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例1 具有固定转轴的矩形线框 abcd 处在直线电流的磁场中,转轴与直导线平行,相距4 r_0,线框的 ab和 d c两边与转轴平行,长度都为5r_0,bc 和 da 边与转轴垂直,长度都为6r_0,转轴通过这两条边的中点,如图1所示,直导线中的电流向上,当导线框垂直于直线电流与转轴构成的平面时,ab 边和 cd 边所在处的
Example 1 A rectangular wire frame with a fixed axis abcd is in a magnetic field of a linear current, the axis of rotation is parallel to the straight wire, 4 r_0 apart, and the ab and dc sides of the wire frame are parallel to the rotation axis, and the lengths are 5r_0, bc and da edges and the rotation axis. Vertically, the length is 6r_0, and the rotation axis passes through the midpoints of the two sides. As shown in FIG. 1, the current in the straight wire is upward. When the wire frame is perpendicular to the plane formed by the linear current and the rotation axis, ab and cd edges are located. At the