A heat lamp emits infrared radiation whose electric field is E_rms = 3500 N/C. (
ID: 1600803 • Letter: A
Question
A heat lamp emits infrared radiation whose electric field is E_rms = 3500 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 3.3 cm. What is the average power delivered to the leg? (c) The portion of the leg being irradiated has a mass of 0.20 kg and a specific heat capacity of 3500 J/(kg C degree). How long does it take to raise its temperature by 1.5 C degree? Assume that there is no other heat transfer into or out of the portion of the leg being heated.Explanation / Answer
a) we know,
Intensity of radiation,
I = Erms^2/(c*mue)
= 3500^2/(3*10^8*4*pi*10^-7)
= 3.25*10^4 W/m^2
b) we know, Intensity = Power/Area
==> Power = Intensity*Area
= 3.25*10^4*pi*0.033^2
= 111.2 W
c) let t is the time taken.
Q = m*c*dT
P*t = m*c*dT
t = m*c*dT/P
= 0.2*3500*1.5/111.2
= 9.44 s
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