ReviewReview starts from beginning and ends aroundthe material that was on exam 5.Remember you are allowed an 8.5x11 sheet ofpaper for notes for the final.
Phases of matter• What makes a Solid• What makes a Liquid• What makes a Gas• Strong bonds between molecules.• Ordered structure.• Molecules do not move past oneanother but do vibrate.• Attraction between molecules.• Ability of molecules to move past one another.• No bond between molecules.• Low density.• Ability of molecules to move past one another.•Rapidity of movement.
The Kelvin Temperature scale• Starts at absolute zero (0 K or –273ºC); as cold as it canget.• Each degree is the same size as a Celsius degree• Ice melts at 273 K• Water boils at 373 KWhat does absolute zero mean?
Properties of Substances• Physical Properties- Tells us about the stuff itself, without it being turnedinto a new substance.(Example: water is a liquid at room temperature.)• Chemical Properties- Tells us what it can change into, and under whatconditions.(Example: water can be split into hydrogen and oxygen gas ifyou run an electric current through it.)
Classify the following as eitherchemical or physical properties:• Is green• Has density 2.1 grams/cm3• Will ignite at 451 degrees Fahrenheit• Boils at 100 degrees Celsius• unstable (as in explodes when dropped)• Is 6 feet tall• Will explode when it contacts water• Will dissolve in water (such as dissolving sugar into water)PhysicalPhysicalChemicalPhysicalChemicalPhysicalChemicalPhysical*
How we describe changes in stuff:Physical and Chemical Changes• Physical change– change in size, shape, etc., but molecules remainthe same.• Chemical change– new stuff gets made.
Classify the following as eitherchemical or physical changes:• It is burning• It is melting• It is evaporating• It is turning brown in the presence of air• It is forming bubbles when I add it to water• It is boiling• It is rustingChemicalPhysicalPhysicalChemicalChemicalPhysicalChemical
Chemical formula• Represent what atoms in acompound• Uses abbreviation fromperiodic table to representeach type of atom• Subscripts following tell howmany of that type of atom(one is implied, so H2O =H2O1).O2CH4H2OCO2
Most materials are mixtures:• Air: Nitrogen (N2), Oxygen (O2), minor amounts ofother gasses• Steel: Iron (Fe), Chromium (Cr), Nickel (Ni), Carbon(C)• Seltzer: Water (H2O) and Carbon Dioxide (CO2)In fact, tap water is a mixture – it contains many things,typically around here carbonates (we have ‘hard’water in this region).
• Mixtures do not react when formed - when youmix salt into water the salt stays salt (NaCl)and the water stays water (H2O).• Mixtures may be separated by physical means:– Solids separated from liquids by filtration– liquids from other liquids by distillation
So we break down matter like this:MATTERASK: Can it be separated by some physical method?YES NOMixture SubstanceYES NOASK: Can it now be separated by someother method?Substance contains morethan one atomCompoundSubstance contains only onekind of atom, has no bondsElement
Substances:Elements and Compounds• Consider a pure substance.“Pure” refers to a substancewhich, from a MACROscopicview appears:• Suppose that you try to dosomething to break it into two ormore different substances.– If we CAN then we say that the originalsubstance was a(n):– If you CAN’T then we say that thesubstance is a(n):uniformCompoundElementCompounds Element
Separation:• (Demo) Mechanical separation, also known as filtration.• Separation by boiling point, also known as distillation.• Centrifuge – a form of mechanical separation by rapidspinning, common for separating the components of blood.
What makes an atom?• Protons– in the Nucleus• Electrons– in a cloud surrounding thenucleus– Equal # of electrons & Protons(atoms are electrically neutral)• Neutrons– in the Nucleus– stabilizes nucleusProtonsNeutronsNucleus
• Atoms are mostly emptyspace.– Electrons in this picturewould be smaller than apinhead.Recall that air is mostlyempty space2 Miles(apparent size of atom)Basketball sized nucleus
Electrons are responsible for chemicalproperties• Why could this be?– Think about the structure...• where are the protons?• the neutrons?• the electrons?• Electrons are the only accessible part of an atom. Theprotons and neutrons are bundled up deep inside.• Unless they are radioactive, atoms will only lose (orgain) electrons – not protons or neutrons.
