http://www.nearingzero.net (nz235.jpg)

We will meet in room 104 Physics (the big lecture hall) on Monday, January 23. Physics 8 will meet Friday of this week.

Announcements

“If we knew what it was we were doing, it would not be called research, would it?”—A. Einstein

I’ll pass graded homework sets around. Please take yours.

If you forgot to put your name on it, do so and hand it back in.

I will read everything you hand in. If you want more than a check mark ...

Old Business: Occam’s Razor

Suppose I put a saucer of milk out before I go to bed. The next morning the milk is gone.

I can think of two theories that explain why the milk disappeared:

1. My cat drank it.

2. The Milk Fairy took it.

Discussion, anybody?

Old Business: Room Length

Here’s our data from last time:

Discussion, anybody?

Room length in inches:552 552 552 552 552 558 551.5552.5553 490 553 549 531.6875 287.5

Old Business: Tacoma

What did you learn about the Tacoma Narrows Bridge?

Make sure your name is on your homework paper, and turn it in when we take a break.

Next, I’ll talk briefly about Environmental Issue 2.

In case I didn’t make it clear what I am trying to accomplish discussing these issues, the point is…

1) to remind you it’s not too early to start thinking about which Issue you are going to do…

2) … and to clarify in some cases how much I expect you to do.

Environmental IssuesIssue 2:

Carrying Capacity and Ecological Footprint

If you choose this Issue, you’ll investigate the number of individuals that can be supported by an area’s resources.You’ll also investigate “sustainability;” whether a population is using resources faster than they can be replenished.This issue is medium length, so I would also ask that you estimate your own personal “ecological footprint.” You can find many sites online that will do this. Be sure to report what site you use, and comment on whether or not you think that site has any ulterior motives for helping you do the calculation.

Physical Science:Matter and Mass

Please bear with me if this seems ridiculously simple. I need to see where we all stand here.

What is matter? Write your definition on a piece of paper and turn it in when we take a break, along with your Tacoma Narrows Bridge homework. Please don’t discuss your definition with anyone else yet. Put your name on the “matter” paper so I can use it to take today’s attendance.

Here’s the grade school definition of matter: something that has mass and takes up space.

During a summer workshop for middle school teachers on “properties of matter” that I helped run a few years ago, the best the teachers could come up with was “the stuff that the universe is made of.”

The four scientists who helped run the project may have snickered a bit at that definition, but couldn’t do much better.Mass tells us how much stuff there is in an object. Weight (on earth, of course) is a measure of the earth's gravitational attraction for an object.

If I go to the moon, do I weigh more or less?

If we were doing physics with math in this class, I might use Newton's second law, F=ma, to “define” mass.

Is my mass more or less?

Mass is measured in grams or kilograms (metric), or slugs (our English system). The kilogram is the SI unit.

If I tossed you a kilogram mass, could you catch it easily, or would it be too heavy?

Forces are measured in dynes or newtons (metric), or pounds (English). The newton is the SI unit.

If I pushed on you with a 1 N force, would you ignore me, or would you fall backwards?

Matter has charged particles in it.

Most of the time the net charge on an object is zero…

there are the same number of positive and negative charges, and the unlike charges “neutralize” each other.

- +

www.nearingzero.net

The earth, to which we are usually "connected" will either take our excess electrons (if we have any) or give us electrons if we have a shortage, until we become neutral. You've experienced that effect on winter days with a carpet and doorknob.

Later on in this course we will develop some degree of familiarity with electric charge and electrical units in order to understand environmental issues related to electrical power.

Environmental IssuesIssue 7:

Oil and Natural Gas

If you choose this Issue, you’ll investigate oil and natural gas supply and cost.

The Environmental Issues book was written before the current global recession.* Instead of answering the “for further thought” questions, answer these: if economies recover soon, what impact will it have on your answers in this issue? If the if economies take years or decades to recover, what impact will it have on your answers in this issue?There is some math in this Issue (natural logarithms)! (You might need to review Issue 0.)

This is one issue that will have a direct impact on your lives!

*Economists seem to think the recession has ended, but ordinary people might have different ideas.

