Electric field amplitude question regarding intensity calculations

AI Thread Summary
The discussion centers on the calculation of intensity using the formula I=P/(4*pi*r^2) and its implications for electric field amplitude. The original poster questions why the formula includes a factor of 4, suggesting it may imply a limited area of calculation, specifically a quarter circle rather than a full sphere. Participants clarify that the intensity is indeed distributed over a spherical surface area, not just a circular one, which resolves the confusion about the factor. The conversation highlights the importance of understanding the geometry involved in these calculations. Ultimately, the correct interpretation leads to a better grasp of how intensity and amplitude relate in electromagnetic fields.
MeatComet
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Homework Statement
A wireless telephone is emitting a constant 10,0mW effect in a homogenous area around the telephone. What is the amplitude of the electrical field 4,0cm from the headset?
Relevant Equations
I=P/A,
I=P/(4*pi*r^2),
I=P/(pi*r^2),

I=(1/2)*√(ε0/μ0)*(E0^2)
My question is specifically with calculating the intensity. The book solution is

I=P/(4*pi*r^2)

but would this not give me a weaker electrical amplitude in the final calculation after plugging it in to
I=(1/2)*√(ε00)*(E02) ?
 
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MeatComet said:
but would this not give me a weaker electrical amplitude in the final calculation after plugging it in to
I=(1/2)*√(ε00)*(E02) ?
Weaker than what? You are asked the find the amplitude at 4.0 cm from the headset.
 
Yes, and that would be the radius in I=P/(4*π*r2), but my question is why there is an extra 4 in the divider, does that not make it so that i only look at the quarter circle and would that imply that the book answer is only looking at the edge of the phone that is touching the face or somesuch?
Because if you only look at the area of the circle the antenna makes, that gives
A=π*r2
=>A=π*42
But according to my book, they do
I=P/A=>
I=1/4*(P/(π*42))
Which is, in fact, cutting the intensity of the field in 4, essentially only looking at a quarter slice?
Because as i understand the equation, the intensity is spread across a circular area, and to find the amplitude at 4.0cm you plug in the intensity per square centimeter across the field that you want, no?
 
MeatComet said:
why there is an extra 4 in the divider
What is the surface area of a sphere? :wink:
 
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Thanks man, i was looking at a circle. Why, why why why. Hahah, wish i could more than like your answer.
 
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