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fw:收音機的原理--電磁波(cited by 作者：國立台灣師範大學 物理系 黃福坤)

收音機的原理：

收音機是怎樣接收到遠方電台所傳來的訊號的呢？

你注意到當收聽 FM 電台時，天線的指向會影響接收的效果。

What pressure oscillation results from oscillating displacements of atoms ?

• We shall consider plane waves for simplicity. The displacement wave is given by the formula y(x,t) = A cos( k_x x - ω t + φ₀ ) where y(x,t) is a displacement in x of the plane
• Last time we have showed on the board that the displacement wave causes local volume compression/rarification with
• In bulk materials, the volume compression corresponds to pressure increase over the ambient pressure, Δ p = P - P_a as described by bulk modulus B
• B plays the role of spring constant in Hooke's Law, but now for volumetric deformations (strains). Units of bulk modulus are Pascales, as that for pressure. Please browse through Chapter 11.4 for discussion of deformations in bulk materials. 
  Notice the difference with Hooke's law. Here it is the relative change of the volume that is involved, while in Hooke's law we deal with the absolute change in the spring length.