* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Electricity - TeacherWeb
Survey
Document related concepts
Magnetic monopole wikipedia , lookup
Casimir effect wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Weightlessness wikipedia , lookup
Maxwell's equations wikipedia , lookup
Newton's laws of motion wikipedia , lookup
Speed of gravity wikipedia , lookup
Atomic nucleus wikipedia , lookup
Nuclear force wikipedia , lookup
Field (physics) wikipedia , lookup
Anti-gravity wikipedia , lookup
Centripetal force wikipedia , lookup
Work (physics) wikipedia , lookup
Electromagnetism wikipedia , lookup
Fundamental interaction wikipedia , lookup
Lorentz force wikipedia , lookup
Transcript
Electricity Charge and Field Static Electricity • • • • • • Comb picks up paper Balloon sticks to wall Sparks when combing hair Shock when touch doorknob “static cling” in dryer lightning Two Kinds of Charge • • • • Unlike charges attract Like charges repel Positive on rubbed glass rod Negative on rubbed plastic rod Conservation of Charge • Net amount of electric charge produced in any process is zero • When positive and negative combine they neutralize • Most every day objects are neutral • Ordinary atoms are neutral Atoms Contain Charge • • • • Protons inside nucleus are positive Electrons outside nucleus are negative Charge on proton same as on electron Electrons can move within solid material Methods of Charging • Rubbing • Induction – Separates charge • Contact • Grounding – Connect with wire leading to ground Charging by Induction Insulators and Conductors • Conductors carry charge easily – Metals have many free electrons – Ionic liquids – Plasmas • Insulators conduct poorly – Dry gases, pure molecular liquids – Wood, paper, cloth, glass, etc • Semiconductors like silicon, germanium Electroscope • Detects charge with leaves that repel or rotating needle Coulomb’s Law • Electric force proportional to product of charges divided by square of distance between them • Q in coulombs • k is Coulomb constant • k = 8.988x109 Nm2/C2 • constant e0 is permittivity of free space 8.85x10-12 C2/Nm2 • Applies to point charges Electrostatic force vs. gravity force • FE = kq1q2/r2 k = 9 x 109 Nm2/C2 • FG =Gm1m2/r2 G = 6.67 x 10-11 Nm2/kg2 • Electrostatic force both attractive and repulsive • Gravitational force only attractive (per Newton, not so certain today) The Smallest Charge • Charge on electron or “elementary” charge qe or e • e = 1.602 x 10-19 Coulombs • Charge is quantized • Discrete amounts only • 1e, 2e, 3e, 4e etc • Quarks of sub-atomic physics have 1/3 and 2/3 charge Example – Calculate Coulomb Force • Find the force between two objects with charge 1 Coulomb at a separation of one meter • F = kq1q2/r2 = 9 x 109 x 1/1 = 9 x 109 N If objects are 100Kg students, what will be initial acceleration? • a = F/m = 9 x 107 m/s/s • Estimate speed reached in one second Example(2) – Coulomb Force • Find force between two 1 micro (10-6) coulomb charges at separation of 20 cm • F = kq1q2/r2 = 9 x 109x 10-6 x 10-6 /(0.20)2 • =9/(.04) x 10-3 2.25 x 10 -1 N Electrostatic Force and Vectors • Fnet = F1 + F2 + F3 + … • Called principle of superposition of forces • Use component method of vector addition • Fx = F1x + F2x Fy = F1y + F2y Component Method Review • • • • • F1X = F1cosq F1Y = F1sinq FX = F1X+F2X FY = F1Y+F2Y F = (FX2 + FY2)1/2 • TanQ = FY/FX F1 q F1X F1Y Three Charges in a Line Assuming the magnitude of all three charges is equal, what is the direction of the net force on the positive charge? To the right Three Charges in a Line Assuming the magnitude of all three charges is equal, what is the direction of the net force on the red positive charge? To the right Three Charges in a Line Assuming the magnitude of all three charges is equal, what is the direction of the net force on the red positive charge? To the left Electric Field kq/r2 • Force acting at a distance vs. field concept • Field E is force on tiny positive test charge divided by magnitude of charge • Direction same as force on + charge • Field line spacing shows strength of E Electric field lines outward from + charge Electric field lines inward to - charge Field due to 2 Like Charges Field due to Unlike Charges You Predict • What would be the direction of the force on a positively charged pith ball near a positively charged Van de Graaf sphere? • What about a negatively charged pith ball? • If the force on a positively charged pith ball is toward the sphere, what must be the field direction? • What is the charge on the sphere? Electric Field Strength • Units newtons per coulomb • E = F/q F = Eq q is “test charge” • E = kQ/r2 due to charge Q Just Coulomb’s law without the q2 kq1q 2 2 r Problem Solving • Draw careful diagram • Apply Coulomb’s Law to get magnitude of forces or fields • Determine direction of net forces by considering like and unlike charges • Show and label each vector force or field • Add vectorially to get resultant • Use symmetry when possible Fields and Conductors • • • • Field inside conductor is zero(static) If not force F=qE would make charges move Charge spreads out maximally on surface Charge Q inside spherical uncharged shell induces –Q on inside surface of shell • Positive charge Q exists on outside • Electric field just outside a conductor is always perpendicular to the surface Why Field Outside Conductor is Perpendicular to Surface • Ask what if there were a parallel component just outside • There would also be one just inside • Electrons would move until equilibrium • Then there could be no field • Contradiction • Therefore the original statement is true Problem • Charged conducting spheres with +3Q and –Q are initially at a distance of L meters apart. They are brought together briefly, then moved back to their original positions. What happens to the force between them? +3Q -Q Problem • Charged conducting spheres with +3Q and –Q are initially at a distance of L meters apart. They are brought together briefly, then moved back to their original positions. What happens to the force between them? +3Q -Q Problem • Charged conducting spheres with +3Q and –Q are initially at a distance of L meters apart. They are brought together briefly, then moved back to their original positions. What happens to the force between them? +Q +Q Problem • Charged conducting spheres with +3Q and –Q are initially at a distance of L meters apart. They are brought together briefly, then moved back to their original positions. What happens to the force between them? +Q +Q Solution • On contact there is neutralization leaving a total of 2Q; each charge gets Q. Upon putting the charges back where they were, the force changes from attractive to repulsive and is 1/3 as strong. Question • What would happen if a positive charge is placed at the center of a neutral conducting spherical shell? + Draw the electric field lines Negative charges induced, inside shell, positive outside as shown on NEXT SLIDE