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實驗 4: 電流天平 ( 課本實驗 18)

實驗 4: 電流天平 ( 課本實驗 18). 目的 : 測量二載流導線間之磁力 原理 : 一長導線 1, 帶定電流 I 1 = I 1 z , 產生磁場 B 1 =  0 I 1 /2  r. f ( 圓柱座標 r , f , z ) ( 磁透常數  0  4  x 10 -7 T  m/A)(SI) 另一平行長導線 2, 帶常電流 I 2 , 相距 r = d, 線段 l 受磁力 F 2 = l 1  I 2 x B 1 =  0 I 1 I 2 l/2  d  r 二導線載流方向相反時 , 磁力為排斥力

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實驗 4: 電流天平 ( 課本實驗 18)

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  1. 實驗4: 電流天平(課本實驗18) 目的: 測量二載流導線間之磁力 原理:一長導線1, 帶定電流I1 = I1z, 產生磁場B1 = 0I1/2r.f(圓柱座標 r, f, z) (磁透常數 0 4 x 10-7 Tm/A)(SI) 另一平行長導線2, 帶常電流I2, 相距 r = d, 線段l受磁力 F2 = l1I2 x B1 = 0I1I2l/2dr 二導線載流方向相反時,磁力為排斥力 二導線載流方向相同時,磁力為吸引力 (例) I = I1 = I2 = 1 A, d = 1 m, F/l = 2 x 10-7 N/m [SI unit基本物理量: 電流(A)之操作定義(1946)]

  2. 實驗: 利用電流天平(current balance)測磁力 靜導線(11): 在固定底座上, 電流I 動導線(12): 形成可動框架(8)之一 部份, 以刀口(2)架在刀口架(1)上, 可自由擺動, 反向電流I受排斥磁力 向上傾斜擺動 傾斜度控制: --砝碼(13): F = mg --平衡質量(3) --感度質量(6): 控制靈敏度 擺動控制: 阻尼磁鐵(9): 利用金屬片在磁場中之旋渦電流變熱來消耗擺動動能 導線間距離: d0 ~ 2 mm 擺動週期: ~2 s

  3. 電流天平(current balance)(保護儀器,注意安全) (4-3)

  4. 直流電源供應器： 輸出(max): 30 V (dc), 3 A (dc) 輸出模式: CV(等電壓輸出), CC(等電流輸出) C. 歐姆計 1. 因電阻R會產生焦耳熱功率P = I2R, 電流不能太大 2. 電阻器(resistor)標示(電阻W及熱功率W) R = a,b,c,d,(e)環 = abc x 10d W  e% error 銀(-2), 金(-1), 黑(0), 棕(1), 紅(2), 橙(3), 黃(4), 綠(5), 藍(6), 紫(7), 灰(8), 白(9)

  5. 兩導線距離d: 用光槓桿(optical level)測量 反射鏡(4): 傾斜角q, 到尺距離b, 到導線距離a, 雷射光高度D D/b = tan2 ~ 2tan (小角度時) = 2(d0/a) d0: 兩導線間隔 d : 兩導線中心距離 d = 2r + d0 (導線半徑r ≠ 0) = 2r + aD/2b I = I1 = I2 F = mg = 0I2l/2d  I2

  6. DVD: 電磁感應 (Electromagnetic Induction)(The Mechanical Universe…and Beyond/MU37) [Annenberg/CPB/www.learner.org] Faraday’s law (法拉弟定律) time-dependent magnetic flux (磁通量) FB(t) = surfaceB.dA induced electromotive force (emf) (感應電動勢) e e = - dFB/dt (Faraday’s law of electromagnetic induction) or induce an electric field E (感應電場) closed loopE.dl = e = -(d/dt)surface B.dA Inductor (電感器) (ex) solenoid inductor (螺線管電感器) self-induced emf from current i(t) e = - (dFB/di)(di/dt) = - L(di/dt) where L is inductance (電感) (henry H = T.m2/A)

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