ELECTRIC-FIELD OF A SLOWLY MOVING RECTANGULAR CURRENT LOOP - A MICROSCOPIC APPROACH

The electric field on the axis of a slowly (v much less than c) moving rectangular current loop is calculated using two concepts from introd uctory level physics textbooks; (1) Lorentz-contraction induced charge density, and (2) relativistic flattening of the electric field patter n of a point charge moving with constant velocity. The correct field, proportional to vI/c(2), on the axis of the moving loop is obtained on ly when both;effects are considered and applied separately to the ions and electrons of the loop. Effect (1) is important only for the longi tudinal sections of the loop, whereas effect (2) must be considered fo r all sections of the loop. The latter point is often overlooked in te xtbooks.