Written records of the idea that matter consists of tiny, indivisible “building blocks” date back to the 5th century BC Greek philosophers Leucippus and Democritus.Of course, being philosophers, they specialized in “thought experiments.” They felt no need to verify that their thoughts were correct.

The modern idea of atoms was slowly (and sometimes painfully) developed over the century between the late 1700’s and late 1800’s.

“I would rather discover one scientific fact than become King of Persia.”—Democritus

Atoms

What can you tell me about atoms?

As late as 1895, the great chemist Wilhelm Ostwald (1909 Nobel Prize winner for work on catalysts, equilibria, and reaction rates), publicly ridiculed the idea of atoms.*

*And contributed greatly to the suicide of Boltzmann, who we will learn of later.

The “discovery” of the electron in 1897 by J. J. Thomson led him to suggest that atoms are positively charged matter with small, negatively charged electrons embedded. Plum pudding.

Some textbook pictures of the plum pudding model do not show what Thomson was thinking of. This one is OK.

You may laugh at the plum pudding model today, but it was taken seriously in the early 1900’s.** The atomic model you probably believe right now is just as laughable.

**“Plum Pudding” Thomson won the 1906 Nobel Prize for his work on the conduction of electricity by gases.

Ahh, the power of plum pudding.*

*There are not now, nor have there ever been, plumbs in plum pudding. And no, that is not a typo. See http://www.whatscookingamerica.net/Cake/plumpuddingTips.htm.

Forgetting about plum pudding for now… if you are a scientist and have a mental picture of an atom, what are you going to do with it?

See if it matches reality!

How?

Develop a theory. Experiment. Compare with theory.

Where do you start?

The simplest atom. Hydrogen.

The plum pudding model for hydrogen.

A lump of + charge (colored like pudding, just for your viewing enjoyment).

A small, negatively charged electron.

-+Where does the electron go?

What if you pull the electron away from the center?

Restoring force!

If you let go, what does the electron do?

Oscillates!

+

-

Even though the + charge is uniformly distributed, in this case it “acts” like it is all concentrated at the center of the atom.

-

What does an oscillating (and therefore accelerated) electron do?

Emits radiation!In this case, visible light.

You can calculate the energy of the emitted radiation (visible light), and the calculated value agrees nicely with some of the radiation emitted by hydrogen.

You think you’re on to something. What do you do next?More complex atoms!

Unfortunately, for anything beyond hydrogen, the modeling and calculations are too difficult! So what else do you try.

Study hydrogen some more?

When you pass a current at high voltage through hydrogen gas, the gas glows. Several colors of light are emitted. Of all these colors, only the single one mentioned on the previous slide is predicted by Thomson’s model.

Dang!

What next?

If you want to find out what’s inside the pudding, do what Rutherford (one of the Plum Pudding believers) suggested…

Stick your finger in the pudding and see what’s there!

Dang! We don’t understand what’s in the pudding.

Ernest Rutherford, a physicist, won the 1908 Nobel Prize in Chemistry for his work on radioactive decay.

“I have dealt with many different transformations with various time-periods, but the quickest I have met was my own transformation from a physicist to a chemist.”—paraphrase of a quote by Rutherford

Rutherford was the world expert on alpha particles, which are produced in radioactive decay. An alpha particle is a helium nucleus—two neutrons, two protons, tightly bound together.

Alpha particles have 8000 times the mass of an electron and Rutherford’s alpha particles were highly energetic. An alpha particle colliding with an electron would be like a human colliding with an ant.

In 1906, Rutherford discovered that alpha particles were only slightly deflected when they passed through matter. This was expected.

Analyzing this weak scattering would give insight into the distribution of charge and mass inside the atom. Rutherford pioneered the technique of studying matter by scattering beams of particles (or waves).

Rutherford’s co-worker was Hans Geiger (“Geiger counter”).

*“Nothing interesting can possibly happen, but let’s have poor Marsden do it anyway. Ha ha ha...”

Rutherford and Geiger had an undergraduate named Ernest Marsden working for them in 1909. They assigned Marsden the job of measuring the scattering at large angles* of alpha particles by a very thin gold foil.

