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• The midterm 1 exam scoreswill be available on OWL at5:00 PM today. Login toOWL and click the “CourseGrades” link to see yourscore. Your score hasalready been scaled tocorrespond to the usualgrading scale:

• OWL Homework 3 will beavailable tomorrow aroundnoon. Homework 3 will bedue Sunday, Oct. 19 beforemidnight.

Announcements Astro 101, 10/9/2008

90 - 100: A80 - 89: B70 - 79: C60 - 69: D<60: F

Do the behaviors of matter and energy,observed on the Earth, apply to the whole cosmos?

What do we know about matter andenergy on Earth?

SUMMARY from previous lectureEnergy comes in different forms.• Kinetic Energy

– Bulk motion– Thermal Energy

• Potential Energy– Gravitational potential energy– Chemical potential energy– Electrical potential energy– Nuclear potential energy– Mechanical potential energy

• Radiative Energy (Light)– We’ll spend at least one entire class on this one.

ENERGY IS ALWAYSCONSERVED.

Some forms of energyare more easily controlledthan others, e.g., chemicalpotential energy can beeasily stored for a rainyday, but heat is hard tohold on to…

Electrical charge: a fundamentalproperty of matter

• Oppositely charged particles attract each other• Similarly charged particles repel each other• Like energy, charge is always conserved• The proton defines the basic unit of charge:

– Proton charge = +1• Electrons have the precise opposite charge of the proton

– electron charge = -1• Most ordinary atoms have equal numbers of protons and

electrons and consequently zero net charge, i.e., they are“electrically neutral”

The (Almost) ModernAtom Model

nucleus electron(negative charge)e-

proton (positive charge)

neutron (“neutral” - no charge)

p+n

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A particular element always has the samenumber of protons, i.e., the #protons definesthe element. Hydrogen has 1 proton. Carbon

has 6 protons, oxygen has 8, etc.

nucleus

electron(negative charge)

e-

proton mass = 1836x electron mass

neutron mass = 1.0008x proton mass

p+n

The vast majority ofthe mass is in thenucleus.

The “size” of an Atom• Although it is the smallest part of the atom, most of

the atom’s mass is contained in the nucleus.• The electrons do not “orbit” the nucleus; they are

“smeared out” in a cloud which give the atom its size.

Hydrogen Atom Magnifiedby 1012

(nucleus)

There are 1022 - 1023 atoms in a drop of water. This is comparableto the number of stars in the entire Universe!

Periodic Table of the Elements

atomic number = #protonsatomic mass no. = #protons + #neutrons

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Hydrogene-

p+

atomic number = 1atomic mass number = 1

Helium

e-

p+

n

e-

np+

atomic number = 2atomic mass number = 4

Hydrogen

e-

p+n

Deuterium

isotope

atomic number = 1atomic mass number = 2

p+

n

e-

np+

atomic number = 2atomic mass number = 4

What if an electron is missing?

ion

He+1

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What if two or more atomscombine to form a particle?

p+ p+

8p+

8n

molecule

H2O (water)

Phases of Matter

Matter can take many forms depending mostly on its temperature, density, and internal energy.

As temperature increases, i.e., as the internal kinetic energy is increased, the strength of the bonding between particles decreases.

Phases of Matter Depend on:TEMPERATURE

• Increasing temperature increasesthe average speed of the particles atthe microscopic level• It is more difficult for particles tobond when moving more rapidly

As temperature increases, i.e., as the internal kinetic energy is increased, the strength of the bonding between particles decreases.

Phases of Matter Depend on:DENSITY

• density = mass

• The higher the density, the greaterthe probability that the particleswill collide and stick together

When matter is compressed, its density increases and it can change from gas to liquid to solid.

volume

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Phases Transitions of Matter

• Many phase transitions are quite familiar:– Freezing: liquid → solid– Melting: solid → liquid– Boiling: liquid → gas

• However, substances never have a single temperature;there is always a range of temperatures in matter.Consequently, we can also have:– Sublimation: particles escape from the solid phase

directly into the gas phase– Evaporation: most particles are cool and remain in the

liquid phase, but a few have enough energy to escapeto the gas phase

Who cares?

• Physicists (astronomers are just a type ofphysicist) seem to be obsessed with boringdetails about temperature, density, blah,blah, blah

• Why is all this stuff important?• Consider what happens when we cool

things down using liquid nitrogen…

If we smack a liquid with a hammer, it canabsorb the energy in a variety of ways...

Energy from impact

After impact, loosely bound particlesin the liquid phase can splash around, and absorb the energy

If we smack a solid with a hammer, there arefewer ways that it can absorb the energy

Energy from impact

Something has to give. If there is a placewhere the bonds are weaker, that’s wherethe solid will break apart.

Weaker bonds here

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The world is strange on the microscopic level.Consider how the energy of an objectcan change on the macroscopic scale:

• A 100 kg truck moving at 100 km/s has an easily calculatedamount of kinetic energy:

KE = 1/2 mv2 = 5 x 105 Joules• We can change the speed of the truck by any amount, e.g.,

we can increase the speed to 101 km/s, 101.5 km/s, 150km/s, (whatever) and the kinetic energy will changeaccordingly.

• Likewise, we can lift the truck off the ground to any height,and its gravitational potential energy will increase accordingto a simple equation:

GPE = mhgGPE = mass x height x gravitational acceleration

One might think an electron on an atom wouldbehave the same way, but it does NOT

e-

p+

atomic number = 1

atomic mass number = 1

Hydrogen

• The electron bound to an atomis not simply a mass moving atany velocity.

• The electron is attracted to thenucleus due to its oppositecharge. Therefore one mightexpect it to have potentialenergy analogous togravitational potential energy.

• The bound electron can storepotential energy, but in amore complicated way thangravitational potentialenergy.

Electrons can be in differentorbits of certain energies, calledenergy levels.

Different atoms have differentenergy levels, set by quantumphysics.

Quantum means discrete! Theelectron energy levels are like astaircase... The electron can“step” from one level up ordown to another level, but thebound electron cannot have anenergy between levels

Atom Energy Levels

Ener

gy

nucleus

“Allowed” energy levels for the boundelectron

Bound electrons prefer to be in thelowest energy level, which is called

the ground state.Electron cloud:

Add just the rightamount of energy,and the electron willjump to a higher energy“excited state”.

If enough energy isgiven to the electron,it will be removedfrom the atom altogether resultingin an ION and aFREE ELECTRON.

Unlike excitation, ionization does notrequire a discrete “quantum” amountof energy. The energy only needs toexceed some amount called the“ionization potential” which dependson the atom being ionized.

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Review: phases of matter

Solid.

Liquid.

Gas.

Dissociation: molecules brokenappart.

Ionization: atoms broken apart.

Plasma: free electrons and freenuclei.