What holds an atom together?• The electric charge ofthe proton and electronhold an atom together.Gravity doesn’t have much sway at thissize – the electric force is muchstronger here.What is the rule for North and South polesof magnets?Positive and negative charges are similar:opposites attract, likes repel.
Like Chargers Repel• Given that like charges repel, why do we havesolids?– Electrons move around, temporary polarization,sharing of electrons between two nuclei• Okay, so they can attract. Why dont the justform a blob?– The nuclei dont like each other.
Conservation of Charge• In the processes you witness today noelectrons or protons are created or destroyed.• Just as energy is conserved, so is chargeconserved – the universe’s net charge is aconstant.• There are no known violations of this principle(it’s more than a theory, we consider it a law).
Coulomb’s Law221 **dqqkF =• Force is proportional to charge.• Charge is measured in Coulombs. 1C = 6.25*1018electrons ofcharge.• Force is inversely proportional to the square of distance.• k is a constant value (think of it as being ‘like pi’ but not 3.14).q1 q2d
Polarization• Electrons are very light(about 1/2000ththe mass of aproton or neutron).• They can easily be pushedaround by the electric force.• Imagine the electron cloudgetting displaced slightlyfrom the nucleus at thecenter...
Demo: Induced Polarization• demo balloon on wall:The wall is not charged, but theballoon sticks – electrons in wallget pushed around.
Example Problem• When you rub a balloon on your hair, does itbecome charged?• Does your hair become charged?• When you then stick the balloon to a wall(assuming it is dry enough to work) is theWALL charged?
Inherent Polarization: Water• Water is a polar molecule• The oxygen carries a partialnegative charge and thehydrogens carry a partialpositive charge.• Oxygen has a stronger ‘hold’on electrons.• Water can ‘hydrogen bond’through these weak partialcharges, which makes waterunusually stable...and allowsus to have some fun.Image courtesy Qwerter, GNU Free Documentation License
Demo: Water and an electric force• Water is a dipole.– dipole means there is a slightcharge separation.• Water, since it is charged, willinteract with another chargedobject.+- - - -
Atoms: attraction and repulsion• Repulsion close-electrons in each atompush against each other• No force far away –atoms are overall neutral• What of the attractionregion? Polarization atwork.-0.500.511.50 1 2 3Force(relative)Distance (relative)Force between two atoms
Example Problem• Is a polarized object charged?
Electric FieldsAn Electric Field is similar to agravitational field (we live inEarth’s gravitational field).It is also similar to a magneticfield (you can see magneticfield lines by pouring ironfilings on a magnet).• A charged particle in anElectric Field will experience aforce.
Conductors• Electrons in conductors are very mobile.• Will always separate so as to cancel theelectric field inside.• Faraday Cage: using of metal to create astructure that shields against electric fields.NoElectricFieldInside----++++++---+
Metals• Highly conductive ofheat and electricity.• Ductile (may be pulledinto wires)• Malleable (may bepounded flat)
Nonmetals• Poor conductors of bothheat and electricity.• Solids are brittle – notmalleable or ductile.• Many nonmetals aregasses at roomtemperature.
Metalloids• Properties in between metalsand nonmetals.• Semiconductors – basis ofmodern civilization.(May be more like a metal ormore like a nonmetaldepending on position –closer to metals, metallic andvice-versa)
A look across the table: periodsSome properties change in a regular way as yougo across a row (natural enough, as thevalence “shell” is filling up as you go).• Size decreases• Electronegativity (how much the atom wants anelectron) goes up.
Columns: Grouping upAll elements in the same column have the samevalence (outer) electron arrangement.• Size goes up as you go down a column• Electronegativity goes down• Elements in the same column tend to have similarpropertiesFor example: Cu, Ag, Au all in one group – and are amongthe few elements to be found naturally in pure forms; theyare unusually nonreactive)
Ionic BondsFairly straightforward:• The Atom with the higher electronegativity steals an electron from the atom with(much) lower electronegativity.• They are then bound by the electric force (which is really strong).• Typically form from elements on opposite sides of the periodic table.Think of it as the bully stealing lunch money from the little kid who then follows himaround begging for it back.
