Question

Physics 2

1. A 7.5 kg box that has an initial speed of 4.0 m / s slides along a rough board and reaches rest. Estimate the total change in the entropy of the universe. Assume that all objects are at room temperature (293 K)

2. What is the efficiency of a Carnot engine that is operating between a hot reserve at 800 K and a cold reserve at 400 K?

Answer 1

1) In this problem, the initial kinetic energy of the box is:KE

we know KE=1/2 ×M *V² ......................(A)

where M=7.5 kg

and V= 4 .0 m/s

and T=293 K

put the all value on equation number (A)

KE=1/2 ×M *V²

  =1/2 ×7.5*(4)²  

=60 J

At the end of the process, the kinetic energy of the box is zero, and all that initial kinetic energy has been transferred to the surroundings as heat,

The entropy change of the universe is then given by:

  

  

  the total change in the entropy of the universe is =0.205 J/K

2)The formula of efficiency of Carnot engine is

   OR

  

Where T_H Is a highest possible heat source =800k

and TL = the lowest heat sink temperature =400 K

Put all value on equation number (2)

  

​​​​​​​=0.5= 50%

the efficiency of a Carnot engine that is operating between a hot reserve at 800 K and a cold reserve at 400 K is =50%