實驗2: 電子受靜電力及磁力作用的偏折 (課本實驗15/16) 實驗2: 電子受靜電力及磁力作用的偏折 (課本實驗15/16) 目的: 觀察陰極射線管(cathode ray tube, CRT)中之電子束在電場(E)及磁場(B)中之偏折運動情形，並測量電子的荷質比(e/m比) 原理: Lorentz force F = q(E + v x B) [SI unit: N = C·(V/m) = C·(m/s).T] 陰極射線管CRT的內部結構 控制柵極G 聚焦陽極F 加速陽極A (總電位V1) 水平偏壓(偏折)平行電極板 垂直偏壓(偏折)平行電極板 Z方向無偏折之電子束 螢光幕 陰極射線管(CRT)內的加熱燈絲 陰極射線管(CRT)內的陰極C 陰極射線管CRT的內部結構

陰極射線管的工作原理 Operation Principle of CRT 經控制柵極G(1), 聚焦陽極F(2)及加速陽極A(3)(總電位V1)後 電子往Z方向(6)做等速運動，mvz2/2 = eV1  vz = (2eV1/m)1/2 電子打到螢光幕(XY面)(7)上的中心位置，形成一亮點。 電子質量 m = 9.11 x 10-31 kg 電荷 q = -e = -1.60 x 10-19 C As V4 = 0 V5 = 0 陰極射線管CRT的內部結構

Cathode Ray Tube (CRT) The electrons are deflected in various directions by two sets of plates. The placing of charge on the plates creates the electric field between the plates and allows the beam to be steered. Commonly used to obtain a visual display of electronic information in oscilloscopes, radar systems, televisions, etc.. Be a vacuum tube in which a beam of electrons is accelerated and deflected under the influence of electric or magnetic fields.

Cathode Ray Tube (CRT) - is one of the main elements of an oscilloscope. A CRT is commonly used to obtain a visual display of electronic information in oscilloscopes, radar systems, televisions, etc. The CRT is a vacuum tube in which a beam of electrons is accelerated and deflected under the influence of electric or magnetic fields. The tubes are produced with electrostatic and electromagnetic control, where electrostatic or magnetic fields deviate the electron beam respectively. The principle scheme of CRT has electrostatic control as well as the motion of the electrons in the beam drawing a sinusoid on the screen of oscilloscope.

Configuration of CRT CRT consists of 6 sections: Glass bulb evacuated to a high vacuum (真空管) Cathode (a source of electrons，電子槍陰極) Cathode heater (陰極加熱器) Electrodes for brightness and focus control, several accelerating anodes (亮度、聚焦、加速控制柵極) Pairs of horizontal and vertical capacitor plates deviating the electron beam Fluorescing screen (螢光幕)

Discharge Tube Discharge Tube Electron Gun It was observed that whenever the glass tube filled with air at low pressure and sealed with electrodes at both the ends was subjected to about 10,000 volts of electricity, a strange glow was produced around the edges of the wide end of the glass tube, irrespective of the type of gases present in it. The glass tube with sealed electrodes is called discharge tube. Discharge Tube Electron Gun

Cutaway rendering of a color CRT Electron guns Electron beams Focusing coils Deflection coils Anode connection Mask for separating beams for red, green, and blue part of displayed image Phosphor layer with red, green, and blue zones Close-up of the phosphor-coated inner side of the screen

CATHODE-RAY TUBE (CRT) - is one of the main elements of an oscilloscope. One of anodes, which accelerate the electrons, is placed close to the screen. The high positive voltage is applied to this electrode. Under the action of the applied voltage the electrons are moved with acceleration from cathode to anode. In the absence of the voltage applied to deviating plates of the capacitor the electron beam will be incident on the screen in the center brightening a point in the fluorescing layer. In oscilloscope the analyzed signal after amplification is applied to vertical deviating plates, while the periodic sawtooth signal is applied to horizontal plates.

http://physics-animations.com/Physics/English/osc_tmp.htm As a result the electron beam "draws"  the dependence of the investigated signal on time on the screen of the tube. Reaching the right side of the screen the beam has to be returned to an initial point at the left side. Thus, if CRT is not blanked during this retrace, then the beam will leave a track crossing the image of investigated signal. For this reason, during retrace a negative voltage is applied to control electrode situated near to cathode and electrons are locked by such a way at the electron gun. As a result, the electron beam will be discontinuous, as shown in animation.

