m Figure 2.4.1 Analogy between the gravitational field g and the electric field E. From the field theory point of view,we say that the charge g creates an electric field E which exerts a forceF=oE on a test charge do. Using the definition of electric field given in Eq.(2.4.1)and the Coulomb's law,the electric field at a distance r from a point charge g is given by E=,19: (2.4.2) 4π60r1 Using the superposition principle,the total electric field due to a group of charges is equal to the vector sum of the electric fields of individual charges: 重-2E-2是 (2.4.3) Animation 2.2:Electric Field of Point Charges Figure 2.4.2 shows one frame of animations of the electric field of a moving positive and negative point charge,assuming the speed of the charge is small compared to the speed of light. Figure 2.4.2 The electric fields of(a)a moving positive charge,(b)a moving negative charge,when the speed of the charge is small compared to the speed of light. 2-8Figure 2.4.1 Analogy between the gravitational field g G and the electric field E . G From the field theory point of view, we say that the charge q creates an electric field E which exerts a force on a test charge . G e 0 F = q G GE 0 q Using the definition of electric field given in Eq. (2.4.1) and the Coulomb’s law, the electric field at a distance r from a point charge q is given by 2 0 1 ˆ 4 q πε r E = r G (2.4.2) Using the superposition principle, the total electric field due to a group of charges is equal to the vector sum of the electric fields of individual charges: 2 0 1 ˆ 4 i i i i i q πε r E E = = ∑ ∑ r G G (2.4.3) Animation 2.2: Electric Field of Point Charges Figure 2.4.2 shows one frame of animations of the electric field of a moving positive and negative point charge, assuming the speed of the charge is small compared to the speed of light. Figure 2.4.2 The electric fields of (a) a moving positive charge, (b) a moving negative charge, when the speed of the charge is small compared to the speed of light. 2-8