3. Reference directions(参考方向) Reference directions for current i(t>0 ()< (2) Reference directions for voltage U()>0 U(t)<07 3. Reference directions(参考方向) i(t) i(t)  0 − − i(t)  0 − − (1) Reference directions for current (t) + − ( t )  0 + + − − (t)  0 + − + − (2) Reference directions for voltage