Page 54 - BASIC CONCEPTUAL OF THERMOFLUID

Example 2

A system is allowed to do work amounting to 500 kNm whilst heat energy amounting to 800 kJ is
transferred into it. Find the change of internal energy and state whether it is an increase or
decrease.

Q in = + 500 kJ W out = + 800 kJ

ΣU = Q - W

ΣU = 500 -800
ΣU = -300

Exercise 1

1. During a complete cycle operation, a system is subjected to the following:
Heat transfer is 800 kJ supplied and 150 kJ rejected.
Work done by the system is 200 kJ.
Calculate the work transferred from the surrounding to the system.

2. A close system undergoes a process in which there is a heat transfer of 200 kJ from the
system to the surroundings. The work done from the system to the surroundings is 75 kJ.
Calculate the change of internal energy and state whether it is an increase or decrease.
Solution exercise 1

Question1

Q in = + 800 kJ W in = - (?) kJ

Q out = -150 kJ W out = + 200 kJ

SQ = SW
Q + Q out = W in + W out
in

+ 800 + (-150) = - W + 200
W = - 450 kJ

Question 2
Q in = - 200 kJ W out = + 75 k

ΣU = Q - W

50
ΣU = - 200 - (+75)