Heat & Thermodynamics

Physics(P)

State Standards

3. Energy cannot be created or destroyed, although in many processes energy is transferred to the environment as heat. As a basis for understanding this concept: a. Students know heat flow and work are two forms of energy transfer between systems. b. Students know that the work done by a heat engine that is working in a cycle is the difference between the heat flow into the engine at high temperature and the heat flow out at a lower temperature (first law of thermodynamics) and that this is an example of the law of conservation of energy. c. Students know the internal energy of an object includes the energy of random motion of the object's atoms and molecules, often referred to as thermal energy. The greater the temperature of the object, the greater the energy of motion of the atoms and molecules that make up the object.

Additional Standards

d. Students know that most processes tend to decrease the order of a system over time and that energy levels are eventually distributed uniformly.

e. Students know that entropy is a quantity that measures the order or disorder of a system and that this quantity is larger for a more disordered system.

f.* Students know the statement "Entropy tends to increase" is a law of statistical probability that governs all closed systems (second law of thermodynamics).

g.* Students know how to solve problems involving heat flow, work, and efficiency in a heat engine and know that all real engines lose some heat to their surroundings

What is the Difference Between Heat and Temperature?

Both are related to energy but there’s a big difference

Temperature

Measure of how hot or cold an object is

Measured by thermometers

Work by expansion of a liquid

Other types use bimetallic strip

Digital Thermometers

Use “thermistors” - temperature dependant semiconductor resistors

Temperature Scales

Fahrenheit T(0F) = 9/5T(0C) + 32

Celsius (centigrade) T(0C ) = 5/9[T(0F) –32]

(degree is 9/5 that of Fahrenheit)

Kelvin (Celsius + 273)

Examples

Zero degrees Celsius is what Kelvin?

Answer: 273o

What is the boiling point of water in degrees Kelvin?

Answer: 373o

200 degrees Celsius is what in Kelvin?

Answer: 473o

Absolute Zero

0 degrees Kelvin = -273 Celsius

Lowest possible temperature

Molecular motion ceases

Courtesy Michigan State University

Kinetic Theory of Heat

All matter is made of tiny atoms and molecules, constantly in motion

Faster is hotter

gas

solid

Temperature and Kinetic Energy

In ideal gas temperature is proportional to average kinetic energy per molecule

Closely related in liquids and gases

Temperature does not depend on the amount

Heat does depend on the amount

Analogy: Heat is like the total height of students in this room, temperature is like their average height.

There is twice as much kinetic energy of moving molecules in two liters of water as in one liter.

Which has more heat?

A swimming pool full of ice water?

A cup full of boiling water?

Answer: the swimming pool, because it has so much more water.

Heat – Energy Transferred

Definition: Energy that transfers because of temperature difference

Heat flows governed by average molecular kinetic energy difference

Thermal Equilibrium

Objects at same temperature are at thermal equilibrium – no heat flows.

Internal Energy

Total of all forms of energy inside something

Includes– Translational kinetic– Rotational kinetic– Potential

Summary

The greater the temperature of an object, the greater the energy of motion of the atoms and molecules that make up that object.

Internal energy of an object includes the energy of random motions of atoms and molecules in the object

Measuring Heat

One calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.Kilocalorie raises the temperature of one kg of water by 10 C (also called Calorie or food calorie)One calorie = 4.186 joulesOne kilocalorie = 4186 joules

Density of Water

Density is mass per unit volume

D = M/V

One gram per cubic centimeter

One kilogram per liter

One thousand kg per cubic meter

Calorie Questions

How many calories are needed to raise the temperature of 10 grams of water by 10C?

10How many calories are needed to raise the temperature of one gram of water by 10 0C?

10

A certain amount of heat raises the temperature of 1 liter of water by 20C. What would be the temperature increase if the same amount of heat were added to 2 liters of water?

Answer 10C

Example

A person consumes and expends about 2000 Calories per day. What is their thermal power output?

2000Cal/d x 1 d/24h x 1h/3600s x 4184J/C

= 96.8J/s = 96.8 Watts

Specific Heat Capacity

Different materials change their temperature by different amounts when they absorb the same amount of heat.Some have more ways of storing energy than othersWater has very high specific heat (capacity)Metals have much less

Q = mcT

Q = mcT expresses how heat absorption works. C is specific heat

Question: A certain rock has a specific heat of 0.25 (water is 1.0) How much heat will be required to heat 5.0 kg rock from 20 to 800C?

Q = 5.0kg x 1000g/kg x 0.25 Cal/g 0C x 60 0C

Q = 75,000 C = 7.5 x 104 Calories

Thermal Expansion

Most materials expand when heated

Only exception is water between 00C and 400C

Expansion joints in bridges, cracks in sidewalks allow for expansion

Bimetallic Strip

How your thermostat works

Don’t Let Your Car’s Engine Overheat

Aluminum expands more than iron

Pistons made of aluminum

Cylinder made of iron

Mechanical Equivalent of Heat

Discovered by James Joule

Falling weight makes

paddle turn

4.186 x 103 J = 1 kcal

Interpretation:

HEAT IS ENERGY

TRANSFER

Courtesy W. Bauer http://lecture.lite.msu.edu/~mmp/kap11/cd295.htm

Joule’s Apparatus

Link to Joule’s original article

Example

When digested a slice of bread yields 100 kcal. How high a hill would a 60 kg student need to climb to “work off” this slice of bread?100 kcal x 4.186 x 103 J/kcal = 4.2 x 105 J

W = mgh

h = W/mg = 4.2 x 105 / (60 kg)(9.80 m/s2) =

714m = 7.1 x 102 m

If the body is only 20 percent efficient in transforming the bread, how high need they climb?

