First and Second Law of Thermodynamics

7/30/2019 First and Second Law of Thermodynamics

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First and Second Laws ofThermodynamics


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RAT 11b


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Class Objectives

Understand and apply:

work, energy, reversibility, heat capacity

First and Second Laws of Thermodynamics


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Reversibility

Reversibility is the ability to run aprocess 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 38.00

Tuesday 100.00 40.00 90.25 34.30

Wednesday 100.00 40.00 81.45 30.95

Thursday 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 theprevious table, except use a

service charge of 10%.

Pair Exercise 1


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Reversibility and Energy

Ifirreversibilities were eliminated, thesesystems 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 eitherdirection, but these processes are rare.

Generally, the irreversibility shows upas waste heat

Not reversible unless

energy is expended

XNot reversible

without expending

energy


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Sources of Irreversibilities

Friction

Voltage drops

Pressure drops

Temperature drops

Concentration drops


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Basic Laws ofThermodynamics

First Law of Thermodynamics

energy can neither be created nordestroyed

Second Law of Thermodynamics

naturally occurring processes aredirectional


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First Law of Thermodynamics

One form of work may be convertedinto 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 flowsfrom higher to lower temperatures and

NOT the other direction.i.e., heat flows downhill just like water

You cannotraise 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 usingheat engines

Jet 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 heatQcold.

Nicolas Carnot (kar n) derived the limits ofconverting heat into work.

High-temperature

Source, Thot

Low-temperature

Sink, Tcold

Heat

Engine

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 ThotTcold, Wmax 0.

This ratio is also called the efficiency, h.

hot

cold

hot

max

T

T

Q

W1


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Pairs Exercise 2

Use Excel to create a graph showingthe amount of work per unit heat for a

heat engine in which the sourcetemperature increases from 300 K to3000 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 amountof heat converted to work, work may be

converted to heat with 100% efficiency.

This is shown by Joules experiment


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Joules Experiment

Joules Mechanical Equivalent of Heat

F

m

Dx

This proved1 kcal = 4,184 J

1 kg H2O

DT= 1oC

E= FDx = 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 theinternal energy of the water.


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Internal Energy

Translation

Rotation

Vibration

Molecular

Interactions


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Heat Capacity

An increase in internal energy increasesthe temperature of the medium.

Different media require differentamounts of energy to produce a giventemperature change.


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Heat Capacity Defined

Heat capacity: the ratio of heat, Q, needed tochange the temperature of a mass, m, by an

amount DT:

Sometimes called specific heat

Tm

QC

D


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Heat Capacity for ConstantVolume Processes (Cv)

Heat is added to a substance of mass m in afixed volume enclosure, which causes a changein internal energy, U. Thus,

Q = U2 - U1 = DU= m CvDT

The v subscript implies constant volume

Heat, Qaddedm m

DTinsulation


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Heat Capacity for ConstantPressure Processes (Cp)

Heat is added to a substance of mass m heldat a fixed pressure, which causes a change ininternal energy, U,AND some PV work.

Heat, Qadded

DT

m m

Dx


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Cp Defined

Thus,

Q = DU + PDV = DH = m CpDT

The 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

DH = DU + PDV


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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 perlbm 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 inK? In oF?

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First and Second Law of Thermodynamics