DEVIL PHYSICS THE BADDEST CLASS ON CAMPUS IB PHYSICS

TSOKOS LESSON 10-1 DESCRIBING FIELDS

Essential Idea: Electic chages and masses each influence the space aound them and that influence can be epesented though the concept of fields.

Natue Of Science: Paadigm shift: The move fom diect, obsevable actions being esponsible fo influence on an object to acceptance of a field s action at a distance equied a paadigm shift in the wold of science.

Theoy Of Knowledge: Although gavitational and electostatic foces decease with the squae of distance and will only become zeo at infinite sepaation, fom a pactical standpoint they become negligible at much smalle distances. How do scientists decide when an effect is so small that it can be ignoed?

Undestandings: Gavitational fields Electostatic fields Electic potential and gavitational potential Field lines Equipotential sufaces

Applications And Skills: Repesenting souces of mass and chage, lines of electic and gavitational foce, and field pattens using an appopiate symbolism Mapping fields using potential Descibing the connection between equipotential sufaces and field lines

Guidance: Electostatic fields ae esticted to the adial fields aound point o spheical chages, the field between two point chages and the unifom fields between chaged paallel plates Gavitational fields ae esticted to the adial fields aound point o spheical masses and the (assumed) unifom field close to the suface of massive celestial bodies and planetay bodies

Guidance: Students should ecognize that no wok is done in moving chage o mass on an equipotential suface

Data Booklet Refeences: W W qv e mv g

Utilization: Knowledge of vecto analysis is useful fo this sub-topic

Aim Models developed fo electic and gavitational fields using lines of foces allow pedictions to be made but have limitations in tems of the finite width of a line

Intoductoy Video The Foce of Gavity

Newton s 2 nd Law Newton s second law (F=ma) implies that if a mass is acceleating, thee must be a foce acting on it An object falls because of gavity What holds planets in thei obits?

Newton s 2 nd Law Newton s second law (F=ma) implies that if a mass is acceleating, thee must be a foce acting on it An object falls because of gavity What holds planets in thei obits? Gavitational Foce

Newton s Law of Gavitation The attactive foce between two point masses is, Whee, F G M M M1 and M2 ae the masses of the attacting bodies is the distance between them 1 G is Newton s constant of univesal gavitation and has a value of 6.667 x 10-11 N m 2 kg -2 2 2

Newton s Law of Gavitation The diection of the foce is along the line joining the two masses,

Newton s Law of Gavitation The fomula applies to point masses, which means the masses ae small in elation to the sepaation between them

Gavitational Field Stength The gavitational field stength at a cetain point is the foce pe unit mass expeienced by a small point mass, m, at that point. M1m F G 2 F ma M1m ma G 2 M a g G 1 2

Gavitational Field Stength The units of gavitational field stength ae Nkg -1 1N = 1 kg m s -2 So units become m s -2 M1m F G 2 F ma M1m ma G 2 M1 a G 2

Gavitational Field Stength Gavitational field stength is a vecto quantity whose diection is given by the diection of the foce a point mass would expeience if placed at the point of inteest.

Gavitational Field Stength The gavitational field stength aound a single point mass is adial which means it is the same fo all points equidistant fom the cente of mass and diected towad the cente.

Gavitational Field Stength On a mico- vesus macolevel (like the pojectile motion of a football), the field stength can be consideed to be unifom with a constant value.

Gavitational Potential Enegy The gavitational potential enegy of two bodies is the wok that was done in binging the bodies to thei pesent position fom infinitely fa apat. Negative sign signifies a foce of attaction M1m F G 2 W Fxd F W E M1m G 2 M1m G P

Gavitational Potential Enegy The gavitational potential at a point P in a gavitational field is the wok done pe unit mass in binging a small point mass m fom infinity to point P. W V V g g G W m m G M1m G M 1 M m 1 GM 1

Gavitational Potential Enegy Gavitational potential is a scala quantity Units ae J/kg (wok pe unit mass) GM1 V g

Gavitational Potential Enegy If a point mass m is moved fom point P to point Q, it has a change in potential It takes wok to do this, thus it also has a change in potential enegy V g W W GM mv E gq P 1 mv mv gp g

Gavitational Potential Enegy The wok done is dependent only on the change in position, not on the path taken The movement must be done at a vey small constant speed so that kinetic enegy is not involved V g W W GM mv E gq P 1 mv mv gp g

Intoductoy Video: Electic Fields and Potential

Electic Field An electic field exists aound any chaged object and extends/adiates eithe into o out of the object By convention, chage flows fom positive to negative so, Fo a positively chaged object, the field lines extend outwad

