A 1500 W portable heater is needed to replace the heat lost through a circular window in the wall of a home that is made of a single pane of glass 0.30 cm thick. Thermal conductivity for the glass is 0.63 W/mK. The temperature inside the house is 
and outside is 
What is the radius of the window?
A. 47 cm
B. 53 cm
C. 27 cm
D. 7.1 cm
E. 84 cm
C. 27 cm
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If the inner accretion disk around a black hole has a temperature of 106 K, at what wavelength will it radiate the most energy? Hint: wavelength in nm =3,000,000/T in K
a. 106 nm b. 3 nm c. 3 x 106 nm d. 1 nm e. 3 x 1011 nm
Tensile Stress and Strain: A 25-kg piece of equipment can be hung by steel wires of length 1.00 m, 2.00 m, or 3.00 m. If all the wires have the same diameter, which one will stretch the greatest percent?
A. the 1.00-m wire B. the 2.00-m wire C. the 3.00-m wire D. They will all stretch by the same percent.
Stellar luminosities vary much more than stellar masses.
Answer the following statement true (T) or false (F)
Captain Jirk reports to headquarters that he left the planet Senesca 1.88 × 10^4 seconds earlier. Headquarters sends back the message: "Was that spaceship proper time?" It will be spaceship proper time if it was
a. measured by one clock fixed at one spot on Senesca. b. measured by one clock fixed at one spot on the spaceship. c. measured by a clock on Senesca at departure and by a clock on the spaceship when reporting. d. measured by a clock on the spaceship when departing and by a clock on Senesca when reporting. e. calculated by dividing the distance from Senesca according to Senesca by the speed of the spaceship.