Thermodynamics
Heat and Temperature
First Law of Thermodynamics and Energy• Efficiency
Second Law of Thermodynamics and time (!)• Even MORE about Efficiency (temperature’s effect)
Unit 03 – Thermodynamics – Slide 1
Temperature Scales
• Heat – technically different
than internal energy
Unit 03 – Thermodynamics - Slide 2
• Temperature – how fast
molecules are moving
- Celsius (oC)
- Fahrenheit (oF)
- Kelvin (K)
Ex 3.1: Temperature
It is 25oC outside on a spring day.
What is the temperature in Kelvin?
What is the temperature in
Fahrenheit?
Unit 03 – Thermodynamics - Slide 3
Temperature and Heat
Heat (Energy) in Temp Up
Unit 03 – Thermodynamics - Slide 4
But how much?
Specific heat (c)
QIN = m*c*DT
Ex 3.2: Specific Heat
How many Joules of energy does it
take to raise the temperature of
5kg of aluminum 30oC?
Unit 03 – Thermodynamics - Slide 5
How about 5kg of
water?
Melting and Boiling
Takes energy to change state
Unit 03 – Thermodynamics - Slide 6
Solid Liquid (fusion)
Latent heat (L)
QIN = m*L
Liquid Gas (evaporation)
Ex 3.3: Latent Heat
How many Joules of energy does it
take to boil our 5kg water?
Unit 03 – Thermodynamics - Slide 7
(Note no change
in temperature.)
First Law of Thermodynamics
Energy is conserved
Unit 03 – Thermodynamics - Slide 8
Heatin – Heatout =
Work Done + Change in
Internal Energy
Qin – Qout = W + DEint
Thermodynamics Engines
W = 30 Joules
QH=100 Joules
QC=70 Joules
Power Plant
Low Temperature
High Temperature30 Joules
100 Joules e = 0.3 =
|All Arrows OUT| = |All arrows IN|
Unit 03 – Thermodynamics - Slide 9
Example 3.4: Efficiency
Unit 03 – Thermodynamics - Slide 10
Your car is around 25% efficient. If a
gallon of gas contains 1.3 X 108 Joules of
energy, how much heat is exhausted into
the air for every gallon of gas burned?
Example 3.5: Efficiency
Unit 03 – Thermodynamics - Slide 11
An coal fired power plant produces 2.5
Gigawatts of electrical power (2.5 GW(e))
and exhausts 4GW of excess heat into the
lake.
What is the total how many Joules/second
from coal must be put into the plant?
What is the efficiency
of the plant?
Question
Unit 03 – Thermodynamics - Slide 12
An coal fired power plant produces 2.5
Gigawatts of electrical power (2.5 GW(e)) and
exhausts 4GW of excess heat into the lake.
What is the efficiency of the plant?
A) 25%
B) 38%
C) 45%
D) 62.5%
E) 80%
Thermodynamics Engines
W = 2.5 Gigawatts
QH= ?? Gigawatts
QC= 4 Gigawatts
Power Plant
Low Temperature
High Temperature
2.5 Gigawatts
6.5 Gigawatts e = 0.38 =
Unit 03 – Thermodynamics - Slide 13
Question
A. 0.3 Gigawatts
B. 0.7 Gigawatts
C. 0.9 Gigawatts
D. 2.1 Gigawatts
E. 7.0 Gigawatts
An coal fired power plant produces
three (3) Gigawatts of electrical power
and has an efficiency of 0.30 and is
cooled using lake water. How much
thermal energy is put into the lake every
second?
Unit 03– Thermodynamics – Slide 14
Coal fired power plant
Is all about how you make steam to
turn the crank of the generator!
To
your
home
Unit 03 – Thermodynamics - Slide 15
You place a 80oC mug of coffee
inside an insulated container that
has an original air temperature of
24oC.
Unit 06 Entropy, Slide 16
80
24 0C 30 0C
30
10 0C
85
?
or
Second Law of Thermodynamics
No engine can
be 100%
efficient
Unit 03 – Thermodynamics - Slide 17
Heat goes from hot to cold
(unless you do work)
Low Temperature
Thermodynamics Engines
W
QH
TC
Power Plant
Low Temperature
High Temperature
TC
TH
emax = 1 -
Unit 03 – Thermodynamics - Slide 18
TH
QCThis is emax
Ex 3.6: Maximum Efficiency
An coal fired power plant
brings in its steam at
200oC and exhausts its
excess heat into the lake
water at 35oC. What is
the maximum efficiency
of the power plants?
Unit 03 – Thermodynamics - Slide 19
Question
A. Make the input to your engine hotter
B. Make the input to your engine colder
C. Make the exhaust from your engine colder
D. Make the exhaust from your engine hotter
You’re builder of an Indy
500 race car. You want
your car to get the most
power it can from the
gasoline it burns. What are
you most likely to do?
Unit 03– Thermodynamics – Slide 20
“Combined Cycle” (two stages)
Unit 03 – Thermodynamics - Slide 21
An modern combined
cycle turbine can take
steam in at 427oC. What
is it’s maximum
efficiency if it exhausts
the steam at 35oC?
TC
TH
emax = 1 -
427oC
35oC
Thermodynamics Engines
W
QH
TC
Power Plant
Low Temperature
High Temperature
TC
TH
emax = 1 -
Unit 03 – Thermodynamics - Slide 22
TH
QC
Why do it???
You could run your
factories by damming
up a stream and using
water power.?
Unit 03 – Thermodynamics - Slide 23
If generating electricity is so incredibly
inefficient, why do we do it? Why don’t see
just use water power like we used to?
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