- #1
tim9000
- 867
- 17
Hi, sorry if this is in the wrong forum but 2 Quick hopefully simple questions about Poynting vector and EM waves.
Q1)
Say I have two antenna (antenni?)...dipoles maybe, both with say, 50W going into them, one oscillating on the kHz range, one on the MHz range (GHz, whatever). But for this example same number of oscilating charges in both antenna.
so ave S = Eo X Bo / 2μ (watts/square meter)
because the average power of sin^2 is a half, irrespective of frequency? (EoSin(kx-ωt) = E)
To maintain equal output power...So either Eo is reduced with increased frequency ω, or there is just less EM per square meter (but since higher frequency the same power) ? Which and how? (It is my understanding that Eo is propertional to intensity, as inensity is proportional to power; and from intensity I imagine that is determined by the amount of oscilating charges and the voltage that induces their movement. Please correct me if I'm wrong. I'd love for someone to explain how U = (epsilon*E)/2 + (B/(2*mu)) relates to U = h.f)
P.S: Ignore Q2:
Q2)
What I've read about TEM waveguides tells me that my mobile phone (GHz λ) shouldn't work inside my bird cage or microwave oven, yet it does. How can this be? (since given that when the oscilating electric field contacts a conductor it creates surface charge)
Q1)
Say I have two antenna (antenni?)...dipoles maybe, both with say, 50W going into them, one oscillating on the kHz range, one on the MHz range (GHz, whatever). But for this example same number of oscilating charges in both antenna.
so ave S = Eo X Bo / 2μ (watts/square meter)
because the average power of sin^2 is a half, irrespective of frequency? (EoSin(kx-ωt) = E)
To maintain equal output power...So either Eo is reduced with increased frequency ω, or there is just less EM per square meter (but since higher frequency the same power) ? Which and how? (It is my understanding that Eo is propertional to intensity, as inensity is proportional to power; and from intensity I imagine that is determined by the amount of oscilating charges and the voltage that induces their movement. Please correct me if I'm wrong. I'd love for someone to explain how U = (epsilon*E)/2 + (B/(2*mu)) relates to U = h.f)
P.S: Ignore Q2:
Q2)
What I've read about TEM waveguides tells me that my mobile phone (GHz λ) shouldn't work inside my bird cage or microwave oven, yet it does. How can this be? (since given that when the oscilating electric field contacts a conductor it creates surface charge)
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