Conceptual Physics

Conceptual PhysicsLuis A. AnchordoquiDepartment of Physics and AstronomyLehman College, City University of New YorkLesson VIIOctober 17, 2017https://arxiv.org/abs/1711.07445L. A. Anchordoqui (CUNY)Conceptual Physics10-17-20171 / 19

Table of Contents1Structure and Properties of MatterThe basic building blocks of matterThe microworldmacroworld connectionPhotonsBurningL. A. Anchordoqui (CUNY)Conceptual Physics10-17-20172 / 19

Structure and Properties of MatterThe basic building blocks of matterAll matter is composed of moleculesand these molecules consist of one or more atomsMolecules retain characteristics of substanceMolecules separate whereas atoms do notunless substance is one of the elementsE.g. water molecules:hydrogen and oxygen separately have nothing to do with waterMolecules of an element contain only one kind of atome.g. hydrogen, oxygen, uranium, ironThere are about 100 elementsand therefore 100 different kinds of atomsSubstances which are not elements are called compoundsat present we know of about one million compoundsL. A. Anchordoqui (CUNY)Conceptual Physics10-17-20173 / 19

Structure and Properties of MatterThe basic building blocks of matterBetween 1900 and 1932 we essentially answered question:“What are atoms themselves made off?”99.97% of atom mass is concentrated in nucleus at its centerconsisting of two kinds of heavy particles (protons and neutrons)Other 0.03% of mass consists of very light electronswhich buzz around in fixed orbits very far from nucleusScale of atom is such that if nucleus were size of golfballelectron orbits would be at a distance of 1/2 mileL. A. Anchordoqui (CUNY)Conceptual Physics10-17-20174 / 19

Structure and Properties of MatterThe basic building blocks of matterBesides mass electron and proton have electric chargeAlthough electron is 2, 000 times lighter than protonit has same charge as protonMore precisely it has an equal and opposite chargeNeutron is neutral it has no chargeRecall electric charges come in two kinds: and Like charges repel one another whereas opposite charges attractForces become much stronger when charges are closed togetherL. A. Anchordoqui (CUNY)Conceptual Physics10-17-20175 / 19

Structure and Properties of MatterThe basic building blocks of matterElectrons are held in orbit by electrical attraction of protonsAtom is neutral always equal numbers of electrons and protonsNuclear forceReason protons in nucleus do not fly appartdue to their strong electrical repulsionMuch stronger attractive force between neutrons and protons(a hundred times as strong)comes into play at these small distancesL. A. Anchordoqui (CUNY)Conceptual Physics10-17-20176 / 19

Structure and Properties of MatterThe basic building blocks of matterWe describe atomic structure in terms of:1atomic number Z number of protons inside nucleus2baryon number A number of nucleons N inside nucleusNuclei of all atoms of particular elementcontain same number of protonsbut often contain different numbers of neutronsNuclei that are related in this way are called isotopesFor isotope with baryon number A of element with symbol Xit is common writing ZA XE.g. when we write 23892 U we mean isotope of uraniumwhich has a total of 238 neutrons plus protons and 92 protons(How many neutrons?)L. A. Anchordoqui (CUNY)Conceptual Physics10-17-20177 / 19

Structure and Properties of MatterL. A. Anchordoqui (CUNY)The microworldConceptual Physicsmacroworld connection10-17-20178 / 19

Structure and Properties of MatterThe microworldmacroworld connectionmacroworldmicroworldMicro Worldatoms & moleculesMacro WorldgramsAtomic mass is the mass of an atom inAtomicmass ) mass of an atom in atomic mass units (u)atomic mass units (amu)By definition:u1 atom 12C “weighs” 12 amuOn this scale1H 1.008 uamu16O 16.00 uamu1L. A. Anchordoqui (CUNY)Conceptual Physics10-17-20179 / 19

TheatomicmicroworldmassmacroworldThe averageis theconnectionweightedaverage of all of the naturally occurringaverage atomic massisotopes of the element.weighted average of all of naturally occurring isotopes of elementStructure and Properties of MatterNaturally occurring lithium is:amu)7.42% 6Li (6.015 u)2amu)92.58% 7Li (7.016 u)Average atomic mass of lithium:7.42 x 6.015 92.58 x 7.016 6.941 uamu100L. A. Anchordoqui (CUNY)Conceptual Physics310-17-201710 / 19

Structure and Properties of MatterThe microworldmacroworld connectionAverage atomic mass (6.941)4L. A. Anchordoqui (CUNY)Conceptual Physics10-17-201711 / 19

Structure and Properties of MatterThe microworldmacroworld connection(mol): A unit to count numbers of particlesThe MoleMole(mol):A unit to count numbers of particlesDozen 12Pair 2Mole amount of a substance that contains as many elementary entitiesTheasmole(mol) is the amount of a substance thatthere are atoms in exactly 12.00 grams of 12Ccontains as many elementary entities as thereare atoms in exactly12.00 grams of 12C1 mol N 6.022 x 1023A6.0221367x 10231 mol NNAA Avogadro’snumberL. A. Anchordoqui (CUNY)Avogadro’snumber (NA)Conceptual Physics10-17-201712 / 195

