• Support PF! Buy your school textbooks, materials and every day products Here!

Cyclic processes

  • Thread starter ariana0923
  • Start date
  • #1
15
0

Homework Statement



Over several cycles, a refrigerator does 1.51 x 10^4 J of work on the refrigerant. The refrigerant in turn removes 7.55 x 10^4 J as heat from the air inside the refrigerator.

a. how much energy is transferred as heat to the outside air?
b. what is the net change in the internal energy of the refrigerant?
c. what is the amount of work done on the air inside the refrigerator?
d. what is the net change in the internal energy of the air inside the refrigerator?


Homework Equations



U=Internal Energy
Q=Heat
W=Work

ΔU = Q-W
ΔUnet = 0
Qnet=Wnet


The Attempt at a Solution



I'm trying to teach myself physics, so I think I'm getting really confused here. First of all, I get mixed up on when to put negative signs in front of numbers or if I even have to in cyclic processes.

I got the right answers (given by my online school) but it may have just been luck.

**I only need help with a and d and somewhat c


a. -7.55 x 10^4 = (1.51 x 10^4 - x)
-9.06 x 10^4= -x
x=9.06 x 10^4 J

Did i do that right at all?

b. I know ΔUnet=0 J in cyclic processes

c. no work done b/c ΔV=0 (how would i know that this is so?)

d. the answer is ΔU= -7.55 x 10^4 but I'm not sure how to mathematically get it
 
Last edited:

Answers and Replies

  • #2
211
0
a) If you remove heat from the fridge, you have to dump it outside. It should just be 7.55*10^4 J

d) Internal energy is just a function of temperature. You remove heat, thus its considered a negative change.
 
  • #3
kuruman
Science Advisor
Homework Helper
Insights Author
Gold Member
8,428
1,925
c) When there is no volume change, there is no displacement of anything due to the force (pressure times area) exerted by the gas. Therefore the gas does no work when its volume is constant.
 

Related Threads for: Cyclic processes

  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
3
Views
6K
  • Last Post
Replies
6
Views
7K
  • Last Post
Replies
18
Views
2K
Replies
1
Views
13K
Replies
2
Views
4K
Replies
2
Views
12K
Top