ThermodynamicsThermodynamics

The study of heat energyThe study of heat energy

through random systemsthrough random systems

Laws of ThermodynamicsLaws of Thermodynamics

0th Law: If Ta=Tb and Tb=Tc then 0th Law: If Ta=Tb and Tb=Tc then Ta=TcTa=Tc

1st Law: ΔE=W+Q (Conservation of 1st Law: ΔE=W+Q (Conservation of Energy)Energy)The increase in thermal energy of a The increase in thermal energy of a

system is determined by the heat of the system is determined by the heat of the system and the work added to it.system and the work added to it.

The idea behind the The idea behind the Heat EngineHeat Engine

EfficiencyEfficiency

WWnetnet=Q=Qnetnet=Q=Qhothot-Q-Qcoldcold

or Eff=Wor Eff=Wnetnet/Q/Qhothot=1-(Q=1-(Qcoldcold/Q/Qhothot))

Ex: A steam engine absorbs 1.98 x10Ex: A steam engine absorbs 1.98 x1055J and J and expels 1.49 x10expels 1.49 x1055J in each cycle. Assume that J in each cycle. Assume that all the remaining energy is used to do work.all the remaining energy is used to do work.a. What is the engine’s efficiency?a. What is the engine’s efficiency?b. How much work is done in each cycle?b. How much work is done in each cycle?

a. Eff=0.247a. Eff=0.247b. W=4.9 x10b. W=4.9 x1044JJ

2nd Law: A system can be defined by 2nd Law: A system can be defined by its entropy. In a closed system its entropy. In a closed system entropy tends to increase.entropy tends to increase.Entropy: A measure of the disorder Entropy: A measure of the disorder

(randomness) of the system(randomness) of the system3rd Law: Temperature and Entropy 3rd Law: Temperature and Entropy

are absolute scales.are absolute scales.At some point no temperature or At some point no temperature or

entropy exists.entropy exists.T→0 K and S→ST→0 K and S→S00

Heat EnginesHeat Engines

Engines allow heat energy to be transformed Engines allow heat energy to be transformed into work or mechanical energy.into work or mechanical energy. Work or Energy Work or Energy Heat increases Heat increases

no big deal. Friction does this.no big deal. Friction does this. Heat Heat Work or Energy Work or Energy

is a big deal. Heat is easy to move around. You could just is a big deal. Heat is easy to move around. You could just bring heat wherever you needed work done and “Boom!” bring heat wherever you needed work done and “Boom!” youyou wouldn’t have to do the work, a machine could. wouldn’t have to do the work, a machine could.

Work or Energy Work or Energy Heat decreasesHeat decreases is also a big deal. Making food cold preserves it and allows is also a big deal. Making food cold preserves it and allows

it to be moved readily. Less spoilage means less disease.it to be moved readily. Less spoilage means less disease.

Any heat engine works on the same Any heat engine works on the same properties.properties. A A hot reservoirhot reservoir is the source of the is the source of the

energy.energy. Both words mean something. Hot means that Both words mean something. Hot means that

there is plenty of heat energy and reservoir there is plenty of heat energy and reservoir means that if heat is removed the temperature means that if heat is removed the temperature doesn’t drop much. doesn’t drop much.

There is also a need for a There is also a need for a cold reservoir.cold reservoir. Again, both words mean something.Again, both words mean something. Cold because it is at a lower temperature than Cold because it is at a lower temperature than

the hot reservoir and reservoir because it must the hot reservoir and reservoir because it must be large enough that you can dump heat into it be large enough that you can dump heat into it without appreciably raising the temperature.without appreciably raising the temperature.

What happens if we put a hot and cold What happens if we put a hot and cold reservoir in contact? Thermal Equilibrium is reservoir in contact? Thermal Equilibrium is

not not the answer!the answer! Heat transfer (or flow) is the answer. Heat transfer (or flow) is the answer.

Remember that these are reservoirs so it Remember that these are reservoirs so it would take a long time for them to come would take a long time for them to come into thermal equilibrium. into thermal equilibrium.

This is great, but we don’t get any work This is great, but we don’t get any work out of it. out of it.

We need to “steal” some of the energy We need to “steal” some of the energy leaving the hot reservoir and make it do leaving the hot reservoir and make it do work for us.work for us.

Heat Engine

• Stealing some energy to do work

High Temp

Low Temp

ΔE

Q

W

Types of Heat EngineTypes of Heat Engine

Steam EngineSteam Engine

Internal Combustion EngineInternal Combustion Engine

Unfortunately we don’t get an Unfortunately we don’t get an even trade!even trade!

