The metric prefix for one one-hundredth is
A) milli.
B) centi.
C) kilo.
D) mega.
E) giga.
B
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A certain utility transformer is designed to step up the emf from 1 kV to 25 kV. If the primary has 1000 turns of wire, how many turns of wire does the secondary have?
A. 25,000 B. 4000 C. 2500 D. 40 E. 25 F. Some other result (specify). T. One needs more information to answer.
We have direct experimental evidence (from large particle accelerators) for the physical conditions in the universe back to about ________ after the Big Bang
A) 3 minutes B) 10-10 seconds C) 1 million years D) 380,000 years E) 300 years
A transformer for charging a power tool steps the line voltage down to 2.9 V. The output current of this transformer is 240 mA. If the transformer is 80% efficient, what is the input current?
A) 300 mA B) 7.3 mA C) 240 mA D) 190 mA E) 4.6 mA
High-strength stainless steel is required for use in building structures and equipment (e.g. , cranes). It is produced by heat treating quench-hardened steel in a process called tempering that reduces brittleness and imparts toughness. In a production facility, alloy steel plates (k=50 W/(m K), c=460 J/(kg K) and ?=7865 kg/m3) of thickness 3 cm have to be tempered in a convective oven by heating them to 5500C. If the plates are initially at 400C and the air inside the heat treating oven is at 7000C with a convective heat transfer coefficient of 45 W/(m2 K), determine how long the plate has to remain in the oven.
GIVEN
• Alloy steel plate of thickness L=3 cm=0.03 m
• Tempering in oven to 5500C
• Initial temperature of the plate (To) = 40°C
• Heat treating oven temperature (T?) = 700°C
• Convective heat transfer coefficient ( ch ) = 45 W/(m2 K)
FIND
• The time required for the plate to reach 5500C.
ASSUMPTIONS
• Constant thermal conductivity
• End effects are negligible
SKETCH
