Physics 2102 Lecture: 10 WED 04 FEB Jonathan Dowling Physics 2102 Lecture: 10 WED 04 FEB Electric Potential III Ch24.11–12

PHYS2102 FIRST MIDTERM EXAM! 6–7PM THU 05 FEB 2009 Dowling’s Sec. 2 in Lockett Hall, Room 6 YOU MUST BRING YOUR STUDENT ID! YOU MUST WRITE UNITS TO GET FULL CREDIT! The exam will cover chapters 21 through 24, as covered in homework sets 1, 2, and 3. The formula sheet for the exam can be found here: http://www.phys.lsu.edu/classes/spring2009/phys2102/formulasheet.pdf KNOW SI PREFIXES n, cm, k, etc. THERE WILL BE A REVIEW SESSION 6–7PM WED 04 FEB 2009 in Williams 103

Continuous Charge Distributions Divide the charge distribution into differential elements Write down an expression for potential from a typical element — treat as point charge Integrate! Simple example: circular rod of radius r, total charge Q; find V at center. r dq

Potential of Continuous Charge Distribution: Line of Charge Uniformly charged rod Total charge Q Length L What is V at position P? x P dx L a Units: [Nm2/C2][C/m]=[Nm/C]=[J/C]=[V]

Electric Field & Potential: A Simple Relationship! Focus only on a simple case: electric field that points along +x axis but whose magnitude varies with x. Notice the following: Point charge: E = kQ/r2 V = kQ/r Dipole (far away): E = kp/r3 V = kp/r2 E is given by a DERIVATIVE of V! Of course! Note: MINUS sign! Units for E: VOLTS/METER (V/m)

E from V: Example Uniformly charged rod Total charge Q Length L We Found V at P! Find E from V? x P dx L a Units: √ Electric Field!

Electric Field & Potential: Question Hollow metal sphere of radius R has a charge +q Which of the following is the electric potential V as a function of distance r from center of sphere? +q V r r=R (b) V r r=R (a) V r r=R (c)

Electric Field & Potential: Example Outside the sphere: Replace by point charge! Inside the sphere: E = 0 (Gauss’ Law) E = –dV/dr = 0 IFF V=constant +q E V

Equipotentials and Conductors Conducting surfaces are EQUIPOTENTIALs At surface of conductor, E is normal (perpendicular) to surface Hence, no work needed to move a charge from one point on a conductor surface to another Equipotentials are normal to E, so they follow the shape of the conductor near the surface. V E

Conductors Change the Field Around Them! An Uncharged Conductor: A Uniform Electric Field: An Uncharged Conductor in the Initially Uniform Electric Field:

Sharp Conductors Charge density is higher at conductor surfaces that have small radius of curvature E = s/e0 for a conductor, hence STRONGER electric fields at sharply curved surfaces! Used for attracting or getting rid of charge: lightning rods Van de Graaf -- metal brush transfers charge from rubber belt Mars pathfinder mission -- tungsten points used to get rid of accumulated charge on rover (electric breakdown on Mars occurs at ~100 V/m) (NASA)

Ben Franklin Invents the Lightning Rod!

Summary: Electric potential: work needed to bring +1C from infinity; units = V = Volt Electric potential uniquely defined for every point in space -- independent of path! Electric potential is a scalar -- add contributions from individual point charges We calculated the electric potential produced by a single charge: V = kq/r, and by continuous charge distributions : V = kdq/r Electric field and electric potential: E= -dV/dx Electric potential energy: work used to build the system, charge by charge. Use W = U = qV for each charge. Conductors: the charges move to make their surface equipotentials. Charge density and electric field are higher on sharp points of conductors.