1Physics 1010:The Physics of Everyday LifeTODAY• Electrostatics• Magnetostatics2Admin Stuff• If you need grade verification for yourscholarship, see me after class• Prof. Finkelstein will give lecture onTuesday (circuits)• HW 7 is due by midnight Monday• Some HW grades have been mixed up, Jingis looking into it3“Kinds” of Forces(forces that we talked about in class)• spring force• normal force: force perpendicular to a surface• Gravity• Friction• Force due to pressure• Centripidal4• person’s force (pushing or pulling)• gravity• spring force• normal force: force perpendicular to a surface• friction: force tangent, or parallel, to a surface• fluid pressure (times area)• tension (of a rope)“Kinds” of Forces(forces that we talked about in class)5• person’s force (pushing or pulling)• gravity• spring force• normal force: force perpendicular to a surface• friction: force tangent, or parallel, to a surface• fluid pressure (times area)• tension (of a rope)However: this is not a very insightful or even useful classification offorces; it merely summarizes some of the forces we’ve dealt with.“Kinds” of Forces(forces that we talked about in class)6The Four Fundamental Forcesand Unifications Leading Thereto and TherefromGravitationalElectromagnetic(the) Weak force(the) Strong forceElectric forceMagnetic forceGUT ?(grandunifiedtheory)ElectroweakinteractionTheory of Everything ?: BIG GUT = GUT + Gravity,explains all forces as aspects of a singleinteraction theory}}}InterplanetaryApples on earth}}today’s subject7Frictions isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force8Frictions isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force9Pulling on a string isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force10Pulling on a string isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force11The spring force isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force12The spring force isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force13The buoyancy force isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force14The buoyancy force isA) GravityB) ElectromagneticC) The Weak ForceD) The Strong Force15• Except for gravity, all of the “kinds” of forces wedraw in force diagrams are due to the Electromagnetic force (mostly electric)• friction• tension• fluid pressure• normal force• “pushing” force• spring force16Lennard-Jones Potential Force ~ - sloperrVAll “contact” forces(spring force, friction,water pressure, etc)are a result of molecules“pushing” or “pulling”against each other. They are all due to the Lennard-Jones potential.This force isELECTROSTATIC17The electric forceElectric charge comes in two “flavors”: it can be positiveor negative. Pick the statement that best characterizesthe “direction” of the electric force:A. Like charges attract (each other); opposites attractB. Like charges repel; opposite charges attractC. Like charges attract; opposite charges repelD. Like charges repel; opposite charges repel18The electric forceElectric charge comes in two “flavors”: it can be positiveor negative. Pick the statement that best characterizesthe “direction” of the electric force:A. Like charges attract (each other); opposites attractB. Like charges repel; opposite charges attractC. Like charges attract; opposite charges repelD. Like charges repel; opposite charges repelAnswer: B. For example, two electrons wouldaccelerate away from each other, and two protonswould accelerate away from other, but an electron anda proton would accelerate toward each other (electronshave negative charge, protons are positive).+ +- -+ -19Elementary Charge• Animal hair (wool, fur) likes to acquire a positivecharge from (or give up a negative charge to)rubber• Silk likes to acquire a negative charge from (orgive up a positive charge to) glass• Where do the charges come from?All macroscopic charges are integer multiples of the electron(negative) or proton (positive) charge.Amounts of charge are measured in Coulombs (C).A proton has a charge 1.60 x 10-19 C; an electron has a charge-1.60 x 10-19 C. That is, 6.25 x 1018 protons make 1 C ofcharge; 1 C is a lot of charge.20Charge Conservation• Although neither electrons nor protons areconserved (both can be created and destroyed) theTOTAL CHARGE IS ALWAYS CONSERVED.• Just as energy and momentum conservation allow usto solve otherwise difficult problems, so chargeconservation will allow us to solve some toughproblems.21Calculating gravitational and electric forces• Gravitational force between two masses,m1 and m2, separated by distance r:+ +- -+ - ! F1on 2,g= Gm1m2r2m1m2rG = 6.67 x 10-11 N m2/kg2• Electric force between two charges, q1 andq2, separated by distance r: ! F1on 2,e= kq1q2r2q1q2rk = 8.99 x 109 N m2/C2“Coulomb’s Law”“Newton’s Law of Gravitation”22Aside: Inverse-Square LawsBoth the electric and gravitational forces obey inverse-squarelaws:F∝1/r2 -- the force (of one mass or charge on another) isproportional to the inverse square of the distance (betweenthem).Most important: the force diminishes with distanceSecond most important: doubling the distance quarters theforceInverse-square laws reflect the three-dimensionality of ouruniverse: the area of a 3D sphere of radius r is A = 4πr2 ∝ r2 .“Stuff” spreading out in 3 dimensions obeys an inverse-squarelaw.23Inverse-Square LawsA special spray-paint nozzle spraysequally in all directions. In 1 min., itpaints a 1mm thick coat on the insidesurface of a sphere of radius 1 m.How thick would the paint be if thesphere had a 2 m radius?A. 1/8 mmB. 1/4 mmC. 1/2 mmD. 1 mmE. 2 mmArea of sphere = 4πr224Inverse-Square LawsB. The 2 m sphere has 4 times the area (A ∝ r2) of the1 m sphere, but receives the same TOTAL amount ofpaint in the same time; therefore, the paint must be 1/4as thick. After 1 min. of spraying, the thickness ofpaint on a sphere of radius r varies as 1/r2 -- an inverse-square law. “Stuff” spreading out in 3 dimensions obeysan inverse-square law (e.g., light and sound intensity).A special spray-paint nozzle spraysequally in all directions. In 1 min., itpaints a 1mm thick coat on the insidesurface of a sphere of radius 1 m.How thick would the paint be if thesphere had a 2 m radius?A. 1/8 mmB. 1/4 mmC. 1/2 mmD. 1 mmE. 2 mmArea of sphere =