The uncertainty principle can be used to relate the uncertainties in which two quantities?
A) position and momentum
B) the force of gravity and the force of electromagnetism
C) spin and charge
D) position and spin
E) mass and energy
A) position and momentum
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What was the temperature of the Universe when the recombination took place that produced the cosmic background radiation we observe today?
a. about 2.7 K b. about 300 K c. about 3000 K d. about 3 million K e. about 3 billion K
Chromatic aberration affects reflector telescopes
Indicate whether the statement is true or false
An automotive catalytic convertor is a packed bed in which a platinum catalyst is coated on the surface of small alumina spheres. A metal container holds the catalyst pellets and allows engine exhaust gases to flow through the bed of pellets. The catalyst must be heated by the exhaust gases to 300°C before the catalyst helps combust unburned hydrocarbons in the gases. The time required to achieve this temperature is critical because unburned hydrocarbons emitted by the vehicle during a cold start comprise a large fraction of the total emissions from the vehicle during an emission test. A fixed volume of catalyst is required but the shape of the bed can be modified to increase the heat-up rate. Compare the heat-up time for a bed 5-cm-diameter and 20-cm-long with one 10 cm diameter and
5-cm-long. The catalyst pellets are spherical, 5-mm-diameter, have a density of 2 g/cm3, thermal conductivity of 12 W/(m K) and specific heat of 1100 J/(kg K). The packed-bed void fraction is 0.5. Exhaust gas from the engine is at a temperature of 400°C, a flow rate of 6.4 gm/s, and has the properties of air.
GIVEN
FIND
The heat-up time (t) for the pellet surface temperature (Tp) to reach 300°C
ASSUMPTIONS
The initial temperature of the bed (To) = 20°C
SKETCH
Arno Penzias and Robert Wilson are famous for____
a. creating the first microwave telescope b. finding the direction of the center of the Universe c. detecting energy released in the splitting of the electroweak force d. detecting the large redshift background radiation from the big bang e. measuring the temperature of the big bang