AP Physics III.A Electrostatics
Law of Conservation of Charge – during any one process, net electrical charge of an isolated system remains constant.
Ex. Two identical isolated conducting spheres, one with charge -6 μC and another with net charge +2 μC are allowed to touch. If the two spheres have the same net charge after touching, what is the net charge on each sphere?
18.4 Charging by Induction and Conduction (also known as, “I wish I had a decent electroscope”)
Ex. An electron “orbits” the proton of a hydrogen atom at an average distance of 0.53 EE 10-10 m. What is the force that the proton exerts on the electron? What is the velocity of the electron for a circular orbit?
Ex. Two charges exert electrical force F on each other. If the magnitude of each charge is doubled and the distance between them is halved, what is the force F′ on each charge in terms of F?
p. 552: 3-5, 7, 11, 15, 19, 21, 23, 79B7 • 3.4 EE -17 kg, mA > mB 79B7 • FBD b)
Charges experience an electrostatic force due to the presence of other charges
Force per Coulomb is the definition of an electric field (“show me the formula”)
An electric field is a vector that has a direction that the force exerts on a positive test charge.
Ex. Find the electric force on a proton placed in an electric field of 2.0 EE 4 N/C that is directed along the positive x-axis.
Electric fields are vectors. The net electric field at a point in space can be determined by considering the contributions of each charged object and adding them together as vectors.
Ex. Electric Field Between Two Point Charges. Two point charges are separated by a distance of 0.100 m. One has a charge of –25.0 μC and the other 50.0 μC. a) What is the magnitude and direction of the electric field at point P between them 0.020 m from the negative charge? b) If an electron is placed at rest at P, what is the magnitude and direction of its initial acceleration?
p. 553: 25, 29, 31, 35-37; 81B3 • a) 182 N/C b) 312 N/C 81B3 • FBD b) E = 5800 N/C, FT = 0.058 N c) drawing
Electric potential decreases or increases not because the field exerts any more or less force (the field is uniform – like gravity near the Earth’s surface). V changes because of distance. A charge released in the field, traveling a greater distance converts more of its Ue to K (like dropping an object from a greater height).
Potential (and therefore potential difference) is scalar (this will simplify some things).
Summary • Electric potential energy – energy a charge has because of its potential in an electric field (so far the field is uniform) • Electric potential – electric potential energy per unit charge • Potential difference – change in electric potential