电场与电势Electric fields & potential

题型概述Question overview

电场、电势与带电粒子运动在本题库中跨多个年份出现，常与力学平衡或能量守恒结合。本模块选取五道各 5–6 分的真题，共 27 分。主要失分点是把场强当标量、把等电势当成等电荷，以及忽略电子带负电。Electric fields, potential and charged-particle motion recur across the years in this collection, often combined with equilibrium or energy conservation. These five questions carry 5–6 marks each, 27 in total. Common errors are adding fields as scalars, confusing equal potential with equal charge, and forgetting the electron’s negative charge.

1先画方向，再叠加电场Draw directions before adding fields

正电荷的场向外，负电荷的场向内。电场是矢量，电势是标量；电势取无穷远为零。Fields point away from positive charges and towards negative charges. Electric field is a vector; potential is a scalar, with zero at infinity.

零场点不一定是零电势点。Zero field does not imply zero potential.

2用能量处理移动电荷Use energy when moving charges

外力缓慢移动电荷时，做功等于电势能的增加。导体相连后电势相同，并保持总电荷守恒；相距很远的球可用孤立球公式。For slow motion, external work equals the gain in potential energy. Connected conductors reach equal potential while conserving total charge; well-separated spheres obey the isolated-sphere formula.

有重力或弹簧时，还要计入对应的势能。Include gravitational or elastic potential energy when present.

3匀强电场连接牛顿定律与运动学Uniform fields connect force and motion

理想平行板间的场强为电压差除以间距。沿电场方向的加速度由电荷符号决定，与场垂直的速度分量保持不变。Between ideal parallel plates, field strength is voltage difference divided by spacing. The sign of the charge sets the acceleration direction; velocity perpendicular to the field stays constant.

2011(i) 沿水平方向加速，不是竖直偏转。In 2011(i), acceleration is horizontal, not vertical.