Electic Field Fo a positively chaged object, the field lines extend outwad + - Fo a negatively chaged object, the field lines extend inwad

Electic Field The field does not exist unless shown to exist by a chage We use a small positive test chage, q, to detemine if a field exists bing the test chage close and if it expeiences a foce, then a field exists

Electic Field Electic field is defined as the foce pe unit chage expeienced by a small positive test chage, q, F k q q 1 2 2 E E F q kq 2 The electic field is a vecto with diection being the same as the foce a positive chage would expeience at the given point +

Electic Field Units fo electic field is N/C E F F q qe

Electic Field The electic field fom a single point chage, Q, at a point a distance away is E F F k qe k E k F q qe Q q Q 1 2 Q q 2 1 2

Electic Potential Enegy Conside an electic field and a positive test chage q In ode to move the chage fom its equilibium position, wok must be done

Electic Potential Enegy If held in that new position, the test chage now has potential enegy like a compessed sping because it wants to go back to its equilibium position

Electic Potential Enegy It takes wok to move the chage fom one places to anothe The amount of wok is equal to the change in potential enegy F k W Fxd E E P P k k Q q 1 2 Q q 1 2 Q q 1 E P

Electic Potential Enegy Just as gavitational potential (V g ) is equal to wok pe unit mass, electic potential (V e ) is equal to wok pe unit chage V V V g e e W m W q kqq 1 q V e kq

Electic Potential Enegy The electic potential at a point P is the amount of wok done pe unit chage as a small positive test chage q is moved fom infinity to the point P. V V e e W q kq The unit of potential is the volt (V), and 1V = 1J/C

Electic Potential V is the electic potential and is defined in tems of the wok, W, needed to bing a positive test chage, q, fom vey fa away to a position close to the chaged body Remembe that wok is based on displacement and not distance tavelled! V e W q qv e W

Potential Diffeence The amount of wok needed to move a test chage fom one point to anothe is equal to the change in potential enegy of the chage W W W U U W qv V Just like gavity B A B qv B U A qv A

Summay Potential Foce Field Potential Enegy Gavity Electicity kq V GM V e g 2 2 kqq F GMm F e g e e P e P g g P g P V m E kqq E V m E GMm E 2 2 kq E GM g

Video: Equipotentials and Fields

Equipotential Sufaces What is this?

Equipotential Sufaces Gavitational potential is given by V GM An equipotential suface consists of those points that have the same potential

Equipotential Sufaces

Equipotential Sufaces Fo electicity, equipotential sufaces o lines ae aeas whee the potential aound a chage ae equal, just like the contou lines on a topogaphical map

Equipotential Sufaces All points a given distance fom the cente of a sphee will have the same potential All points a given pependicula distance fom a paallel plate will have the same potential

Equipotential Sufaces Equipotential lines fo two opposite chages of diffeent magnitudes Movement along an equipotential line equies no wok because thee is no change in potential

Connection Between Electic Field and Electic Potential The electic field stength (E) is equal to the change in potential divided by the distance ove which that change takes place Thus the field stength is equal to the potential gadient E V If the potential is constant, the field stength is zeo Potential inside a sphee is constant so the field is zeo

Connection Between Electic Field and Electic Potential Since thee is no potential diffeence along an equipotential line, the field is zeo along those lines

Connection Between Electic Field and Electic Potential Since thee IS a potential diffeence BETWEEN equipotential lines, and because field stength is based on displacement, the electic field is must be nomal to those lines

Connection Between Electic Field and Electic Potential The sufaces of conductos ae aeas of equipotential Field lines un pependicula to the suface of conductos If they didn t thee would be a component paallel to the equipotential suface and that can t happen

Connection Between Electic Field and Electic Potential

Electicity Vs Gavitation Compaison of Newton s Law of Gavitation and Coulomb s Law

Paallel Plates Unifom Electic Field exists when the field has a constant magnitude and diection such as that geneated by two oppositely chaged paallel plates.

Paallel Plates The field lines at the edges begin to cuve The field is unifom if the length of the field is lage compaed to the distance between the plates

Electic Field between paallel plates The electic field, E, between two paallel plates is equal to the potential diffeence between the plates, V, divided by the distance between the plates, d Note that E is the electic field E does not stand fo enegy! E V d

Undestandings: Gavitational fields Electostatic fields Electic potential and gavitational potential Field lines Equipotential sufaces

Guidance: Students should ecognize that no wok is done in moving chage o mass on an equipotential suface

Data Booklet Refeences: W W qv e mv g

Essential Idea: Electic chages and masses each influence the space aound them and that influence can be epesented though the concept of fields.

QUESTIONS?

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