ChemE 260 The 2 nd Law of Thermodynamics April 26, 2005 Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 6: C CB 5: 5 & 6

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 Cold Reservoir HP QCQC QCQC Hot Reservoir Cold Reservoir HP QCQC QCQC HE QHQH QCQC W HE 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 Baratuci ChemE 260 April 26, 2005

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 ? This heat engine violates the Kelvin-Planck Statement of the 2 nd Law. The HP complies with both the 1 st and 2 nd Laws The combined system transfers heat from the cold reservoir to the hot reservoir without any work interaction. Hot Reservoir Cold Reservoir HE QHQH W HE Hot Reservoir Cold Reservoir HP Q H,B Q C,B WBWB HE Q H,A WAWA Baratuci ChemE 260 April 26, 2005

Perpetual Motion Machines 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 Kind 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 Produce and consume no work Have no friction Run forever A bit harder to de-bunk Baratuci ChemE 260 April 26, 2005

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

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. Baratuci ChemE 260 April 26, 2005

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