More on Ionic Bonds• Must have charges balanced• May be more complex thanNaCl –– CaF2– Al2O3It all depends on how manyelectrons each is capable ofgiving up/receiving.• Crystal shape determined byinteractions– break a salt crystal and theshards will tend to have thesame shape
Covalent Bonds• Both atoms want one* more electron• Solve problem by sharing (neither is strong enough to take itfrom the other)Imagine two kids playing with (sharing) the same toy.*sometimes more than one, but in this picture only one.
Polar Covalent BondsUnequal sharing.• Atom with larger electronegativity has a partial negative charge.• Atom with smaller electronegativity has partial positive charge.• Electrons are still shared, but spend more time around largerelectronegativity atom.
Metallic Bonds• Metals have a very weak hold on outer electrons• In a lump of metal, these outer electrons escape• Resulting metal ions stabilized by a ‘sea’ of free electrons(This is why metals can mix into alloys so well – metal ions,regardless of type, stabilized in a sea of electrons.)
Chemical EquationsThe general form of a chemical equation:reactants → productsA specific example:C(s) + O2(g)→ CO2(g)
A simple rule for all chemicalequations:Law of Mass Conservation:Matter is neither created nor destroyed.What this means:There must be the same number of each type ofatom on each side of a chemical equation.
Two more rules:• You cannot change the identity of a substance.– Changing H2O to H2O2 is not allowed.The first is water the second is hydrogen peroxide, a poisonwhen pure.You may change the number in front of a chemical. Itmultiplies the number of atoms of each type you have.For instance H2O changed to 2 H2O. First case has 2 hydrogenand one oxygen, second has 4 hydrogen and two oxygen.
Suggested Procedure1) Always balance one element at a time. Startwith the most complex reactant.2) If you unbalance previously balancedelements don’t worry about it now.3) After each element is done, then go back andfix previous ones.4) Repeat step 3...5) Minimize coefficients if necessary (you mayget 2:4:2 – reduce to 1:2:1).
Speed• Average speed measures how far you travel in aperiod of time:• Instantaneous speed also measures how quickly youare going – but right now.Which does a speedometer measure?requiredtimecovereddistancespeedaverage =
The difference between average andinstantaneous• Imagine you covered a 40 mile distance in 60minutes.– What would your average speed be?– If you started and ended at a rest, would this alsobe the highest speed reported by yourspeedometer?40 MPHNo, the highest speed reported would be considerably higher. Why?
Constant Velocity• Both speed AND direction must be constant.• Are any of these examples of constantvelocity?– A car travels 40 MPH North– A car comes to a stop– A car accelerates– A car heading south makes a right turn
Motion is relative• Right now, relative to the earth we are stationary.• Relative to the center of the Earth we are moving1600 KPH.• Relative to the sun we are moving 100,000 KPH.• Relative to the center of the galaxy we are movingeven faster.When we talk about motion we must always specify thepoint from which we are making our measurements!
AccelerationHow can a velocity change?In order to change there must be an acceleration:As with velocity we may have an instantaneousacceleration – we won’t deal with that in this class,though, and we will instead stick to constantaccelerations.elapsedtimeyin velocitchangeonacceleratimean =
Acceleration and velocity• Let’s imagine you have a car that goes 0-60MPH in 10 seconds. What is its acceleration?acceleration = (60MPH – 0 MPH)/10s = 6 MPH/s6 miles per hour per second.How about something sportier? Tesla electric car: 0-60 MPH in 3.9 seconds (let’s call it 4).
Freefalling• Gravity acts as aconstant acceleration:ag = -10 m/s2In each second we gain10m/s in speed.What would the speed beat 3 seconds?– 4 seconds?– 5 seconds?
Thrown ObjectsPrelude to Projectiles• In each second the ball loses10 m/s velocity.• v = v0 + a*tHow can we tell how long it takes toreach the highest point?When at highest point, v = 0.So when a*t = v0 or t = v0/a
Projectiles: 2d motion• How does gravity act?Only affects verticalmotion.• Horizontal speed isconstant.• Ball will movehorizonally equalamounts in equal times.
Note that the horizontal components are always the same and the verticalcomponents change.