Structure and Properties of MatterThe microworldmacroworld connectioneggsMolar mass is the mass of 1 mole of shoes in gramsmarblesatomsegMolar massis thegmass of 1 mole of shoeggsx 1023 atoms1 mole 12C atoms 6.022 12.00maratoMolar mass is the mass of 1 mole of shoes in gramsmarbles 12.00 amu1 mole 12C atoms 6.022 x 1023 atoms1 12C atomatomsu1 mole 12C atoms 6.022 x 1023 atoms 12.00 g112C1 12C atom 12.00 amuu12C atoms 12.00 g 12C1atommole 12.00amuu1 mole lithium atoms 6.941 g of Li1 mole 12C atoms 12.00 g 12C1 mole 12C atoms 12.00 g 121 mole lithium atoms 6.941 g1 112moleC atom12.001.66 x 10-24 g 12.00 glithiumx atoms 6.941g of gLi1 12 C atom x 2312For any element12C atomsForx any12.00 amu 12.00Cxatoms6.0221 amu6.0221023 element1 12C atomg x 10 1.6610-24 g 12.00 amux 23 element12C atoms(u) molar mass (atomic mass (amu)Forx any12.00 amu atomic6.022101 amumass (grams)(u) molarmass(amu)(u) molar mass (grams)atomic mass (amu)1uamu 1.66 x 10-24 g1uamu -241.66 x 10-24 g or 1 23g 6.022 x 1023 amuu 61uamu 1.66 x 10 g or 1 g 6.022 x 10 amuuor 1 g 6.022 x6M molar mass in g/molM molar mass in g/molmass inConceptualg/mol PhysicsM molarnumberNA Avogadro’sL. A. Anchordoqui(CUNY)NA Avogadro’s number10-17-201713 / 19

Structure and Properties of MatterPhotons Two atoms of C and O combine to form a CO molecule as follows L. A. Anchordoqui (CUNY)Conceptual Physics10-17-201714 / 19

Structure and Properties of MatterPhotonsIn one way or another (which we will discuss soon)C and O atoms are driven up against one anotherNegative electrons repel each other and atoms fly appartBut once in a while pair of atoms come together so hardthat electrons are driven past each otherNegative C electrons get far enough past negative O electronsand begin to experience attractive force of positive O nucleusSame history for oxygen electronsWhen this happens C and O combine into stable CO moleculeL. A. Anchordoqui (CUNY)Conceptual Physics10-17-201715 / 19

Structure and Properties of MatterPhotonsAs atoms snap together something very important happens: Small packet of energy (called photon) is emitted(sort of like when 2 magnets snap together little heat is generated)For each molecule of CO formed a photon is emittedand all photons have same energySo every time 3 grams of C combines with 4 grams of Osame amount of energy (6 Cal) is released(in form of million billion billion photons)Reason C and O always combine in proportions 3-g-C to 4-g-Ois that 3 g of C contain same number of atoms as 4 g of Oand atoms just combine one-to-oneL. A. Anchordoqui (CUNY)Conceptual Physics10-17-201716 / 19

Structure and Properties of MatterBurning Now we come to a crucial consideration chain reaction Let us suppose following two considerations are fulfilled:12There are many C and O molecules togetherC and O molecules are well interspersedThen γ originating in combination of one pair of C and Ohas good chance of hitting another C (or O)and driving it with enough force to combine again with O (or C)Combination will go on without outside energyso long as there are enough C and O atoms close togetherand they are interspersed enough [conditions (1) and (2)]This is what we call burningCondition (1) can be translated to mean “we need enough fuel”Condition (2) “give the fire some air!”L. A. Anchordoqui (CUNY)Conceptual Physics10-17-201717 / 19

Structure and Properties of Matter BurningHow do we ignite the reaction? We must agitate C-O mixture so that C’s really slam into O’sovercoming repulsion of outer electronsAgitating group of molecules to larger average velocityis definition of raising temperature of groupWe can raise temperature of mixture by:12introducing another source of photons (a light match)physically agitating the C’s (friction)e.g. as rocket nose cone burns when it rushes through airL. A. Anchordoqui (CUNY)Conceptual Physics10-17-201718 / 19

Structure and Properties of Matter BurningWhere does the photon come from? Unfortunately complete revelation of these secretswould require you to take graduate physics courseAt this point we can simply provide following explanationPhoton does not exist in atoms before it appearsBut when it appears something else does disappear: massMass of CO molecule is less than sum of C and O atom massesphoton energy EEinstein discovered relation between:disappearing mass mE mc2c speed of lightIn energy units you have learned thatappearance of 2 Cal of energy in photonsis associated with disappearance of 10 10 g of massRoughly speaking 10 8 -th of mass of fuel burned disappearsL. A. Anchordoqui (CUNY)Conceptual Physics10-17-201719 / 19

Atomic mass is the mass of an atom in atomic mass units (amu) Micro World atoms & molecules Macro World grams microworld macroworld Atomic mass ! mass of an atom in atomic mass units (u) u u u L. A. Anchordoqui (CU