The solar energy coming to the earth by radiation is 1,340 W/m2 . If the sun has a radius of 7.000 x 108m, is a perfect radiator and is located 1.500 ? 1011m from the earth, then what is the temperature of the sun?
A. 6430 K
B. 5760 K
C. 4230 K
D. 3670 K
E. 3210 K
B. 5760 K
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Dielectric heating, also known as RF or high frequency heating, is a process in which a high- frequency alternating electric field or microwave electromagnetic radiation heats a dielectric material. An important application of this phenomenon is in the heating of food in a microwave oven. Microwave ovens heat food by bombarding it with electromagnetic radiation in the microwave spectral range, causing polarized molecules in the field to rotate and build up thermal energy, thus cooking or heating the food. However, when the food is initially frozen, because the molecules in the solid do not rotate readily, volumetric heat generation is an order of magnitude less than if the material were in the liquid form. Microwave power not absorbed in the food is dissipated to the microwave generator
in the form of heat. Estimate the time it takes to heat a 2 lb roast, initially at a temperature of -20°C, after it is placed in a 1-kW total power microwave oven with an interior temperature of 30°C. Assume the meat has a spherical shape and a heat transfer coefficient of 12 W/(m2 K) from its surface. First estimate the time required to reach a uniform temperature of 0°C when the water in the meat is in the form of ice. Assume that 4% of the oven power is absorbed in the food. After the meat thaws (all ice is melted), the meat increases in temperature. Now estimate the time required for the meat to reach 75°C if 90% of the microwave oven power is absorbed in the meat. Assume that the physical properties of meat are the same as those of liquid water. GIVEN • Wight of the roast (m) =2 lb=0.907 kg • Initial temperature (T0)=-200C • Microwave oven power( P)= 1000 W • Microwave interior temperature (T?)=300C • Heat transfer coefficient ( ch )= 12 W/(m2 K) FIND • Time required reaching uniform temperature of 00C considering 4% of oven power is absorbed in food. • Time required to reach 750C is 90% of oven power is absorbed in food when ice is melted. ASSUMPTIONS • Ambient temperature is constant at T? • Beef has properties of water and ice. • Heat transfer coefficient is constant at hc • The material properties of the wire are constant ? Thermal conductivity = k ? Thermal diffusivity = ?
The point in time at which oxygen started to build up in the Earth's atmosphere is referred to as the
A) great oxidation event B) great oxygen crisis C) great oxygenation event D) great photosynthetic event
If an object's mass (M) is doubled, the gravitational force between it and other objects will be
A. increased by a factor of 2. B. decreased by a factor of 4. C. increased by a factor of 4. D. decreased by a factor of 2.
A particle (charge = 5.0 ?C) is released from rest at a point x = 10 cm. If a 5.0-?C charge is held fixed at the origin, what is the kinetic energy of the particle after it has moved 90 cm?
A. 1.6 J B. 2.0 J C. 2.4 J D. 1.2 J E. 1.8 J