No field Electric field E +9 F=gE d Torque =dx gE =pxE p=gd Electronic polarization Orientation and ionic polarization (a) (b) Figure 3-1 An electric dipole consists of two charges of equal magnitude but opposite sign,separated by a small vector distance d.(a)Electronic polarization arises when the average motion of the electron cloud about its nucleus is slightly displaced.(b)Orien- tation polarization arises when an asymmetric polar molecule tends to line up with an applied electric field.If the spacing d also changes,the molecule has ionic polarization. Courtesy of Krieger Publishing.Used with permission. + da dV d-da Surface S + AQ qNdV d.da >0 Positive charge leaves qNd-da b =P.da d.da <0 Positive charge enters V d-da>o d.da <o Volume V 6.641,Electromagnetic Fields,Forces,and Motion Lecture 7 Prof.Markus Zahn Page 2 of 276.641, Electromagnetic Fields, Forces, and Motion Lecture 7 Prof. Markus Zahn Page 2 of 27 Courtesy of Krieger Publishing. Used with permission