Power: A certain ac signal at 1000 Hz is applied across an inductor and a 100-? resistor. If the power factor of the circuit is 0.400, what is the impedance of this circuit?
A. 150 ?
B. 200 ?
C. 250 ?
D. 100 ?
E. 300 ?
Answer: C
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An economizer is to be purchased for a power plant. The unit is to be large enough to heat 7.5 kg/s of pressurized water from 71 to 182°C. There are 26 kg/s of flue gases (cp = 1000 J/(kg K)) available at 426°C. Estimate (a) the outlet temperature of the flue gases and (b) the heat transfer area required for a counterflow arrangement if the overall heat transfer coefficient is 57 W/(m2 K).
GIVEN
Counterflow heat exchanger - flue gases heating water Water flow rate ( m w)= 7.5 kg/s Water temperatures
? Tw,in = 71°C
? Tw,out = 182°C Gas flow rate ( m g) = 26 kg/s Gas specific heat (cpg) = 1000 J/(kg K) Gas inlet temperature (Tg,in) = 426°C Overall heat transfer coefficient (U) = 57 W/(m2 K)
FIND
(a) Outlet gas temperature Tg,out
(b) Heat transfer area (A) required
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of water at the mean temperature of 126.5°C (cpw)= 4240 J/(kg K)
From Voyager 2, Neptune's clouds appeared much more active than did Uranus', surprising since Neptune is smaller, colder, and farther from the Sun
Indicate whether the statement is true or false
When water of mass m and specific heat c is heated at constant pressure from absolute temperature T1 to absolute temperature T2, its change in entropy is what?
a. cm ln(T2/T1) b. cm (T2/T1) c. cm (T2 ? T1)/T1 d. cm (T2 ? T1)/T2 e. cm (T2 ? T1)/(T2 + T1)
When a charged particle is moved along an electric field line,
a. the electric field does no work on the charge. b. the electrical potential energy of the charge does not change. c. the electrical potential energy of the charge undergoes the maximum change in magnitude. d. the voltage changes, but there is no change in electrical potential energy. e. the electrical potential energy undergoes the maximum change, but there is no change in voltage.