1、CHAPTER 20 GLENCOE PHYSICSStatic ElectricityA.Charge 1.Charge-basis for electricity a.two types-“+”or“-”based on the excess or deficiency of electrons.Named by Ben Franklin b.can be created chemically,mech.,etc c.like repel,unlike attract2.Microscopic view of Charge a.Atom is neutral-electrons balan
2、ced by protons.Electrons in orbit around nucleus b.With addition of energy,electrons removed leaving a positive ion.Freed electrons become attached to other material and have a negative ion.(1)neutral objects charged by rubbing(2)conservation of charge is critical3.Conductors and Insulators a.Insula
3、tors-materials through which charges do not move easily.Charges removed from one area are not replaced by charges from another area b.Conductors-materials such as metals that allow charges to move(1)Charge is carried by the movement of electrons.(2)metals have an electron that can be easily removed
4、as electrons act as though they belong to the entire whole-an electron cloud(3)Air is an insulator,but under certain conditions sparks can occur.4.Concentration of Charge a.charged body distributes charge over the surface b.distributed to bring about equilibrium c.pointed surfaces have a higher conc
5、entration of charge d.Grounding-same charge per unit area,removal of excess charge.B.Electric Force 1.more powerful than gravity and both attractive and repulsive 2.Charging by Conduction-charging a neutral body by touching it with a charged body.Positive or negative 3.Charged by induction-charge an
6、 object without touching3.Coulombs Law a.1785-Charles Coulomb devises an experiment to measure the electric force.b.“The magnitude of the force that a tiny sphere with charge q exerts on a second sphere with charge q,separated by a distance d,is given by the equationF kqqd2(1)Force that q exerts on
7、q and that q exerts on q(2)What concept is this analogous to?c.Hold for point charges or spherically shaped charges.Distance is between the centers of such objects.d.Unit of charge is the Coulomb(C)(1)one coulomb is the charge on 6.25 x 1018 electrons(2)lightning bolt is about 7-10 C(3)elementary ch
8、arge is the charge on one electron-negative 1.60 x 10-19 C e.For charge in Coulombs,distance in meters,and force in Newtons k=9 x 109 N-m2/C2 f.Electric Force-a vector quantity,must show direction.(1)repulsive forces-positive signs,force on object is away from second object.(2)attractive forces-nega
9、tive signs,force on object is toward second object.g.A charged object is always attracted by a neutral object(and,by 3rd Law,a neutral object is attracted to a charged object)4.Problems a.Two point charges of magnitude 3 x 10-9 C and 6 x 10-9 C are separated by 30 cm.Find the force of repulsion.F=(9
10、x109)(3x10-9)(6x10-9)/.32 =1.8 x10-6 N F kqqd2 b.A 4.5 x 10-9 C charge is located at 3.2 m from a-2.8 x 10-9 C charge.Find the electrostatic force.c.An electron and a proton of a hydrogen atom are separated by a distance of 5.3 x 10-11 m.An electron has a charge of-1.60 x 10-19 C and a mass of 9.11
11、x 10-31 kg.A proton has a mass of 1.67 x 10-27 kg.Find the electrostatic and gravitational forces between the two.How could you find the speed of the electron?Find it.a.F=-1.11 x 10-8 N Attractive b.FG=3.6 x 10-47 N attractiveFE =-8.2 x 10-8 N attractiveFc=FG+FE=m v 2/rtherefore v=2.18 x 10 6 m/sF k
12、qqd25.Principle of Superposition a.When two or more charges affect a different charge,the resultant force acting on the charge equals the vector sum of the various individual charges present.b.A 2.2 x 10-9 C charge is located at x=-1.5m,a 5.4 x 10-9 C is located at x=2 m,and a 3.5 x 10-9 C charge is
13、 located at the origin.Find the net force acting on the charge at the origin.F 3-2=3.08 x 10-8 NF 3-1=4.25 x 10-8 N Fnet=F 3-1 -F 3-2=1.17 x 10-8 N towards q2+q1 1.5 q32 m q2F3-2F3-1 c.For extra credit.Determine the resultant force acting on charge q 3 in the following situation.q 1=6 x 10-9 C q 2=-2 x 10-9 C 4m q 3=5 x 10-9 C 3 mq 2q 3q 1
