The diagram shows a heat engine. In which area of the diagram is unusable thermal energy detected?
W
X
Y
Z

Z is the area of the diagram which has unusable thermal energy
detected.
This is also referred to as heat energy and it is responsible for
temperature differences in a body or area.
The Z region which is denoted by blue is referred to as the cold
region while the W region is the hot region. We know that since
heat is usually lost to the surroundings , then heat has been lost
to the Z region which is unusable and detected
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