• Provided they have the same initial speed, objects willalways travel farthest when thrown at 45º.• Naturally, reality interferes – air resistanceGoing the Distance: long range
Faster and faster: getting to orbit• What happens if youthrow an object fasterand faster?– Goes further beforehitting ground.– What if ground fallsaway?And the Earth is curved...
Everlasting freefall• An orbiting object isfalling around the Earth.• Tricky question toanswer: is gravity stillpulling on an astronaut?
Newtons First LawAn object at rest will remain at rest unless actedupon by a force;An object in motion will remain in motion unlessacted upon by a force.Consider a ball left on a flat surface: will it move?Remember footage of astronauts playing – what isthe motion of a tossed object like?
Net ForceTypically more than oneforce act on an object.The force observed(acceleration observed)will be sum of all forcesacting on object.May be split intohorizontal and verticalcomponents.
Newtons Second LawThe acceleration of an object isproportional to the force applied andinversely proportional to the mass ora = F / m Or more commonly F = m * aa and F are vectors (have both a directionand a magnitude).
If the velocity isconstant what is thenet force?Why?If the net force isnon-zero, how wouldyou know?v = 1000 km/hrThrust is 100,000 N
Not free fallTerminal velocity:when an objects force fromair resistance equals itsweight.150-200 km/hr for askydiver.Canopy adds airresistance:terminal speed 15-25 km/hr.what if one diver has thesame size chute but weighsmore?
Newtons Third LawFor every action there is an equal and oppositereaction.Some action/reaction pairs:I push on the floor, the floor pushes back on mewith equal force (unless it breaks).Tug of war (when at a standstill) – west grouppulls rope with a force equal in size but oppositein direction to east group.
Simple Rule:Action: Object A exerts a force on object BReaction: Object B exerts a force on object AA BF on Adue to BF on Bdue to A
3rdlaw example problemA 5 kg and a 1 kg mass collide.Which experiences the larger force?Explain.
CannonAction forces and reactionforces must be the samesize.Why does the cannonballfly so fast?Why doesn’t the cannonmove as much as theball?Recall Newton’s 2ndlaw:acceleration = force/massFma = Fma =
Wrapping up Newton’s LawsAn object will not accelerate (change directionor speed) unless acted upon by an unbalancedforce.The acceleration of an object is proportional tothe force on it and inversely proportional tomass (a = F/m).So what will happen to acceleration if you doubleforce with same mass?If you double mass with same force?If object A exerts a force on object B thenObject B exerts an equal in magnitude butoppositely directed force on A.
Momentum:Momentum = mass * velocityMomentum has a direction and thatdirection matches the direction of motion.Changing direction requires a change inmomentum.
Coming to a stop:Braking: Low force over long timeCrash: High force over short time.What is the momentum change for a truck braking to astop? For a truck hitting a wall?Result is the same: same mass, same starting velocity andfinal velocity is zero.crunch!vFvv
CollisionsImagine two 1-kgcarts.Momentum before =1kg*10m/s = 10kg*m/smomentum aftermust be equal.2 kg * v = 10 kg*m/sv = 5 m/s
WorkWork: force takingplace over distance.W = F * dWork is only donewhen an objectmoves against aforce.No move, no work.No force, no work.
Gravitational Potential EnergyPE = m*g*hm = massg = 10 m/s2h = distance abovesurfaceDouble m double PEDouble h double PE
Kinetic Energy• Energy of motion –– KE = ½*m*v2– So the more massive the moreenergy– but more dramatic, the morevelocity the MORE energy.Potential Energy and KineticEnergy freely interconvert:dropped objects, the releaseof a bow, etc.
Example Kinetic Energy Problem• Which has more kinetic energy – a car traveling at 30km/hr or a car half the mass traveling at 60 km/hr?• A little more complicated: KE depends directly onmass so halving the mass would have the kineticenergy BUT it depends on the square of the velocityso twice the velocity would be 4 times the KE. Theresult is that the lighter car at 60 km/hr has twice thekinetic energy of the heavier, slower car.
So a pendulum is kinda interesting...• All potential energy at either end.• All kinetic at the lowest point.• Knowing how far the bob falls you can calculateexactly how quickly it will be going at the bottom.
