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Feb. 27Ozone III

Issue Topic Due

March 1Energy

Thermodynamics

March 6Energy

Thermodynamics

March 8Energy

Thermodynamics

March 13Energy

Thermodynamics

March 16Energy

Thermodynamics

March 20Energy

Thermodynamics

March 22Energy

Thermodynamics

Physics 6 Schedule

Labmeets Friday

!

Turn in Issue!

“Homework” through 3/22 due on 22nd!

“…if I were a heathen, I would rear a statue to Energy and fall down and worship it!”—Mark Twain

I have a special favor to ask of all those who look at these notes in advance…

…please do not read the material on slides 24 and 25. I want to see your answer to several questions, not what you think I think you should say.

Thank you for your attention and now back to our show…

The Plan

Yes, there is a Plan!

Little by little, I have introduced you to some of the major ideas of physics.

All semester has been building up to Energy.

Once I finish Energy, I might lecture a bit on nuclear energy, but the rest of the semester is applications. “Case Studies.”I spent a lot of time on ozone depletion. I could have covered the whole topic in two sentences. Instead, I sacrificed a lot of class time exploring the process of discovery and the complexity of the science.

The upcoming “Case Studies” are just as complex, but I will devote less class time to them.

Get out a piece of paper. On the piece of paper, write down something that goes.

Then give the piece of paper to someone else in the class.After the pieces of paper have changed hands once, give them to someone else in the class.

On the piece of paper you just received... write down where the energy comes from that makes the something go. Give me the piece of paper when you take a break. No names needed.

Physical Science:Energy (Part II)

Homework Assignment #5.* Find out about the Energy Patrol.

Can you tell me what you found out about the Energy Patrol?

Do you remember this homework assignment ?

How much money did it save?

Why don’t more institutions have an Energy Patrol?

*You can still turn this assignment in, if you haven’t done so already.

Is it wise to give kids keys to the building?

Why doesn’t every human on earth do something like this?

I left you on February 15 with the Law of Conservation of Energy…

Energy is neither created nor destroyed, but only changed from one form into another

…and these thoughts…

Qwertyuiop

m

H

PE=mgH

Initially, the book has a gravitational potential energy mgH.

While it is falling, it has kinetic energy mv2/2.

At the end, it is not moving, and has no energy.

Qwertyuiop

m H

PE=0

Qwertyuiop

Oops, what happened to conservation of energy?

KE=0

Energy is conserved. You just have to include all forms of energy.

Qwertyuiop

Red bar represents energy initially “stored” in book.

Energy is conserved. You just have to include all forms of energy.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

Red bar represents energy initially “stored” in book.

Energy is conserved. You just have to include all forms of energy.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

Energy is conserved. You just have to include all forms of energy.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

Energy is conserved. You just have to include all forms of energy.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

Energy is conserved. You just have to include all forms of energy.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

Energy is conserved. You just have to include all forms of energy.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

later…

Energy is conserved. You just have to include all forms of energy.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

still later…

This analysis didn’t account for the chemical energy of the bonds between atoms and molecules in the book…

…or the nuclear energy of the neutrons and protons bonded together…

…or the book’s mass-energy (from E=mc2).

I could have included those energies, but they would not have changed (on the scale of my diagram) during the process.

“internal” vs “thermal” energy

Hyperphysics is a good web site for understanding how the different parts of physics fit together.

It is “technical,” but you might want to take a look at it. You might be surprised by how much of it you can understand.

Hyperphysics says this about “internal” energy:

“Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale.”

Hyperphysics says this about “thermal” energy:

“The average translational kinetic energy possessed by free particles … is sometimes called the thermal energy per particle.”

I may use the terms loosely in speaking, but thermal energy refers to the kinetic energy of particles too tiny for you to see… and internal energy includes both the kinetic and potential energy of particles too tiny for you to see.

In my discussion of the falling book, “should” I have included potential energies resulting from the deformation of book and floor?

Technically, “yes.” I’m glad I didn’t. My neck hurts enough already.

book

KE

book

PE a

ir

KE soun

d

therm

al

(book)

therm

al

(floor)

the r

est

of

the

univ

ers

e

Qwertyuiop

pote

nti

al

(book)

pote

nti

al

(floor)

Or I could have lumped these into “internal (book).”

I should label these “internal potential” but I have no space!

Reviewing what I have talked about so far:

I defined energy as the capability of doing work, and defined work in terms of a force moving a mass through some distance.

