An electric clothes dryer draws 20 A at 220 V. If the clothes put into the dryer have a mass of 7.0 kg when wet and 3.0 kg dry, how long does it take to dry the clothes? (Assume all heat energy goes into vaporizing water, Lv = 2.26 × 10^6 J/kg.)
a. 55 min
c. 16 min
b. 26 min
d. 34 min
D
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In general, fast reactions
a. are exothermic. b. are endothermic. c. have low activation energies. d. have high activation energies.
A three-cell flashlight has 3 batteries in series each with an emf of 1.5 V. If the bulb has a resistance of 50 
what current is flowing when the flashlight is operating? (Assume the internal resistance of the batteries is negligible.)
A. 30 mA B. 10 mA C. 27 mA D. 90 mA E. 11 A
A myopic (nearsighted) dancer wears eyeglasses that allow him to have clear distant vision. The power of the lenses of his eyeglasses is -3.00 diopters. Without eyeglasses, what is the far point of the dancer?
A) 0.50 m B) 0.17 m C) 0.42 m D) 0.33 m E) 0.25 m
A flat roof is modeled as a flat plate insulated on the bottom and placed in the sunlight. If the radiant heat that the roof receives from the sun is 600 W/m2 , the convection heat transfer coefficient between the roof and the air is 12 W/(m2 K), and the air temperature is 27°C, determine the roof temperature for the following two cases: (a) Radiative heat loss to space is negligible. (b) The roof is black (? = 1.0) and radiates to space, which is assumed to be a black-body at 0 K.
GIVEN
FIND
ASSUMPTIONS
- Steady state prevails
- No heat is lost from the bottom of the plate
SKETCH
