A heat lamp emits infrared radiation whose rms electric field is E_rms = 3700 N/

Question

A heat lamp emits infrared radiation whose rms electric field is E_rms = 3700 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 4.1 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 (kg C degree). How long does it take to raise its temperature by 2.8 C degree? Assume that there is no other heat transfer into or out of the portion of the leg being heated.

Explanation / Answer

(A) average intensity = Erms^2 / c u0

= (3700)^2 / (3 x 10^8 x 4pi x 10^-7)

= 36313.85 W/m^2

(B) P = I A

= (36313.85) (pi x 0.041^2)

= 191.77 W

(c) Q = 0.20 x 3500 x 2.8 = 1960 J

t = Q/P = 10.22 sec

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