Spontaneity of Chemical and Physical Processes: The Second and Third Laws of Thermodynamics 1.
First and Second Laws of Thermodynamics
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Transcript of First and Second Laws of Thermodynamics
First and Second Laws of Thermodynamics
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RAT 11b
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Class Objectives
Understand and apply:work, energy, reversibility, heat capacityFirst and Second Laws of Thermodynamics
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Reversibility Reversibility is the ability to run a
process backwards and forwards infinitely without losses.
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Reversible Irreversible (no service fee) (5% service fee)Day Dollars Pounds Dollars PoundsMonday 100.00 40.00 100.00 38.00Tuesday 100.00 40.00 90.25 34.30Wednesday 100.00 40.00 81.45 30.95Thursday 100.00 40.00 73.51 27.93Friday 100.00 40.00 66.34 25.20
Each morning, dollars are converted to pounds.Each evening, pounds are converted to dollars.
Money analogy
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Using Excel, reproduce the previous table, except use a service charge of 10%.
Pair Exercise 1
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Reversibility and Energy
If irreversibilities were eliminated, these systems would run forever.Perpetual motion machines
Electric Current
Generator Motor
Voltage
Pump Turbine
Fluid Flow
Pressure
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Example: Popping a Balloon A “reversible process” can go in either
direction, but these processes are rare. Generally, the irreversibility shows up
as waste heat
Not reversible unless energy is expended
XNot reversible
without expendingenergy
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Sources of Irreversibilities Friction Voltage drops Pressure drops Temperature drops Concentration drops
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Basic Laws of Thermodynamics
First Law of Thermodynamicsenergy can neither be created nor
destroyedSecond Law of
Thermodynamicsnaturally occurring processes are
directional
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First Law of Thermodynamics One form of work may be
converted into another, or, work may be converted to heat, or, heat may be converted to work, but, final energy = initial energy
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2nd Law of Thermodynamics We intuitively know that heat flows
from higher to lower temperatures and NOT the other direction.i.e., heat flows “downhill” just like waterYou cannot raise the temperature in
this room by adding ice cubes. Thus processes that employ heat are
inherently irreversible.
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Heat/Work Conversions Heat transfer is inherently irreversible.
This places limits on the amount of work that can be produced from heat.
Heat can be converted to work using heat enginesJet engines (planes), steam engines
(trains), internal combustion engines (automobiles)
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Heat into Work
A heat engine takes in an amount of heat, Qhot, and produces work, W, and waste heat Qcold.
Nicolas Carnot (kar nō) derived the limits of converting heat into work.
High-temperatureSource, Thot
Low-temperatureSink, Tcold
HeatEngine
W
Qhot Qcold(e.g., flame) (e.g., cooling pond)
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Carnot Equation: Efficiency Given the heat engine on the previous
slide, the maximum work that can be produced is governed by:
where the temperatures are absolute temperatures.
Thus, as Thot Tcold, Wmax 0. This ratio is also called the efficiency, .
hot
cold
hot
max
TT
QW
1
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Pairs Exercise 2 Use Excel to create a graph
showing the amount of work per unit heat for a heat engine in which the source temperature increases from 300 K to 3000 K and the waste heat is rejected to an ambient temperature of 300 K.
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Work into Heat Although there are limits on the
amount of heat converted to work, work may be converted to heat with 100% efficiency.
This is shown by Joule’s experiment…
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Joule’s ExperimentJoule’s Mechanical Equivalent of Heat
F
m
x
This proved 1 kcal = 4,184 J
1 kg H2O
T = 1oC
E = Fx = 4,184 J
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Where did the energy go? By the First Law of
Thermodynamics, the energy we put into the water (either work or heat) cannot be destroyed.
The heat or work added increased the internal energy of the water.
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Internal Energy
Translation
Rotation
Vibration
MolecularInteractions
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Heat Capacity An increase in internal energy
increases the temperature of the medium.
Different media require different amounts of energy to produce a given temperature change.
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Heat Capacity Defined Heat capacity: the ratio of heat, Q,
needed to change the temperature of a mass, m, by an amount T:
Sometimes called specific heatTm
QC
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Heat Capacity for Constant Volume Processes (Cv)
Heat is added to a substance of mass m in a fixed volume enclosure, which causes a change in internal energy, U. Thus,
Q = U2 - U1 = U = m Cv TThe v subscript implies constant volume
Heat, Qaddedm m
Tinsulation
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Heat Capacity for Constant Pressure Processes (Cp)
Heat is added to a substance of mass m held at a fixed pressure, which causes a change in internal energy, U, AND some PV work.
Heat, Qadded
T
m m
x
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Cp Defined Thus,
Q = U + PV = H = m Cp TThe p subscript implies constant pressure
Note: H, enthalpy. is defined as U + PV, so dH = d(U+PV) = dU + VdP + PdV
At constant pressure, dP = 0, so dH= dU + PdV
For large changes at constant pressure H = U + PV
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Experimental Heat Capacity
Experimentally, it is easier to add heat at constant pressure than constant volume, thus you will typically see tables reporting Cp for various materials (Table 21.2 in Foundations of Engineering).
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Pair Exercise 3
1. Calculate the change in enthalpy per lbm of nitrogen gas as its temperature decreases from 500 oF to 200 oF.
2. Two kg of water (Cv=4.2 kJ/kg K) are heated using 200 Btu of energy. What is the change in temperature in K? In oF?