Second Law of Thermodynamics
Engines and
Refrigerators
1
Heat Engine
• Any device that transforms heat
partly into work or mechanical
energy
*working substance – matter
inside the engine which undergoes
inflow and outflow of heat,
expansion and compression, and
sometimes phase change
2
how it works
1.
2.
3.
Young
Working substance
absorbs heat from
the hot reservoir
Performs some
mechanical work
Discards the
remaining energy
in the form of heat
into the cold
reservoir
*cyclic process
3
1st law applied to WS
QH>0: WS absorbs heat from HR
QC<0: WS discards heat into CR
W>0: WS performs mechanical work
U=0 after one cycle
1st law: Q = U + W
Qnet = W
QH + QC = W
|QH| - |QC| = |W|
4
thermal efficiency
e = W/QH
= (|QH|-|QC|)/|QH|
= 1 –(|QC|/QH|)
5
Refrigerator
• takes heat from a cold place and
gives of heat to a warmer place
• requires a net input of mechanical
work
working substance – refrigerant
fluid
cold reservoir – inside of refrigerator
hot reservoir – outside of
refrigerator
6
how it works
1.
2.
3.
fluid absorbs heat
from the cold
reservoir
work is done on
the fluid
energy from heat
transfer and work
done is discarded
into the hot
reservoir
Young
*cyclic process
7
Young
1. the fluid in the evaporator
coil is colder than the
inside of the ref, so it
absorbs heat
2. the compressor takes in
fluid and compresses it
adiabatically (work is done
on the fluid)
3. fluid is delivered to
condenser at high
pressure, fluid temperature
is higher than that of
surrounding air, fluid gives
of heat and condenses
4. fluid expands adiabatically
(expansion valve) into
the evaporator and cools
considerably
8
1st law applied to fluid
QC>0: fluid absorbs heat from CR
QH<0: fluid discards heat into HR
W<0: work is done on fluid
U=0 after one cycle
1st law: Q = U + W
Qnet = W
QH + QC = W
-|QH| + |QC| = -|W|
|QC| + |W| = |QH|
9
coefficient of performance
K = |QC|/|W|
= |QC|/(|QH|-|QC|)
10
• A heat engine takes in 1200 J of
heat from the high-temperature
heat source in each cycle and
does 400 J of work in each cycle.
How much heat is released into
the environment in each cycle?
A. 400 J
B. 800 J
C. 1200 J
D. 1600 J
• A heat engine takes in 1200 J of
heat from the high-temperature
heat source in each cycle and
does 400 J of work in each cycle.
What is the efficiency of this
engine?
A. ¼
B. 1/3
C. 3
D. 4
• In one cycle, a heat engine takes
in 900 J of heat from a hightemperature reservoir and
releases 600 J of heat to a lowertemperature reservoir. How much
work is done by the engine in
each cycle?
A. 300 J
B. 600 J
C. 900 J
D. 1500 J
• In one cycle, a heat engine takes
in 900 J of heat from a hightemperature reservoir and
releases 600 J of heat to a lowertemperature reservoir. What is its
efficiency?
A. 3/9
B. 6/9
C. 3/15
D. 6/15
• In one cycle a heat engine does
400 J of work and releases 500 J
of heat to a lower-temperature
reservoir. How much heat does it
take in from the highertemperature reservoir?
A. 100 J
B. 400 J
C. 500 J
D. 900 J
• In one cycle a heat engine does
400 J of work and releases 500 J
of heat to a lower-temperature
reservoir. What is the efficiency
of the engine?
A. 4/9
B. 4/5
C. 5/9
D. 1
• In one cycle, a heat engine takes
in 1000 J of heat from a hightemperature reservoir, releases
600 J of heat to a lowertemperature reservoir, and does
400 J of work. What is its
efficiency?
A. ¼
B. 1/3
C. 2/5
D. 3/5
• A heat pump takes in 300 J of heat
from a low-temperature reservoir in
each cycle and uses 150 J of work per
cycle to move the heat to a highertemperature reservoir. How much heat
is released to the higher-temperature
reservoir in each cycle?
A. 150 J
B. 300 J
C. 450 J
D. 600 J
• A heat pump takes in 300 J of
heat from a low-temperature
reservoir in each cycle and uses
150 J of work per cycle to move
the heat to a higher-temperature
reservoir. What is the coefficient
of performance (COP)?
A. 1/3
B. 1/2
C. 2
D. 3
• In each cycle of its operation, a
refrigerator removes 18 J of heat
from the inside of the refrigerator
and releases 30 J of heat into the
room. How much work per cycle
is required to operate this
refrigerator?
A. 12 J
B. 18 J
C. 30 J
D. 48 J
• In each cycle of its operation, a
refrigerator removes 18 J of heat
from the inside of the refrigerator
and releases 30 J of heat into the
room. What is the COP of this
refrigerator?
A. 18/12
B. 30/12
C. 12/30
D. 18/30