Thermodynamics A Physics Approach (Chapters 10-12)

Thermodynamics

A Physics Approach

(Chapters 10-12)

What is Temperature

• Temperature is a measure of the kinetic energy of matter.

• Collision between molecules causes energy transfer

• Motion of molecules causes pressure (collisions with container).

• KE = (½mv2)av = 3/2 kBT

Zero Degrees

• Absolute Zero (Kelvin Scale) (-273.15 oC)

• No motion of molecules• Pressure is Zero.

No motion =No Energy=No Temperature

•T related to energy.

•0 Celcius is pretty HOT = 273 Kelvin

•Can’t really get to zero Kelvin.

•milliKelvin is easy, any more is hard (Noble Prize awarded for laser cooling)

•Deep Space is about 3K (background radiation from the Big Bang warms the universe

Ideal Gas Law (properly)

• Chemistry:

PV=nRT• Pressure in atm

(1 atm = av. air pressure)• Volume in Liters• n is number of moles• T in Kelvin• R=0.0821 L·atm/mol·K

• Physics:

PV=NkBT• Pressure in Pascals

( 1 Pa = 1 N/m2 )• Volume in m3 • N is number of molecules• T in Kelvin• R = 8.31 J/mol·K or

• KB = R/NA = 1.38 ·10-23 J/K

Kinetic Theory of Gases

• See powerpoint on Kinetic Theory of gases (quickly)

KE = (½mv2)av = 3/2 kBT

What is Heat?

• Heat is Thermal Energy• Energy transferred between two objects because of

temperature difference.• 1 calorie is the heat required to raise the temperature of

one gram of water by one degree Celcius.• (1 Btu is heat required to raise one pound of water by one degree Fahrenheit)

• 1 cal = 4.186 Joules• Work is Energy. Heat is generated in mechanical

processes. • Known as MECHANICAL EQUIVALENCE OF HEAT

Thermal Energy Transfer

• Specific Heat Q=mcT or Latent Heat Q=mL• Heat transfers between different objects based

on temperature difference. The greater the T, the greater the rate.

• Does not matter which has more energy (ice berg vs. coffee cup), only T

Three Ways to Burn Yourself•Conduction (Direct thermal

transfer through atomic excitation.)

•Convection (Heat Rises.

Thermal Energy is carried by

moving particles (air, water, etc.

Caused by density changes)

•Radiation (photons carry energy

and collide with other material)

0th Law of Thermodynamics

• Thermal Equilibrium is the condition where there is no net heat flow between two bodies that have been placed in thermal contact.

• If A is in thermal equilibrium with B and

• C is in thermal equilibrium with B then

• A is in thermal equilibrium with C

Thermal Transfer

• Prevent Radiation with reflector

• Prevent Convection with vacuum or tiny air pockets

• Prevent Conduction with insulating materials (vacuum is best)

Work done BY a gas

• Work is Energy. • Heat is Energy

• (½ mv2av) = (3/2)kBT

• W = Fy = PA y =P V• Raise piston, lift weight.• Now the gas has less

Energy in it. • Work done by the gas =

Area under the curve(Note, the PATH MATTERS)

Path Does Matter

• Work is Area under the Curve.• For a repeatable process, must return to initial

state.

FIRST LAW OF THERMODYNAMICS

• Q = Heat transferred TO a system

• W = Work done BY a system

• U = Internal Energy of a system

U = Uf – Ui = Q – W

First Law

• PRINCIPLE of CONSERVATION of ENERGY

• In a cyclic system (where the system is the same at the end as at the beginning)

U = 0

• Q = W

2nd Law of Thermodynamics

• Heat Energy, on its own, flows from Hot to Cold

• Heat Energy does NOT spontaneously flow from cold to hot

Carnot CycleIdealized repeatable cycle.• A-B Isothermal expansion• B-C Adiabatic (free) expansion• C-D Isothermal Compression• D-A Adiabatic Compression

Heat Engine

• Generalized Thermodynamic Engine

• Must be T to get work done

• There is always loss of Heat to environment

2nd Law of thermodynamics Fancier wording

• It is impossible to construct a heat engine that, operating in a cycle, produces no other effect than the absorption of heat from a reservoir and the performance of an equal amount of work.

Thermal Efficiency

h

c

h

ch

Q

Q

Q

QQ

Q

WE

1

Power Plant as Heat Engine

Thermal Efficiency for an ideal Carnot Cycle

h

c

h

chcarnot T

T

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Carnot Cycle Java script

Heat Engine vs. Heat Pump

• Heat Engine extracts work from the transport of heat from hot to cold

HOT

COLD

WOut

HOT

COLD

WIn

• Heat Pump is a Heat Engine running backwards. Work is put in to extract heat from cold

Space Heater vs. Heat Pump• Space Heater

does work to create heat (electrical Energy) and dump it into hot space.

HOT

COLD

WIn

HOT

COLD

WIn

• Heat Pump uses electrical energy to move pre-existing heat from the cold into the hot. The energy required to do the work is also dumped as heat.

100 Joules of Work = 100 Joules of Heat

100 Joules of Work = 300 Joules of Heat