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29.7. Producing Electromagetic Waves 產生電磁波 Any changing E or B will create EM waves. E 或 B 的任何變化都會產生電磁波 Any accelerated charge produces radiation. 任何電荷加速就會產生輻射。 Radio transmitter: e’s oscillate in antenna driven by LC circuit. 無線電發射器：e由LC電路驅動而在天線振動。 X-ray tube: accelerated e’s slammed into target. X-射線管：e 加速後撞上標靶。 MW magnetron tube: e’s circle in B. 微波磁控管：e 在 B 中繞圈。 EM wave 電磁波: f = f of q motion 運動 ?Most efficient: ? ~ dimension of emitter / reciever 効率最高： ? ~ 發射 / 接收器 大小 Waves emit / receive ? axis of dipole. 發射 / 接收的波 ? 雙極的軸。 Source replenishes radiated energy 能源補充輻射掉的能量 LC oscillator drives I in antenna LC 振動器驅動天線的 I. Outgoing EM waves 外出電磁波 29.8. Energy Momentum in Electromagetic Waves電磁波的能量和動量 Consider box of thickness dx, face A ? k of EM wave. 假想一個盒子，厚 dx ，其面 A ? 電磁波的 k。 Energy densities: 能量密度： Energy in box 盒內能量 : Rate of energy moving through box: 能量通過盒子的速率： Intensity S = rate of energy flow per unit area 強度 S = 每單位面積通過能量的速率 Plane waves 平面波: ? Plane waves:平面波： In general: 通用： Poynting vector 坡印廷向量 see Prob 64 Average intensity for plane waves : 平面波的平均強度： E, B in phase 同相 Example 29.3. Solar Energy 太陽能 The average intensity of noontime sunlight on a clear day is about 1 kW/m2. 晴天午時陽光的平均強度約為 1 kW/m2。 What are the peak electric magnetic fields in sunlight ? 陽光的電和磁場的峰值為何？ At this intensity, what area of 40% efficient solar collectors would you need to replace a 4.8-kW water heater ? 據此強度，一個 40% 効率的陽光收集器需要多大面積才能取替一個 4.8-kW 的熱水器？ (a) (b) Area needed is 需要面積 Waves from Localized Sources來自小範圍源頭的波 Afar from localized source, wave is spherical : 離小範圍源頭甚遠處，波呈球狀： Intensity = power / area 強度 = 功率 / 面積 ? wave fields dominates static fields away from the sources. 離源頭甚遠處，波的場蓋過靜態場。 Example 29.4. Cell Phone Reception 手機收訊 A cell phone’s typical average power output is about 0.6 W. 一個手機的平均輸出功率通常是 0.6 W 左右。 If the receiver at a cell tower can handle signals with peak electric fields as weak as 1.2 mV