A heat lamp emits infrared radiation whose rms electric field is E rms = 2700 N/
ID: 1400017 • Letter: A
Question
A heat lamp emits infrared radiation whose rms electric field is Erms = 2700 N/C. (a) What is the average intensity of the radiation? (b) The radiation is focused on a person's leg over a circular area of radius 3.1 cm. What is the average power delivered to the leg? (c) The portion of the leg being irradiated has a mass of 0.28 kg and a specific heat capacity of 3500 J/(kg·C°). How long does it take to raise its temperature by 2.3 C°? Assume that there is no other heat transfer into or out of the portion of the leg being heated.
Explanation / Answer
a)
Intenisty of light wave, I = Erms^2/(c*mue)
= 2700^2/(3*10^8*4*pi*10^-7)
= 19337 W/m^2
b) power = Intensity*Area
= 19337*pi*0.031^2
= 58.38 Watts
c) heat required, Q = m*c*dT
= 0.28*3500*2.3
= 2254 J
time taken, t = Q/P
= 2254/58.38
= 38.6 s
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