Indeed, most of Marsden’s alpha particles passed straight through the gold foil.

A few were scattered at large angles.

Some even bounced straight back.

“It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you.”—Ernest Rutherford

USS Washington

15 inch guns

In the Thomson model, electric charge is smeared out over the atomic volume, and minimal interaction is expected between the charged alpha particles and the gold atoms. That’s because there’s no local electric field to deflect a charged particle.

OK, I lied on the previous slide (for dramatic effect). The battleship Washington had a main battery of nine 16-inch guns. I spent a long time looking for pictures of 15-inch guns and shells, with humans in them to illustrate the scale.

Lots of 16-shell pictures, like this one. No 15-inch. But if you see one of these coming at you, are you going to argue about that extra inch?

“It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you.”—Ernest Rutherford

15-inch shell: weight—2700 pounds; muzzle velocity—1570 miles per hour; range—21 miles

Splat!

Scattering experiments “demanded” the model that Rutherford then invented for the atom.

See http://micro.magnet.fsu.edu/electromag/java/rutherford/ for a “toy” to play with.

Expected.

Observed. There must be concentrations of massive, highly charged matter to deflect the alpha particles.figures from: http://www.as.utexas.edu/astronomy/education/spring01/lambert/classnotes9.html

So a “snapshot” of Rutherford’s atom at some instant in time looks like this. -

++++

-

-

-Of course, the electrons and nucleus are very small, but I had to draw them big enough to see.

Rutherford didn’t use the word “nucleus” in describing this model. He used the term “charge concentration.”*

*See http://dbhs.wvusd.k12.ca.us/AtomicStructure/Rutherford-Model.html

Rutherford was confident he had the atom figured out. "The question of the stability of the atom proposed need not be considered at this stage, for this will obviously depend upon the minute structure of the atom, and on the motion of the constituent charged parts."*

About 16 slides back, you probably told me atoms are made of neutrons, protons and electrons. The neutrons and protons are "squeezed" together inside very small nuclei. The electrons "orbit" somewhere outside the nucleus. Protons are positively charged particles, and neutrons are neutral.

http://www.valleystream13.com/Wheeler/science/jan/atom

So your atom looks something like this…

http://www.csmate.colostate.edu/cltw/cohortpages/viney/atomhistory.html

Did it ever bother you when a science teacher told you that the nucleus was made of neutrons and protons? How can lots of positively charged protons be squeezed together inside a small volume? I thought like charges repel.

You have a problem here!

It is true that matter is made of atoms, which are made of neutrons, protons, and electrons.

Did any of you raise your hand and protest that finding a bunch of protons crammed together is not consistent with “like charges repel?”

It is also true that like charged protons repel each other, like charged electrons repel each other, and unlike charged electrons and protons attract each other. The force between charged particles is called the "electro-static" force; "electro-" for obvious (I hope) reasons and "static" because the force exists between particles when they are stationary (as well as when they are in motion).

+ +

This force must be stronger than the electrostatic force.

What does the fact that atomic nuclei are held together tightly imply to you?

There must be some other force in operation besides the electrostatic force.

The force that binds electrons to atoms is much weaker. That's why you can easily "scrape" electrons off atoms.

The force that hold nuclei together is, in fact, extremely strong. That's why atomic bombs and nuclear reactors are so "powerful." I'll talk about it later.

Atoms can bond together to form molecules or crystals. The bonds that hold atoms together in crystals are a result of the behavior of electrons.

I wonder what role the neutrons in atoms play. It seems now as if they are excess baggage. Anybody ever wonder about them? (You’ll have to wait for the answer.)

The bonds can be very weak, as between oxygen molecules (at room temperature it is far too warm for oxygen molecules to form solid oxygen), or quite strong, as in diamonds.

Environmental IssuesIssue 18: Grain Production

and Beef Consumption

If you choose this Issue, you’ll investigate global grain production and the effect of beef consumption on grain usage.

As you do these Issues, the text author will refer you to external sources. You might wish to investigate these sources further. Do these sources of information have a particular axe to grind?

You don’t need to do the “for further study.”