Bullet in Block

When a 10 g bullet traveling 500 m/s is stopped inside a 1kg wood block nearly all its KE is transformed to heat. How many kcal are released?

KE = ½ mv2 = 0.5 x 0.010 kg x (500)2 = 1250 J

1250 J x 1 kcal/4186 J = 0.30 kcal

Thermodynamics

Study of heat and its transformation into mechanical energy

Based on conservation of energy

Explains how engines like car motors work

First Law of Thermodynamics

Generally, when you add heat to a system it changes into an equal amount of some other form of energy

Heat added = increase in internal energy + external work done by the system

Work Done On and By

Compressing a gas by pushing down on a piston = work done on

A gas expands by pushing a piston up = work done by

Questions

20 J of heat is added to a system that does no work. What is the change in internal energy?

Answer +20 J20 J of heat is added to a system that does 10 J of work. What is the change in internal energy?

Answer +10 J

20 J of heat is added to a system that does 30 J of work. What is the change of internal energy?

Answer -10 J20 J of heat is added to a system that has 10 J of work done on it. What is the change of internal energy?

Answer +30 J

Bicycle Pump

What do you think happens when you operate the pump. Where does the work you do go?It goes to heat, some through friction, some to adiabatic compression of the air inside the pumpWhat does “adiabatic” mean?Answer: No heat enters or leaves Q=0

Adiabatic Processes

Compression or expansion of a gas so that no heat enters or leaves

Example: gas in cylinder of car or diesel engine

Why adiabatic? Because it happens too fast for much heat to enter or leave.

In adiabatic compression, temperature rises. – In diesel engine, enough to ignite gas without spark

plug

A process can also be adiabatic if it happens inside a well insulated conatiner.

Courtesy “How Stuff Works”

Courtesy Shell Canada

Adiabatic Expansion

Produces cooling

Example: blow on your hand first with wide open mouth, then with puckered lips

How do you explain the results?

The Chinook

What would you expect to happen if cold air moves down the slopes of mountains

Hint: it will be compressed by atmosphere into smaller volume

Chinook wind is warm

Common in Rocky mountains

Second Law of Thermodynamics

Heat flows from hot to cold. By itself it will never flow from cold to hot.

Question: Would it violate the First Law of Thermodynamics (energy conservation) if heat flowed from a cold object to a warm object touching it?

Answer: No

Second Law Applied to Engines

It is impossible to build a heat engine that changes heat completely into work.

Courtesy University of Oregon

Such an engine would be 100% efficient!

Allowed by 1st law, forbidden by 2nd law

Heat Engine

Some heat is converted to useful workThe rest is exhausted on at a lower temperature (cause of thermal pollution)Efficiency = useful work / heat input– About 20-25% for gasoline engine– About 35-40% for diesel engine

The energy exhausted is waste, cannot be recovered

Ideal (Carnot) Engine

Ideal (Maximum possible efficiency) = (Thot – Tcold)/Thot (Kelvin temperatures)

What is the efficiency of a steam turbine (assumed ideal) operating between 400K (1270C) and 300K (270C)?(400 – 300)/400 = ¼ or 25%What would be the efficiency if the turbine could operate at 600K?What would the exhaust temperature need to be for an engine to be 100% efficient?

1/2

0 K

Steam Turbine

Limits to Technology

What factors limit the efficiency of an engine?– Friction– Temperature at which parts melt– Carnot efficiency

What would be the advantages of a ceramic engine? Disadvantages?

Courtesy University of Colorado

Can operate at 3000 degrees without cooling, is light and doesn’t need much cooling, but…

Heat Engine Summary

Work done is difference between heat flow in at high temperature and the heat flow out at a lower temperature (conservation of energy)

Order and Disorder

Useful energy tends to degenerate and become less usefulAlternate statement of 2nd Law: Natural systems tend toward disorderQuestion: Could all the air molecules in this room spontaneously concentrate at the top of the room (more orderly system)?

Entropy

A measure of how much change occurs when energy spreads out according to the second law.

More generally (and less accurately) a measure of disorder

When disorder increases, entropy increases

Mess to Neat?

Will this mess become neat all by itself?

No way!

Will this dish reassemble all by itself?

No Way, the Second Law of Thermodynamics prohibits it

Entropy Summary

Entropy is a quantity that measures the order or disorder of a systemThis quantity is larger for a more disordered systemThe Second Law of Thermodynamics says that entropy tends to increaseAll real engines lose heat to their surroundings

Courtesy California Science Standards in Physics

Courtesy University of Oregon

Global Warming