I find it hard to analyze the system (1)+(2), the system (3)+(4), and the system (6).

thank u in ad:)

*Th8,9 Question 8-9 refer to the following processes involving systems labeled by numbers (1) through (8).

A bar of iron (1) at 300K is brought into thermal contact with a body (2) at 400K, the two being thermally isolated from all other systems.

An ideal gas (3) is compressed reversibly while in contact with a reservoir (4), the two being thermally isolated from all other systems.

A body of water (5) freezes reversibly.

A container of water (6) is stirred and its temperature increases by 1K.

A chemical reaction takes place in an isolated system (7).

A Carnot engine (8) operates in a cycle.

8. For which of the following systems does the entropy decrease?

(A) 1

(B) 4

(C) 5

(D) 6

(E) 7

9. For which of the following systems does the entropy increase?

(A) 2

(B) 3

(C) 8

(D) 1 and 2 combined

(E) 3 and 4 combined

## one thermodynamic problem

from

dS = dQ/T

since Q = m c dT, once u integrate it, u get mc ln(T_2/T_1)

assuming that the bodies are the same mass, you can calculate the

final temperature T_2 from averaging the two initial temperatures ..

(T_1 is the initial temperature of each object)

for sys 3 and 4, you have an ideal gas expanding in an isothermal

process. this means that the internal energy is zero. (to wit: u = mc

delta T for ideal gas) thus, by the first law, you have dq = dw) work

for isothermal process is just P_1V_1 ln (V_2/V_1) ... there are

different forms to this formula if you recall that because the

process is isothermal, by the ideal gas equation, u have nRT_1 =

nRT_2 = P_1V_1 = P_2V_2

(so.... once u have dq ... plug it into the dS equation above)

(so.... once u have dq ... plug it into the dS equation above)

PS: P_1 stands for P subscript 1 ...

ok hopefully this is clear.