Physics 205A Quiz 7, fall semester 2017
Cuesta College, San Luis Obispo, CA
Stars can be modeled as spherical blackbodies that radiate heat out to an environment assumed to have a temperature of 0 K. The sun has a surface temperature of 5,772 K and a surface area of 6.09×1018 m2. The sun radiates heat per time at a rate of:
(A) 3.45×1011 watts.
(B) 1.11×1015 watts.
(C) 1.99×1015 watts.
(D) 3.83×1026 watts.
Correct answer (highlight to unhide): (D)
The net power radiated by the sun (where the negative sign indicates that heat is continuously leaving the surface of the sun) is given by:
Power = –e·σ·A·((T)4 – (Tenv)4),
where the emissitivity e = 1 (for an ideal blackbody), the Stefan-Boltzmann constant σ = 5.670×10–8 watts/(m2·K4), the surface area A = 6.09×1018 m2, the surface temperature T = 5,772 K, and the temperature of the environment is assumed to be Tenv = 0 K, such that the rate of heat radiated by the sun can be solved for:
Power = –(1)·(5.670×10–8 watts/(m2·K4)·(6.09×1018 m2)·((5,772 K)4 – (0)4),
Power = –3.832707188×1026 watts,
or to three significant figures, the rate of hate radiated per time from the sun is 3.83×1026 watts.
(Response (A) is A·σ; response (B) is T4; response (C) is e·σ·A·T.)
Sections 70854, 70855
Exam code: quiz07Whu7
(A) : 1 student
(B) : 3 students
(C) : 18 students
(D) : 20 students
Success level: 48%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66
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