ChemE 260 The 2

nd

Law of Thermodynamics

Dr. William Baratuci

Senior Lecturer Chemical Engineering Department University of Washington TCD 6: C CB 5: 5 & 6 April 26, 2005

The 2

nd

Law of Thermodynamics

• Colloquial Statement of the 2 nd Law – Heat flows spontaneously from hot objects to cold objects.

• The Clausius Statement of the 2 nd law – A thermodynamic cycle cannot transfer heat from a cooler body to a hotter body and produce no other effect on the universe.

– The “other effect on the universe” means refrigerators and heat pumps require a work input !

Baratuci ChemE 260 April 26, 2005

Application of the Clausius Statement

• Can a heat engine have an efficiency of 100% ?

That is, can a HE completely convert heat into work ?

Hot Reservoir Q C HP Q C HP Hot Reservoir Q H HE W HE Q C Cold Reservoir Q C Q C Cold Reservoir

Baratuci ChemE 260 April 26, 2005 • This heat pump violates the Clausius Statement of the 2 nd Law.

• The HE complies with both the 1 st and 2 nd Laws • The combined system absorbs heat from a single reservoir and converts it completely into work

Kelvin – Planck Statement

• A thermodynamic cycle cannot produce a net amount of work while exchanging heat with just one thermal reservoir.

– This means that heat engines cannot completely convert heat into work. 

< 100%

• We showed, on the previous slide, that a cycle which violates the Clauisus Statement also violates the K-P Statement.

• If we can show that the converse is also true (that a cycle which violates the K-P Statement must also violate the Clausius Statement) then we can conclude that the two statements of the 2 nd Law are equivalent.

Baratuci ChemE 260 April 26, 2005

A HE That Violates KP Violates Clausius

• Does a HE that completely converts heat into work violate the Clausius Statement of the 2 nd Law ?

Baratuci ChemE 260 April 26, 2005

Hot Reservoir Q H HE W HE Cold Reservoir

• This heat engine violates the Kelvin-Planck Statement of the 2 nd Law.

Q H,A Hot Reservoir Q H,B HE HP W A W B Q C,B Cold Reservoir

• The HP complies with both the 1 st 2 nd Laws and • The combined system transfers heat from the cold reservoir to the hot reservoir without any work interaction.

Perpetual Motion Machines

Baratuci ChemE 260 April 26, 2005 • Three kinds of PMMs – 1 st Kind • Violate the 1 st Law or conservation of mass • Create of destroy mass or energy without a nuclear reaction • Usually fairly easy to identify – 2 nd • Violate the 2 nd Law of Thermodynamics • It is easiest to identify these using the Kelvin-Planck Statement of the 2 nd Law – 3 rd Kind Kind • Produce and consume no work • Have no friction • Run forever • A bit harder to de-bunk

Next Class …

• Reversibility – Reversible Processes • No friction, etc • No real ones exist, but some processes are nearly reversible.

• We will compare real processes to the reversible process in order to evaluate its performance – Irreversible Processes • All real processes – Internally Reversible Processes • All the irreversibilities fall outside of the system boundary.

Baratuci ChemE 260 April 26, 2005

Baratuci ChemE 260 April 26, 2005

Example #1

• An inventor claims to have developed a resistance heater that supplies 1.2 kW-h of energy to a room for each kW-h of electricity it consumes. Is this a reasonable claim or has the inventor developed a perpetual motion machine ? Explain.

Example #2

• It is common knowledge that the temperature of air rises as it is compressed. An inventor thought about using this high-temperature air to heat buildings. He used a compressor driven by an electric motor. The inventor claims that the compressed hot-air system is 25% more efficient than a resistance heating system that provides an equivalent amount of heating. Is this claim valid, or is this just another perpetual motion machine ? Explain.

Baratuci ChemE 260 April 26, 2005