† Ch. 23 Electrostatics. Coulombs Law: F=(k Q1 Q2/r^2) rˆ ! (k = 1 will of course be given on the exam) 4pe0 † Electric charge is conserved. Charge of electron is negative, -e (e will of course be given on the exam) Electric Field: E=F/q (force per unit charge)! [Units: N/C] points away from +, towards - charges E-fields superpose ( => know how to add vectors, find components) Terminology: Conductor, insulator, dielectric, dipole... Ch. 24 Gauss' law Interpret field line diagrams: (Higher density of field lines means stronger E field, counting lines to find charges) Definition and computation of flux, F= Ú E⋅dA Gauss' Law: Ú E⋅dA = Q(enclosed)/ e0 † (dA points outward) † When/how does symmetry help? When/how does superposition help? E field is zero inside conductors in steady state.! All excess electric charge sits on surfaces of conductors At surface of conductor, E is normal, and |E|=s /e0 (Do you know WHY all of the above are true?) † Ch 25 Electric potential Work is F*d*cos(q) [Unit is J = N*m] V = U/q , Voltage = Potential energy/charge, [V = J/C] B DVAB =VB -VA =- Ú E⋅dL A Isolated systems conserve energy, PEi+KEi=PEf+KEf. (Where KE = 1/2 m v^2) If speed is constant: Wext = DU = q D V. Point charge: V= k Q/r. Use superposition if more than one charge, adds up like a number. (V is neg. if Q is neg. ) You choose where you want to call PE=0, or V=0. (Here, it was at infinity) Constant E means DVAB = -E⋅DL 1 eV = 1.6E-19 J! =unit of energy. † be given on the exam) (This will of course (Equipotential graphs not on this exam) Point P in empty space is equidistant from 3 charges as shown. (Note: the 4 "answer" arrows are all 45 degrees from the axes) What is the direction of the electric field at point P? PURPLE: Some completely other angle/ not sure. +y +Q +x GREEN +2Q r YELLOW r P BLUE r +Q PINK B) If a negative test charge -q is now placed at P, which direction is the force on that charge? A point charge Q is at is the origin. Point P is located at the point (x=3, y=4) What is the x component of Electric field at point P? BLUE: (kQ/25) PINK: (kQ/9) YELLOW: (kQ/25)*(3) GREEN: (kQ/25)*(3/5) PURPLE: None of these What is the voltage at point P? BLUE: kQ/3 PINK: kQ/5 YELLOW: (kQ/5)*(3/5) GREEN: kQ/25 PURPLE: None of these The diagram shows a positive point charge, Q=1 mC, near a metal bar. The electric field in the vicinity of the point charge and the bar are shown by the field lines in the figure. A) From the figure, what can you say about the net charge on the bar? BLUE: YELLOW: PINK: GREEN: PURPLE: Q(bar)= +1 mC Q(bar) = -1 mC Q(bar) = 0.25 mC Q(bar) = -.25 mC None of these + Qbar Q Three closed surfaces enclose a point charge. Which surface has the smallest outward flux through it? PINK: Small cube green yellow YELLOW: smaller sphere GREEN: larger sphere pink + PURPLE: Need more info BLUE: All three have the same flux. If the pink cube was moved to the side, so that it no longer enclosed the charge, which surface now has the smallest outward flux?