We lose energy to We lose energy to randomness/Entropyrandomness/EntropyThis is the 2nd Law of ThermodynamicsThis is the 2nd Law of Thermodynamics

Automobile engines are only about 15% Automobile engines are only about 15% efficient. That means for every 100J of efficient. That means for every 100J of heat energy, 15J worth of work is done on heat energy, 15J worth of work is done on the piston and 85J of heat are discarded. the piston and 85J of heat are discarded. Still, this is the source of energy for most Still, this is the source of energy for most of our transportation. of our transportation.

h

c

h

ch

h

net

Q

Q

Q

QQ

Q

Weff

1

If a steam engine takes in 2.254 x 10If a steam engine takes in 2.254 x 1044 kJ kJ of heat and gives up 1.915 x 10of heat and gives up 1.915 x 1044 kJ of kJ of heat to the exhaust, what is the engines heat to the exhaust, what is the engines efficiency?efficiency?

h

c

Q

Q1eff

15%or 10 x 2.254

10 x 1.9154

4

15.

1

kJ

kJeff

We can go backwards!

• This is a refrigerator or air conditioner

High Temp

Low Temp

ΔE

Q

W

This requires Energy/WorkThis requires Energy/Work

This is why your refrigerator must be This is why your refrigerator must be plugged in.plugged in.It is constantly dumping heat into your It is constantly dumping heat into your

kitchenkitchenDue to the 2nd Law of Due to the 2nd Law of

Thermodynamics more heat is Thermodynamics more heat is dumped than is removeddumped than is removedIf you left the refrigerator door open you If you left the refrigerator door open you

would heat up the kitchenwould heat up the kitchen

Perfect Heat EnginePerfect Heat Engine

Qcold

Qhot

ΔE

W

Qcold

Qhot

ΔE

W

EfficiencyEfficiency

WWnetnet=Q=Qnetnet=Q=Qhothot-Q-Qcoldcold

or Eff=Wor Eff=Wnetnet/Q/Qhothot=(1-Q=(1-Qcoldcold/Q/Qhothot))

Ex: A steam engine absorbs 1.98 x10Ex: A steam engine absorbs 1.98 x1055J and J and expels 1.49 x10expels 1.49 x1055J in each cycle. Assume that J in each cycle. Assume that all the remaining energy is used to do work.all the remaining energy is used to do work.a. What is the engine’s efficiency?a. What is the engine’s efficiency?b. How much work is done in each cycle?b. How much work is done in each cycle?

a. Eff=0.247a. Eff=0.247b. W=4.9 x10b. W=4.9 x1044JJ

More 2More 2ndnd Law Law

EntropyEntropy

S=entropyS=entropy Q=Heat (Joules)Q=Heat (Joules) T=Temperature (In Kelvin)T=Temperature (In Kelvin)

Entropy in a system must increase or Entropy in a system must increase or at least stay the same!!!!!!!!!!!!at least stay the same!!!!!!!!!!!!

T

QS

An engine has a hot reservoir at An engine has a hot reservoir at 1000 K and uses the atmosphere at 1000 K and uses the atmosphere at 300 K as the cold reservoir. You take 300 K as the cold reservoir. You take 2500 J from the hot reservoir to do 2500 J from the hot reservoir to do 1900 J of work.1900 J of work.A. How much heat goes into the A. How much heat goes into the

atmosphere?atmosphere?B. Is this engine possible? (Does the B. Is this engine possible? (Does the

entropy increase?)entropy increase?)

WWnetnet=Q=Qnetnet=Q=Qhothot-Q-Qcoldcold

QQcoldcold=Q=Qhothot-W-Wnetnet=2500J -1900J =600J=2500J -1900J =600J

EntropyEntropy

Engine is not possible.Engine is not possible. What is the maximum amount of work we can take What is the maximum amount of work we can take

out?out? How much work is done?How much work is done?

5.21000

2500

K

J

T

QShot

0.2300

600

K

J

T

QScold

Carnot EngineCarnot Engine

Maximum efficiency Maximum efficiency In theory could run backwardsIn theory could run backwards

All temperatures in KelvinAll temperatures in Kelvin What is the efficiency of an ideal steam What is the efficiency of an ideal steam

engine with steam at 685 K and engine with steam at 685 K and exhaust at 298 K?exhaust at 298 K? What is QWhat is Qcold cold ??

h

ch

T

TT

inputHeat

outputWorkeff

Carnot EngineCarnot Engine

Maximum efficiency Maximum efficiency In theory could run backwardsIn theory could run backwards

All temperatures in KelvinAll temperatures in Kelvin What is the efficiency of an ideal steam What is the efficiency of an ideal steam

engine with steam at 685 K and engine with steam at 685 K and exhaust at 298 K?exhaust at 298 K? What is QWhat is Qhothot if Q if Qcoldcold is 450J? is 450J?

h

ch

T

TT

inputHeat

outputWorkeff