If your body were equally efficient at running and walking, you would burn as many calories walking a mile as running a mile.

We found that there are two main forms of energy, kinetic and potential, and I implied that I could categorize all kinds of energy into one of those two forms.

We saw how thermal energy really comes from the motion of atoms or molecules.

Internal energy includes both thermal energy and the potential energies between atoms and molecules.

Energy: kinetic or potential!

heat and temperature

Your attendance assignment for today. Define these two terms: “heat” and “temperature.” One simple sentence each, please!

While you are waiting for the rest of the class, consider your response to this simple question:

Imagine a cauldron of boiling oil.

Would you, for $10,000, allow me to place a single drop of this boiling oil on your hand? (I’ll count the yes’s and no’s.)

Next, very carefully consider your response to this simple question:

Would you, for $10,000, allow me to pour (all at once!) the entire contents of the cauldron of boiling oil on your hand? (Again, I’ll count the yes’s and no’s.)

If—as I suspect will be true—you vote mostly for the $10,000 and the single drop, and reject overwhelming the $10,000 and the entire cauldron, then you understand the difference between temperature and heat……even though I suspect many—or most—of you to equate heat and temperature in your definitions.

You should be safe if you think of heat as just another name for kinetic energy.

The boiling oil is not my original idea. See this page for a discussion of misconceptions about heat and temperature (in response to a question from a 6th grade teacher).

You put a cauldron of oil on the fire and heat it up; in doing so you give the oil molecules more kinetic energy and they move around faster.

You say you heated up the oil. (The pot also gets hotter because its atoms are vibrating faster.)

In chemistry, heat may have its own special definition, which may look different but ultimately has the same meaning: heat is just a form of energy.

Some things you can do with energy:

make a bang (book hitting floor)

boil oil

burn skin using the energy of the boiling oil

rip a molecule apart

No—I’m thinking about ozone, not your skin!

You can also "use" heat energy by putting cold matter in contact with hot matter.

You don't actually "use up" the energy; some of it just moves from the hot matter to the cold matter.

Remember, energy doesn’t disappear; it just gets spread out or transformed from one form to another.

Temperature is a measure of the energy of a system.

Temperature is usually defined in terms of the thermal energy of an ideal gas (ideal gas molecules are in constant motion, with their speeds increasing as the temperature goes up).Hyperphysics points out that “temperature” is actually a very complex topic, and gives this definition of temperature:

“Temperature is a measure of the tendency of an object to spontaneously give up energy to its surroundings. When two objects are in thermal contact, the one that tends to spontaneously lose energy is at the higher temperature. (Schroeder, Thermal Physics, Ch 1.)”

Let me finish this section on work and energy by reminding you again of the law of conservation of energy.

You "use" energy but you don't "use it up."

Energy is neither created nor destroyed, but only changed from one form into another

If we never use up energy, we can never run out of energy, so what's all this fuss I hear about the need for energy conservation?

When you measure the temperature of something with a thermometer, you actually measure the local energy content of the molecules that are in contact with the thermometer bulb.

The fuss is not about conserving energy; it is about conserving substances which store useful forms of energy.

I will discuss thermodynamics soon. You will see that whenever we use energy to do something useful, we have taken a useful form of energy (such as the energy stored in oil) and converted some of it into a less useful form of energy (such as wasted heat).

Everything we do, the mere act of living and breathing, results in consumption of the useful forms of energy in the universe.

Depressing, right? It's really not that bad; the universe has plenty of high-grade energy for us to use, as long as we are willing to make some compromises when we use it.

Every second, the sun radiates 1350 joules of energy into every square meter of space at the earth’s distance from the sun.*

Power

In a bit, I’ll give you the opportunity to calculate the energy your body needs to keep functioning for one second.

*I expressed it a bit awkwardly because not all of this 1350 joules/second/m2 reaches the surface of the earth.

Power is the rate at which energy is being used… or the rate at which energy is being delivered so that we can do things with it.

The unit of power is the watt:

energy usedPower=

time during which energy was used

energy transformed from one form to anotherPower=

time during which energy was transformed

Mathematically:

energyPower=

time

1 joule1 watt=

1 second

If I use 1 joule of energy every second, I am using 1 watt of power (a watt is a joule per second).

A 100 watt light bulb takes 100 joules of electrical energy every second and converts it to light and heat.

A kilowatt (kW) is a thousand watts, and a megawatt (MW) is a million watts. Typical power plant outputs are measured in tens or thousands of megawatts.

