b) 3.27

 

 

 

 

 

 

 

 

 

. mengompres a – c adiabatic membutuhkan 1000J

. mengompres b – c membutuhkan panas 1500J keluar system

µ = (1000J / 1500J) x 100%

  = 66.7%

K_p = 1500J / 1000J

      = 1.5