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PH 122 (46)

Electromagnetism, energy and field strength, intensity, polarization

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PH 122
Brian Jones

26 March The speed of electromagnetic waves is the same for all waves regardless of wavelength A typical analog cell phone has a frequency of 850 MHz; a digital phone a frequency of 1950 MHz. Compared to the signal from an analog phone, the digital signal has shorter wavelength and higher photon energy. A radio tower emits two 50 W signals, one an AM signal at a frequency of 850 kHz, one an FM signal at a frequency of 85 MHz. Which signal has more photons per second? FM: higher frequency → higher photon energy Therefore the AM signal has more photons per second Energy and Field Strength Intensity of an electromagnetic wave with field amplitudes 0 and B0 P 2 c 2 I = A = ½ ϵ 0 0 ½ μ 0 B 0 Amplitude of magnetic field typically small 2I E 0 √cε 0 E 0 B 0 c E 0 cB 0 The Microwave Oven Inside the cavity of a microwave oven, the 2.4 GHz electromagnetic waves have an intensity of 5.0 kW/m2 What is the strength of the electric field? 3 2 I = 5.0 x 10 W/m 1,940 V/m The magnetic field? 6.5 μT Intensit
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