1,000 MW = 1,000,000,000 W = 10,000,000

When you buy power from the power company, you don't really buy power, you buy energy.

You are billed for the energy you use, typically expressed as the number of kilowatt-hours you use.

Remember, power is energy per time, so power times time gives you energy:

energypower=

time

energypower×time= ×time=energy

time

If you leave a 100 watt light bulb on for one hour, you use 0.1 kilowatts for 1 hour, or 0.1 kilowatt-hour.

To find the energy in joules, multiply the power in kWh by the 3600 seconds in an hour.

63600 seconds 1000 watts joule/second1 kilowatt - hour× × × = 3.6×10 joules

hour kilowatt watt

If you leave ten 100 watt light bulbs on for one hour, you use 1 kilowatt-hour of energy.

Homework assignment #8.* Due one week from today. You can pick any one of the following to turn in. You may work with up to three other people; make sure all your names are on the paper. Some choices are for the research-minded, others for the number-happy.

*See the grades spreadsheet for a list of homework assignments.

There are three choices for this assignment. Each one begins on a separate slide.

Handy fact: a calorie is a measure of energy. One calorie is equivalent to 4.19 joules of energy—a “physics” calorie. A “food” Calorie is equal to 1000 calories.

Research plus simple calculation.

Choice #1

Look up your utility bills for the last year (or a reasonable length of time if you can't find a year's worth). How many kilowatt-hours of electricity did you use?

Using the fact that a kilowatt is 1000 watts and a watt is a joule per second, how many joules of energy did you use? If your utility bill has enough information, find out how much you were charged per kilowatt-hour, and calculate how much you were charged per joule.

The next part is easy math. However, this is supposed to be a “nonmathematical” course. Therefore, you may, if you wish, write “No Math!” on you paper and turn it in without the math.

Interesting, but simple calculation. Are you worth a light bulb?

Choice #2

A calorie is a measure of energy. One calorie is equivalent to 4.19 joules of energy. A food calorie is equal to 1000 calories. Suppose your body uses 2000 food calories of energy per day. If all that energy could be made to light up a light bulb all day long, what wattage bulb could you light up?If you know your actual daily calorie intake, e.g., 1650 calories, you may replace my “typical” 2000 calories with your actual energy intake, to find out how much of a light bulb you are worth.

I saw a diet infomercial quite a few years back. I only saw it once, and I think I know why.

Choice #3

Interesting diet calculation. It will take 2 slides to present this choice.

It was a new miracle diet. You had to buy the diet plan, of course, but the idea was simple. According to this diet, all you have to do to lose substantial amounts of weight is drink a few glasses of ice water a day.

The reasoning goes like this: your body takes in the ice water and heats it up to body temperature. That "burns" calories. The claim was that it "burned" enough calories to result in substantial weight loss.

Here's the problem (I will give you all of the needed numbers at the end): suppose you drink 5 extra 12-ounce glasses of ice-cold water (water at 0 C)* every day. How many pounds of fat will you lose in a week.

According to the 12-ounce soda can I am looking at right now, 12 ounces is 355 ml, or 355 cubic centimeters. It takes one calorie to heat one gram of water (which has a volume of 1 cubic centimeter) by 1 degree C. Body temperature is 36 C.

*Water at 0 C would be “just about” to freeze, but would still be a liquid. Remove any additional heat energy, and it would start to freeze.

One pound of fat is equivalent to 3500 kilocalories. (Caution: the Calorie, with a capital "C," that you read about in diet books is actually 1000 calories, or one kilocalorie.)

I told you earlier that heat is just a form of energy.

Thermodynamics:The Transfer of Heat

That may or may not seem obvious to you, but for a long time it was not a "given" for scientists. It took until the middle part of the 1800's before an experiment could be devised to show that heat is a form of energy.

Because I am going to lecture on heat transfer, let's talk for a minute about how heat can be transferred from one place to another.

You have probably heard of the three means of heat transfer (have you?).

Conduction: molecules in contact receive excess energy from neighbors and pass it on to cooler neighbors. Heat always flows from warmer substances to cooler substances.

They are... err, what are they?

conduction

convection

radiation

Some materials are very good conductors of heat, and others are very poor conductors of heat.

If you held a copper rod in your right hand and a wood branch in your left hand, and stuck them both in the fireplace at the same time, which would you let go of first?In general, metals are good conductors of heat, and electrical insulators are poor conductors of heat.

Not a good building material!