●■”■■■■■”● 0 r P dl 00 0 r<R 解: E= r>R 4πer2 (1)r>R,U,=Edi=Ecoso-dl -品 4π8o' (2)r<R E R/OrP M di 00 U,-Edi-Ecos0dl-EcosodEcos0d 4πER Q Q r<R 4πER U= 4π8R Q r>R 4πEr 电势是连续的 R 例:无限长均匀带电直线 : di E 解E2,,-Ed-5o0 55 Q O r P dl R 解: r R r Q r R E 2 4 0 0 (1)r R, = P P U E dl P Ecos dl = r dr r Q 2 4 0 r Q 4 0 (2)r R Q O r P M dl = = + P P U E dl P Ecos dl M P Ecos dl M Ecos dl = = , R dr r Q 2 4 0 R Q 4 0 U r R r Q r R R Q U 0 0 4 4 电势是连续的 R r 例:无限长均匀带电直线 dl r P E 解: , = r E 2 0 P P U E dl P Ecos dl = = ,无意义 r dr r 2 0 r r ln 2 0 E E R Q 4 0 R