Conservation of energy• In the absence of external work input or output,the energy of a system remains unchanged.Energy cannot be created or destroyed.
Power• Energy Expended over Time:• The unit of power is the watt.• Power is also the rate at which energy is changedfrom one form to another.• You are familiar with 60 watt lightbulbs, the spaceshuttle can develop 33,000 MW.intervaltimedoneworkpower =
Gravitational Force• The gravitational force isalways attractive.• Proportional to each massinvolved (doubling m1 OR m2doubles the force).• Inversely proportional tosquare of the distancebetween masses (double d,¼ force).221dm*m*GF =
The Moon “Falls” Around the Earth• Newton imagined the moonwould travel in a straight line– unless pulled downward byEarth.• Ultimately, he proved that,just as the projectile fallsunder its ideal path, so doesthe moon.• The force that makes anapple fall and the moon orbitare the same.
Example ProblemImagine an impossibleladder that extends intothe sky.• A girl, weighing 600 N,stands at the bottom. Shethen climbs to four timesthe earth’s radius.– What is her weight now?Distance goes from 1*earth radius to 5*earth radius (ladder adds 4x earth radius).Distance therefore increases 5 times.Distance squared will go up 25 times.Force depends on the inverse of distance squared so it will go down 25 times:600/25= 24 N
Possible Test Problem• Consider a 1-N apple in atree.– If the tree were twice as tall,would the weight of the applebe ¼ as much?– Why or why not?– The problem is one of “whereis our reference point?” Similarto the problem withtemperature – We need tocount from the ‘right’ zero.The right zero is the center of the earth, some 6000 Km (6,000,000 meters)Increasing the height by 2 meters is negligible (6 million vs. 6 million and 2 meters).
• When you fall you accelerate.• If your acceleration matchesgravitational acceleration (10m/s2) you will feel weightless.• This does not mean gravityis no longer acting on you.– If gravity were no longer acting onyou then you would notaccelerate.m*am*g
Weight and Gravity not necessarilyequal:What do you feel inside anelevator?• Going up, as you accelerateyou feel heavy.• Going down, as youaccelerate, you feel light.• When you stop you feelnormal weight.• (If the elevator were to breakand you were to drop youwould feel brieflyweightless).
Weightlessness• If you accelerate freelyyou have no weight.• Freefall, no matterwhere it occurs, resultsin ‘no weight’.• You may still beundergoing agravitationalacceleration, such as anastronaut in orbit.
Heat• Heat always flows from warmer to colder.• Heat is the flow of thermal energy.• Thermal energy is a property of an object.• Heat is measured in calories1 calorie = 4.28 joules• Food is measured in Calories.1 Calorie = 1000 calories
Specific Heat Capcity• Measured in Joules pergram-Kelvin or Joules pergram-Celsius.• To calculate heat required tochange temperature:– Heat = c*m*ΔT• Water has a high heatcapacity. This has wonderfuleffects on our climate.
Example Problem• The specific heat capacity of copper is more thantwice that of silver. This means that if we had 1-kg ofcopper and 1-kg of silver, and each started at thesame temperature and each received 1000 Joules ofenergy, thena) the copper would end up hotter than the silverb) the silver would end up hotter than the copperc) both pieces of metal would end up having the sameincrease in temperature
Heat• Heat always moves from warmer to cooler.• All objects radiate energy– if energy radiated is less than absorbed, object heats up– If energy radiated is more than absorbed, object cools off– If energy radiated is equal to that absorbed the object is at thermalequilibrium (as all things at room temperature are).Heating Up Cooling Off Equilibrium
Sample Problem• The picture to the leftrepresents an object doingwhat? (heating, cooling,equilibrium.)• Why?Heating. It is receiving more energy than it is radiating.
Heat is a transfer of thermal energy• All objects contain thermal energy.– Energy contained is proportional to temperature– Energy is also proportional to ‘size’ of material– Two objects of the same ‘size’ at the sametemperature contain the same thermal energy.
• Why is it that something like a speck of hotwater or oil kicked out of a pan will rarely burnyou?– While it is much hotter, it is very small and containslittle thermal energy.