真空中有一点电荷Q,在与它相距为r的a点处有一试验电荷q.现使试验电荷q从a点沿半圆弧轨道运动到b点,如图<img src="/lqwvjeImg/4f/4fa1f6058e175cf80a4de06d53748cf5.png"style="width:112px;height:81px;">所示.则电场力对q作功为( )<br>·<img src="/lqwvjeImg/54/54fe44acb4bd79af60fd6904b3d857c5.png"style="width:85px;height:48px;">.<br>· <img src="/lqwvjeImg/a8/a8a3348d953e4673d0a0008c69e00bf2.png"style="width:70px;height:45px;">.<br>· <img src="/lqwvjeImg/d4/d417b78e6f246d22a19f4e06000e58a6.png"style="width:72px;height:45px;">.<br>· 0.