Question

如圖所示，兩塊豎直放置的平行金屬板A、B，兩板間距d=0.04m，兩板間的電壓U=400V，板間有一勻強(qiáng)電場(chǎng)．在A、B兩板上端連線的中點(diǎn)Q的正上方，距Q為h=1.25m的P點(diǎn)處有一帶正電小球，已知小球的質(zhì)量m=5×10^﹣6kg，電荷量q=5×10^﹣8C．設(shè)A、B板足夠長(zhǎng)，g取10m/s²．求：                                                                  

（1）帶正電小球從P點(diǎn)開(kāi)始由靜止下落，經(jīng)多長(zhǎng)時(shí)間到達(dá)Q點(diǎn)？                                       

（2）帶正電小球從Q點(diǎn)進(jìn)入電場(chǎng)后，經(jīng)多長(zhǎng)時(shí)間與金屬板相碰？                                      

（3）相碰時(shí)，離金屬板上端的距離多大？                                                                    

                                                                                                               

                                                                                                                                     

Answer 1

（1）設(shè)小球從P到Q需時(shí)間t₁，由h=得

  t₁==0.5s

（2）設(shè)小球進(jìn)入電場(chǎng)后運(yùn)動(dòng)時(shí)間為t₂

則qE=ma

  E=

則小球水平方向的加速度a=

水平方向作勻加速運(yùn)動(dòng)，則有

解得t₂=0.02s

故總時(shí)間為t=t₁+t₂=0.52s

（2）小球由P點(diǎn)開(kāi)始在豎直方向上始終是自由落體運(yùn)動(dòng)，則y==2.45m

答：

（1）帶正電小球從P點(diǎn)開(kāi)始由靜止下落，經(jīng)0.5s時(shí)間到達(dá)Q點(diǎn)；

（2）帶正電小球從P點(diǎn)開(kāi)始由靜止下落，經(jīng)0.52s和金屬板相碰；

（3）相碰時(shí)，離金屬板上端的距離為2.45m．