1 thermodynamics. 2 a few reminders temperature determines the direction of flow of thermal energy...
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Thermodynamics
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A few reminders
TEMPERATURE determines the direction of flow of thermal energy between two bodies in thermal equilibrium
HOT COLD
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A few reminders
TEMPERATURE is also a measure of the average kinetic energy of particles in a substance
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A few reminders
INTERNAL ENERGY is the sum of the kinetic energy and potential energies of particles in a substance
K.E. + P.E.
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Internal energy
The sum of the KE and PE of the particles in a system
NOTE, THIS IS NOT THE SAME AS THE TOTAL ENERGY.
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A few reminders
In an ideal gas, the INTERNAL ENERGY is all kinetic energy.
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What is THERMODYNAMICS?A study of the connection between
thermal energy entering or leaving a system and the work done on or by the system.
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A few words to consider
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Thermodynamic system
The system/machine that we are considering the flow of heat energy in/out of and work done on/by the system.
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The surroundings
Everything else!
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Heat
The quantity of heat/thermal energy (transferred by a temperature difference).
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Work
The energy transferred (changed)
E.g. Work = Force x distance
or
Work = VIt
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Example
Finding the work done on or by a gas when it expands at constant pressure (i.e. a small change in volume!)
(most of the systems we consider will involve the compression or expansion of gases under different conditions)
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Work done by a gas (constant pressure)
Work = force x distanceWork = force x Δx
(Pressure = F/A so F = PA)
Work = PAΔx
(AΔx = ΔV)
Work = pΔV
P
Δx
A
P
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The 1st law of thermodynamics
Q = ΔU + W
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The 1st law of thermodynamics
Q = ΔU + W
Q = The thermal energy given to a system (if this is negative, thermal energy is leaving the system)
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The 1st law of thermodynamics
Q = ΔU + W
ΔU = The increase in internal energy (if this is negative the internal energy is decreasing)
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The 1st law of thermodynamics
Q = ΔU + W
W = The work done on the surroundings (if this is negative the surroundings are doing work on the system)
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The 1st law of thermodynamics
Q = ΔU + W
This is really just another form of the principle of energy conservation
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Ideal gas processes
In most cases we will be considering changes to an ideal gas (this will be the “system)
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pV diagrams and work done
Changes that happen during a thermodynamic process can usefully be shown on a pV diagram
p
V
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pV diagrams and work doneThe area under the graph represents
the work donep
V
A
B
This area represents the work done by the gas (on the surroundings) when it expands from state A to state B
What happens if the gas is going from state B to A?
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ISOCHORIC (isovolumetric) processes
These take place at constant volume
V = constant, so p/T = constant
Q = negative
ΔU = negative
W = zero
p
V
A
B
Isochoric decrease in pressure
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ISOBARIC processes
These take place at constant pressure
p = constant, so V/T = constant
Q = positive
ΔU = positive
W = positive
p
V
A B
Isobaric expansion
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ISOTHERMAL processes
These take place at constant temperature
T = constant, so pV = constant
Q = positive
ΔU = zero
W = positive
p
V
A
B
Isothermal expansion
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ADIABATIC processes
No thermal energy transfer with the surroundings (approximately a rapid expansion or contraction)
Q = zero
ΔU = negative
W = positive
p
V
A
B
Adiabatic expansion
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Heat engines and heat pumps
A heat engine is any device that uses a source of heat energy to do work.
Examples include the internal combustion engine of a car.
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Heat engine
Below is a generalised diagram showing the essential parts of any heat engine.
Hot reservoir
Thot
Cold reservoir
TcoldThermal energy Qhot
Thermal energy
Qcold
Work done
ΔW
Engine
“Reservoir” implies a constant heat source
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A simple example of using an ideal gas in a heat engine
p
V
Isobaric expansion
Isovolumetric decrease in pressure
Isobaric compression
Isovolumetric increase in pressure
Heat in
Heat out
Area = work done by gas
ΔU = (3/2)nRΔT
Heat out
Heat in A B
CD
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Let’s read!
Page 191 to 192 “An example of a heat engine”
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Heat pump
Simply a heat engine run in reverse! (Put work in, transfer heat from cold reservoir to hot reservoir)
Hot reservoir
Thot
Cold reservoir
TcoldThermal energy Qhot
Thermal energy
Qcold
Input work
ΔW
Engine
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Heat pump
p
V
Isobaric compression
Isovolumetric increase in pressure
Isobaric expansion
Isovolumetric decrease in pressure
Heat out
Heat in
Area = work done on gas
Heat in
Heat out
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Questions
Page 193 Questions 1 to 5 Page 194 Questions 10
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2nd Law of Thermodynamics and entropy
There are many ways of stating the 2nd law, below is the Kelvin-Planck formulation
“No heat engine, operating over a cycle, can take in heat from its surroundings and totally convert it totally into work” (some heat has to be transferred to the cold reservoir)
This is possible in a single process however
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2nd Law of Thermodynamics and entropyOther statements of the 2nd law
include• No heat pump can transfer thermal
energy from a low temperature to a higher temperature reservoir without work being done on it (Clausius)
• The entropy of the universe can never decrease
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Entropy This is a measure of the
disorder of a system Most systems, when left,
tend towards more disorder (think of your bedroom!
This is why heat spreads from hot to cold.
Entropy can decrease in a small part of a system
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Entropy
Thot TcoldΔQ
Decrease in entropy = Q/Thot
Increase in entropy = Q/Tcold
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1st and 2nd laws These laws MUST apply in all
situations A refrigerator does transfer heat from
cold to hot, but work must be done (electricity supplied and some converted into heat) to do this
A boat could use the temperature difference between the sea and atmosphere to run, but eventually the two reservoirs would reach the same temperature
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Degradation
The more spread energy becomes, the less useful it is. The heat produced in the brakes of a car is still energy, but not really in a useful form. We call this energy degradation
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That’s it!
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Now let’s try some questions
Page 193Questions 1 to 5Page 194Questions 10 to 13.
Let’s also have a test on 4th November