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-Electric Potential due to a Charged Conductor -The Millikan Oil Drop Experiment -Applications of Electrostatics

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## -Electric Potential due to a Charged Conductor -The Millikan Oil Drop Experiment -Applications of Electrostatics

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**-Electric Potential due to a Charged Conductor-The Millikan**Oil Drop Experiment-Applications of Electrostatics AP Physics C Mrs. Coyle**Electric Potential –What we used so far!**• Electric Potential • Potential Difference • Potential for a point charge • Potential for multiple point charges • Potential for continuous charge distribution**Is the surface of a charged conductor an equipotential?**• Is the electric potential constant everywhere inside a charged conductor and equal to its value at the surface?**Electric Potential Difference on the Surface of a Charged**Conductor in Equilibrium • Let A and B be points on the surface of the charged conductor • Let ds be the displacement from A to B. • E is always perpendicular to the displacement ds. So, E ·ds = 0 • Therefore, the potential difference between A and B is also zero**Electric Potential Difference on the Surface of a Charged**Conductor in Equilibrium • V is constant everywhere on the surface of a charged conductor in equilibrium • ΔV = 0 between any two points on the surface • The surface of any charged conductor in electrostatic equilibrium is an equipotential surface**What about the inside of a charged conductor?**• E=0 inside the conductor in equilibrium • E ·ds = 0 • Therefore, the electric potential is constant everywhere inside the conductor and equal to the value at the surface.**Solid Conducting Sphere**• r<R V=kq/R E=0 • r=R V=kq/R E=kQ/R2 • r>R V=kq/r E=kQ/r2 Note: • V is a Scalar related to energy • E is a Vector related to force.**Irregularly Shaped Conductors**• The charge density is high where the radius of curvature is small • The electric field is high at sharp points**Irregularly Shaped Conductors**• The field lines are perpendicular to the conducting surface • The equipotential surfaces are perpendicular to the field lines**Quick Quiz 25.10**Draw a graph of the electric potential as a function of position relative to the center of the left sphere. (Left sphere 1, radius a),(Right sphere 2, radius c) The centers of the spheres are a distance b apart.**Quick Quiz 25.10**Answer: See below. Notice the flat plateaus at each conductor, representing the constant electric potential inside a solid conductor.**Ex 25.9: Two Connected Charged Spheres**• The separation distance of the spheres is much greater than the radius of either sphere so their fields do not affect each other. • Show**ΔV= O in a Cavity in a Conductor, so Equipotential to Body**of Conductor • Assume no charges are inside the cavity • E=0 inside the conductor • The electric field inside does not depend on the charge distribution on the outside surface of the conductor**Corona Discharge**• If the electric field near a conductor is sufficiently strong, electrons resulting from random ionizations of air molecules near the conductor accelerate away from their parent molecules • These electrons can ionize additional molecules near the conductor**Corona Discharge**• The glow that is observed near a charged conductor of a strong E-field. • It results from the recombination of freed electrons with the ionized air molecules • Most likely to occur near very sharp points**Millikan Oil-Drop Experiment**• Robert Millikan measured e, the charge of the electron e = 1.60 x 10-19 C • He also demonstrated the quantized nature of this charge**Oil-Drop Experiment**• With no electric field between the plates: The drop reaches terminal velocity with FD = mg**Oil-Drop Experiment**• When an electric field is set up between the plates the drop moves upwards and reaches a new terminal velocity • Fe = mg +FD =qE • Solve for q • Observed thousands of times and always found e=multiple of 1.6x10-19 C**Electrostatic Precipitator**• It removes particulate matter like ashes from combustion gases • Corona discharge ionizes particles of air • Most of the dirt particles are negatively charged and are drawn to the walls by the electric field of the negative potential coil.**Xerography: uses photoconductive material coating(Selenium)**a drum