Heat and the Second Law of Thermodynamics

Chapter  4  

Great Idea: Heat is a form of energy that flows from

warmer to cooler objects

1st and 2nd law of Thermodynamics crucial for understanding all sciences

Chapter Outline •  Nature’s Direction •  Coming to Terms with Heat •  Heat Transfer •  The Second Law of Thermodynamics •  Consequences of the Second Law

Nature’s Direction

Second  Law  of    Thermodynamics  

Coming to Terms with Heat

Heat and Temperature •  Heat

–  Energy in motion –  Warm to cool

•  Temperature –  Atomic movement

•  Temperature Scales –  Fahrenheit scale –  Celsius –  Kelvin

•  Absolute zero

         Comparison  of  Temperature  Scales  

Temperature Conversions •  Conversion to Fahrenheit: – °F = (1.8 x °C) + 32

•  Conversion to Celsius: – °C = (°F – 32)/1.8

Specific Heat Capacity

•  Specific heat capacity – Quantity of heat needed to raise 1g of a

substance by 1 °C •  Water – Highest specific heat – 1 cal/g °C

Heat Transfer

Heat Transfer •  Conduction – Movement of heat by atomic-scale collision – Thermal conductivity

•  Convection – Bulk transfer of molecules – Convection cell

•  Radiation

Conduc9on  

Infrared Imaging Detects Heat Loss

Convec9on  

Technology •  Home Insulation

The Science of Life •  Animal Insulation and Feathers

The Science of Life •  Temperature Regulation

The Second Law of Thermodynamics

The Second Law of Thermodynamics

– Heat does not flow spontaneously from cold to hot

– No engine can exclusively convert heat to work – Every isolated system tends toward disorder

Heat Will Not Flow Spontaneously from a Cold

to a Hot Body

Can you do it???

Efficiency of an Engine

•  Determined by the second law of thermodynamics

Every Isolated System Becomes More Disordered with Time

•  Ordered System – Regular predictable pattern

•  Disordered System – Random

•  Entropy – Measure of disorder – The entropy of an isolated system remains

constant or increases

Highly Ordered Regular vs. Disordered Patterns

Consequences of the Second Law

The Arrow of Time •  Four dimensions – 3 have no directionality – The 4th has direction

•  Time – Second law and time

Built-in Limitations of the Universe

•  The Second Law’s consequences – Some things cannot happen – Fossil fuels – Hierarchy of energy

The Science of Life •  Evolution and the Second Law of

Thermodynamics

Let’s go back to Energy…. How do we get energy from fossil fuels?

ThinkQuest  

One example….

How Do Fossil Fuels and Biomass Pollute? All fossil fuels and biomasses consist of carbon and hydrogen atoms. When these fuels are burned, or "combusted," carbon atoms unite with oxygen in the air to form carbon dioxide.  

Energy content of various fuels: Energy from fuels Direct heat, lighting

Run an engine….Heat, Other work

Unit  of  energy  is  Joules  

What is the problem???????? Efficiency

Engines: Efficiency = work done/energy put into the system

= What you got out/What you put in

A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat.

100% efficiency not possible…. friction, viscosity insulation other

Think of an engine as operating between two thermal reservoirs… One at High Temperature…engine takes the heat (QH from hot reservoir) does work with it and returns the rest to the cooler reservoir…

Energy Conservation………………………. QH=Work + QC

Efficiency  =  W/QH  

If you want engine to operate you must provide QH

No perfect engines………….. We can only convert some of the heat to work

not all of it……… 2nd Law of Thermodynamics……………… Entropy ……………Time Travel …………… “Beam me up Scotty” ………….. “perfume “

Example: In the US, 85% of the electricity is generated by burning fossil fuels to produce steam, Which in turn drives alternators that produce electricity. Power plants can produce steam with a temperature as high as 6000C by pressurizing the steam. The resulting waste heat must be exhausted into the environment at a temperature of 200 C (1- Tcold/Thot)x 100%= maximum efficiency 600+273=873 degree Kelvin 20 + 273=293 1-(293/873) x 100%=66.4%  

Real power plants…………….40%

So…what does this mean for energy?

Electricity is a manufactured product. It is not something you pump out of the ground or mine or collect from the sun or wind. Electric power is manufactured from a rotating machine that we call an electrical generator. After it is generated, (manufactured) it is then delivered through copper wires to where it is utilized

But the invention of the machine to generate power is right next to the invention of the printing press in the list of major contributions to the advancement of human civilization.

A "generator" and "motor" are essentially the same thing: what you call it depends on whether electricity is going into the unit or coming out of it.

Electric generators are essentially very large quantities of copper wire spinning around inside very large magnets, at very high speeds. Example: A commercial utility electric generator -- for example, a 180-megawatt generator -- can be quite large. It is 20 feet in diameter, 50 feet long, and weighs over 50 tons. The copper coils (called the "armature") spin at 3600 revolutions per minute. Although the principle is simple (copper wire and magnets), it's not necessarily easy! Steam turbine generators, gas turbine generators, diesel engine generators, alternate energy systems (except photovoltaics), even nuclear power plants all operate on the same principle - magnets plus copper wire plus motion equals electric current. The electricity produced is the same, regardless of source.