An electron (m = 9.1 × 10?31 kg, q = ?1.6 × 10?19 C) starts from rest at point A and has a speed of 5.0 × 106 m/s at point B. Only electric forces act on it during this motion. Determine the electric potential difference VA?VB.
A. ?71 V
B. +71 V
C. ?26 V
D. +26 V
E. ?140 V
Answer: A
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In a heat exchanger, as shown in accompanying figure, air flows over brass tubes of 1.8- cm-ID and 2.1-cm-OD that contain steam. The convection heat-transfer coefficients on the air and steam sides of the tubes are 70 W/(m2 K) and 210 W/(m2 K), respectively. Calculate the overall heat transfer coefficient for the heat exchanger (a) based on the inner tube area, (b) based on the outer tube area.
GIVEN
• Air flow over brass tubes containing steam
• Tube diameters
? Inside (Di) = 1.8 cm = 0.018 m
? Outside (Do) = 2.1 cm = 0.021 m
• Convective heat transfer coefficients
? Air side h c= 70 W/(m2 K)
? Steam side h i= 210 W/(m2 K)
FIND
• The overall heat transfer coefficient for the heat exchanger based on
(a) the inner tube area (Ui) and
(b) the outer tube area (Uo)
ASSUMPTIONS
• The heat transfer coefficients are uniform over the transfer surfaces
SKETCH
PROPERTIES AND CONSTANTS
the thermal conductivity of brass at 20°C (kb) = 111 W/(m K)
The current supplied by a battery as a function of time is I(t) = (0.88
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One astronomical unit, or one AU is defined as the
What will be an ideal response?