（2010?南通模拟）如图所示，RP是一块长为L=4m的绝缘平板固定在水平地面上，整个空间有一个平行于PR的匀 （2005?海淀区一模）如图所示，PR是一长为L=0.64m...

\uff082012?\u5e73\u9065\u53bf\u6a21\u62df\uff09\u5982\u56fe\u6240\u793a\uff0cPR\u662f\u4e00\u5757\u957f\u4e3aL=4m\u7684\u7edd\u7f18\u5e73\u677f\u56fa\u5b9a\u5728\u6c34\u5e73\u5730\u9762\u4e0a\uff0c\u6574\u4e2a\u7a7a\u95f4\u6709\u4e00\u4e2a\u5e73\u884c\u4e8ePR\u7684

\uff081\uff09\u7531\u9898\uff0c\u7269\u4f53\u8fd4\u56de\u65f6\u5728\u78c1\u573a\u4e2d\u4ecd\u505a\u5300\u901f\u8fd0\u52a8\uff0c\u6d1b\u4f26\u5179\u529b\u65b9\u5411\u5411\u4e0a\uff0c\u5219\u77e5\u7269\u4f53\u5e26\u6b63\u7535\uff0e\uff082\uff09\u7269\u4f53\u8fd4\u56de\u540e\u5728\u78c1\u573a\u4e2d\u65e0\u7535\u573a\uff0c\u4ecd\u505a\u5300\u901f\u8fd0\u52a8\uff0c\u6d1b\u4f26\u5179\u529b\u4e0e\u91cd\u529b\u5e73\u8861\uff0c\u5219\u6709 mg=qBv2 \u2026\u2460\u79bb\u5f00\u78c1\u573a\u540e\uff0c\u7531\u52a8\u80fd\u5b9a\u7406\u5f97\uff1a-\u03bcmgL4=0-12mv22\u2026\u2461\u89e3\u5f97\uff1av2=22m/s\uff083\uff09\u4ee3\u5165\u2460\u5f97\uff1aB=22T\uff084\uff09\u7531\u4e8e\u7535\u8377\u7531P\u8fd0\u52a8\u5230C\u505a\u5300\u52a0\u901f\u8fd0\u52a8\uff0c\u53ef\u77e5\u7535\u573a\u65b9\u5411\u6c34\u5e73\u5411\u53f3\uff0c\u4e14\u6709\uff1a\uff08Eq-\u03bcmg\uff09L2=12mv21-0\u2026\u2462\u8fdb\u5165\u7535\u78c1\u573a\u540e\u505a\u5300\u901f\u8fd0\u52a8\uff0c\u5219\u6709\uff1aqE=\u03bc\uff08qBv1+mg\uff09 \u2026\u2463\u8054\u7acb\u2462\u2463\u89e3\u5f97\uff1a\uff0cv1=42m/s\uff0cE=2.4N/C\u7b54\uff1a\uff081\uff09\u7269\u4f53\u5e26\u6b63\u7535\uff0e\uff082\uff09\u7269\u4f53\u4e0e\u6321\u677f\u78b0\u649e\u649e\u524d\u540e\u7684\u901f\u5ea6v1\u548cv2\u5206\u522b\u4e3a42m/s\u548c22m/s\uff0e\uff083\uff09\u78c1\u611f\u5e94\u5f3a\u5ea6B\u7684\u5927\u5c0f\u662f <div style="width:6px;background: url('http://hiphotos.baidu.com/zhidao/pic/item/c2cec3fdfc0392458726adbb8494a4c27c1e25f6.jpg'

\uff081\uff09\u7269\u4f53\u8fd4\u56de\u540e\u5728\u78c1\u573a\u4e2d\u65e0\u7535\u573a\uff0c\u4ecd\u505a\u5300\u901f\u8fd0\u52a8\uff0c\u6d1b\u4f26\u5179\u529b\u4e0e\u91cd\u529b\u5e73\u8861\uff0c\u5219\u6709 mg=qBv2\u2026\u2460\u79bb\u5f00\u78c1\u573a\u540e\uff0c\u7531\u52a8\u80fd\u5b9a\u7406\u5f97\uff1a-\u03bcmgL4=0-12mv22 \u2026\u2461\u89e3\u5f97\uff0cv2=0.8m/s\u4ee3\u5165\u2460\u5f97\uff0cB=0.125 T\u7531\u4e8e\u7535\u8377\u7531P\u8fd0\u52a8\u5230C\u505a\u5300\u52a0\u901f\u8fd0\u52a8\uff0c\u53ef\u77e5\u7535\u573a\u65b9\u5411\u6c34\u5e73\u5411\u53f3\uff0c\u4e14\u6709\uff08Eq-\u03bcmg\uff09L2=12mv21-0\u2026\u2462\u8fdb\u5165\u7535\u78c1\u573a\u540e\u505a\u5300\u901f\u8fd0\u52a8\uff0c\u5219\u6709 qE=\u03bc\uff08qBv1+mg\uff09\u2026\u2463\u8054\u7acb\u2462\u2463\u89e3\u5f97\uff0cv1=1.6m/s\uff0cE=6.0\u00d710-2N/C\uff082\uff09\u7269\u4f53\u4eceP\u5230D\u505a\u5300\u52a0\u901f\u76f4\u7ebf\u8fd0\u52a8\uff0c\u540e\u505a\u5300\u901f\u76f4\u7ebf\u8fd0\u52a8\uff0c\u6839\u636e\u8fd0\u52a8\u5b66\u516c\u5f0f\u4e0e\u725b\u987f\u7b2c\u4e8c\u5b9a\u5f8b\uff0c\u5219\u6709qE-\u03bcmg=ma\u8fd0\u52a8\u5b66\u516c\u5f0f\uff0cL?d\uff1d12at21\u4ee3\u5165\u6570\u636e\uff0c\u89e3\u5f97\uff1at2=0.4s\u800c\u5300\u901f\u76f4\u7ebf\u8fd0\u52a8\uff0c\u5219\u6709\uff1at2\uff1ddv1=0.321.6s\uff1d0.2s\u7531\u9898\u610f\uff0c\u4ee3\u5165\u6570\u636e\uff0c\u89e3\u5f97\uff1a\u4eceP\u8fd0\u52a8\u5230R\u7684\u65f6\u95f4t=t1+t2=0.4s+0.2s=0.6s\u7b54\uff1a\uff081\uff09\u7535\u573a\u5f3a\u5ea6E\u7684\u5927\u5c0f6.0\u00d710-2N/C\uff1b\uff082\uff09\u7269\u4f53\u4eceP\u8fd0\u52a8\u5230R\u7684\u65f6\u95f40.6s\uff0e

（1）由题，物体返回时在磁场中仍做匀速运动，洛伦兹力方向向上，则知物体带正电．
（2）物体返回后在磁场中无电场，仍做匀速运动，洛伦兹力与重力平衡，则有
   mg=qBv₂…①
离开磁场后，由动能定理得：-μmg

L

4

=0-

1

2

m

v

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