Related Web Sites for CRT Animation http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html http://physics-animations.com/Physics/English/osc_tmp.htm http://www.sciencemuseum.org.uk/on-line/electron/section2/shockwave2.asp The Cathode Ray Tube site: http://members.chello.nl/~h.dijkstra19/page3.html How Television Works: http://www.howstuffworks.com/tv.htm/printable

2. 電子在直流電場中之偏折： X：等速直線運動 Y：等加度運動 X, Y等速直線運動 利用水平電板X(4)及垂直電板Y(5)所提供的電壓，產生均勻電場(E)，使電子束偏折，如電板Y長L, 兩板距離d, 直流電位差V2 (E = V2/d)，電子離開電板後距螢光幕D，則電子在Y方向偏折SE，使螢光幕(XY面)在Y軸 SE 處產生一亮點。 X：等速直線運動 Y：等加度運動 X, Y等速直線運動

3. 電子在交流電場中之偏折： 若在電板Y加交流電頻率 f = /2 = 1/T 因高頻，電子在螢光幕成一長為l的亮線 Note：一般家電 Vrms = 110 V, f = 60 Hz

4. 電子在磁場中之偏折 5. 同時在電場及磁場的作用下 並使電子不偏折 利用共軸雙線圈螺線管之均勻磁場(B)來偏折電子束 如磁場在x方向 (長L)，電子離開磁場後距螢光幕 D 則電子在Y方向偏折 SB ( Sm) 螢光幕(XY面)Y軸上SB有一亮點 5. 同時在電場及磁場的作用下 並使電子不偏折 SE + SB = 0 eE = evzB vz = E/B 代入 SE 或 SB 即可求電子的荷值 (e/m) 比

實驗步驟： I. 陰極射線管電源 1. 陰極射線管(X = 60 mm, Y =54 mm, D = 115 mm, L = 11 mm, d = 1.65 mm) 插入CRT基座(Unilab 031.502) X為X電板輸入端 Y為Y電板輸入端 A1為最後加速陽極 Z接柵極 2. CRT基座接CTR電源供應(032.332) 之五腳CRT插座(tube supply) 注意：高壓電源! 接電路前, 確定電源供應器 開關為關(off)(燈暗) 3. 調整CRT亮點之 亮度(brill)(加速陽極) 及焦距(focus)(聚焦陽極)

II、電場偏折 1. 放大器(amplifier)(2)(032.842)接CRT電源供應器(1) (032.332)之aux unit直流電源(6 V) 2. 產生交流電場: 電源供應器(1)之交流電壓(6 Vrms)接到放大器之 交流耦合A (alternating coupling)輸入端 +共用端C (common) AC: 只有ac信號進入(串聯電容器) DC: 直流耦合D (direct coupling): ac/dc信號均可進入 3. 放大器之X/Y輸出端接CRT基座(3) (031.502)之垂直Y電板輸入 4. 利用放大器之增益(gain)及 移動 (shift) 鈕，觀察及調整Y方向 之亮線位置及長度/電壓振幅值 l = 2Vm (校正後) Y靈敏度(sensitivity): 23 V/1 cm (Q):如何量週期T? (1) (3) (2)

III.交流電壓波形及頻率 1. 時基產生器(2)產生掃描鋸齒波 (掃描週期可調) 2. 時基產生器(2)同步掃描鋸齒波 (掃描週期可調)輸出接水平 放大器(3)輸入DC端(下頁) 3. 水平放大器(3)之輸出端接 CRT基座之水平電板輸入(5) (水平輸入為時間t)(校正後)

3. (5)垂直Y電板輸入不變 但垂直放大器(4)輸入接DC端 (垂直輸入為電壓V) 4. 放大器輸入A端同步接 (1) 3. (5)垂直Y電板輸入不變 但垂直放大器(4)輸入接DC端 (垂直輸入為電壓V) 4. 放大器輸入A端同步接 時基產生器(2)(032.892) (time base generator) 同步輸入端(sync) 5. 觀查CRT之波形Y(X) = V(t), 量振幅(Vm)及週期(T) (2) (3) (4) (5)

IV. 磁場偏折 產生磁場：直流電源供應器(5) 接到共軸雙線圈螺線管(2) (031.402)，線圈數為N = 3000圈 + 3000圈。 利用可變電阻(4)及安培計(3) 調整及測量電流(I)大小，因磁場B  NI，所以可求算B值。 3.共軸雙線圈螺線管放於CRT 基座(1)之CRT上。 4.利用不同方式測量電子的e/m比值。 (1) (2) (5) (3) (4)