Question

如圖所示，兩根不計電阻的金屬導(dǎo)線MN與PQ放在水平面內(nèi)，MN是直導(dǎo)線，PQ的PQ₁段是直導(dǎo)線，Q₁Q₂段是弧形導(dǎo)線，Q₂Q₃段是直導(dǎo)線，MN、PQ₁、Q₂Q₃相互平行，M、P間接入一個阻值R=0.25Ω的電阻。一根質(zhì)量為1.0 kg不計電阻的金屬棒AB能在MN、PQ上無摩擦地滑動，金屬棒始終垂直于MN，整個裝置處于磁感應(yīng)強度B=0.5T的勻強磁場中，磁場方向豎直向下。金屬棒處于位置（I）時，給金屬棒一個向右的速度v₁=4 m/s，同時方向水平向右的外力F₁ ="3" N作用在金屬棒上使金屬棒向右做勻減速直線運動；當(dāng)金屬棒運動到位置(Ⅱ)時，外力方向不變，大小變?yōu)镕₂，金屬棒向右做勻速直線運動，經(jīng)過時間t ="2" s到達(dá)位置(Ⅲ)。金屬棒在位置(I)時，與MN、Q₁Q₂相接觸于a、b兩點，a、b的間距L₁=1 m，金屬棒在位置(Ⅱ)時，棒與MN、Q₁Q₂相接觸于c、d兩點。已知s₁="7.5" m。求：（1）金屬棒向右勻減速運動時的加速度大小?
（2）c、d兩點間的距離L₂=?
（3）外力F₂的大��？
（4）金屬棒從位置(I)運動到位置(Ⅲ)的過程中，電阻R上放出的熱量Q=？

Answer 1

（1）金屬棒從位置(I)到位置(Ⅱ)的過程中，加速度不變，方向向左，設(shè)大小為a，在位置I時，a、b間的感應(yīng)電動勢為E₁，感應(yīng)電流為I₁，受到的安培力為F_安1，則
E₁=BL₁ v₁，，F_安1···································①
F_安1="4" N·································································②
根據(jù)牛頓第二定律得
F_安1－F₁ =ma·····························································③
a=" 1" m / s²·································································④
（2）設(shè)金屬棒在位置(Ⅱ)時速度為v₂，由運動學(xué)規(guī)律得
=－2a s₁···························································⑤
v₂=" 1" m / s·································································⑥
由于在(I)和(II)之間做勻減速直線運動，即加速度大小保持不變，外力F₁恒定，所以AB棒受到的安培力不變即F_安1=F_安2
···························································⑦
m······················································⑧
（3）金屬棒從位置(Ⅱ)到位置(Ⅲ)的過程中，做勻速直線運動，感應(yīng)電動勢大小與位置(Ⅱ)時的感應(yīng)電動勢大小相等，安培力與位置(Ⅱ)時的安培力大小相等，所以
F₂= F_安2="4" N······························································⑨
(4) 設(shè)位置（II）和(Ⅲ)之間的距離為s₂，則
s₂= v₂t="2" m ································································⑩
設(shè)從位置(I)到位置(Ⅱ)的過程中，外力做功為W₁，從位置(Ⅱ)到位置(Ⅲ)的過程中，外力做功為W₂，則
W₁= F₁ s₁="22.5" J ···························································11
W₂= F₂ s₂="8" J······························································12
根據(jù)能量守恒得W₁+ W₂·······························13·
解得Q =" 38" J ·····························································14

略