1.2 Couloumb's law 1)Model: point charge 2) Couloumb's law F=k21O2F-102-To ro-4兀Eor COULOMB kc=9×10Nm2/c2, En=885×1012c2/Nm2 F 21
1.2 Couloumb’s law 1) Model:point charge 2) Couloumb’s law q1 2 q r 21 r F21 2 0 1 2 0 2 0 1 2 4 1 r r Q Q r r Q Q F k = = k=9×109Nm2 /c2 ， 0=8.85×10-12c 2 /Nm2
Caution 1)valid for point charge in air 2)obey newton's law 3)in general Fe> F8 Henry Cavendish Experiment Gleichgewicht Torsionsfaden Gleichgewichts position 需( Position 2
3) in general Fe Fg Caution: 1)valid for point charge in air 2) obey newton’s law Henry Cavendish Experiment
Quiz 1. Even though electric forces are very much stronger than gravitational forces, gravitational forces determine the motions in the solar system. Why? 2. When we approach another person, we are not aware of the gravitational and electric forces between us. What are the reason in each case?
1. Even though electric forces are very much stronger than gravitational forces, gravitational forces determine the motions in the solar system. Why? 2. When we approach another person, we are not aware of the gravitational and electric forces between us. What are the reason in each case? Quiz
s2 Electric field E 2.1 Electric field Electric source Electric field- Electric charge E、F Electric field for point charge F 1 goro 0 4丌六 qo4兀E0r Collection of point charge F E ∑Ek=∑ 4兀 r superposition of electric field
§2 Electric field E 2.1 Electric field Electric source Electric field Electric charge q0 F E = Electric field for point charge 0 2 0 4 0 1 r r qq F = 0 2 0 4 0 1 r r q q F E = = = = = k k k k k r r q E q F E 0 2 0 0 4 1 k k k Collection of point charge superposition of electric field
Charge distribution dE 1 d e g-O 4丌Er ndl de dq da pdv de E g-O 4丌E Step to solve e. dE e=dE →dE→ → E= dE
Charge distribution 0 2 0 d 4 1 d r r q E = = 0 2 4 0 d r r q E dq = dq r E d P dl dS dV Step to solve E: dq dE y x dE dE Ex = dEx Ey = dEy