A styrofoam ball of mass 0.120 g is placed in an electric field of 6000 N/C pointing downward. What charge must be placed on the ball for it to be suspended?

A) -16.0 nC
B) -18.0 nC
C) -57.2 nC
D) -125 nC
E) -196 nC


E

Physics & Space Science

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Series/Parallel Circuits: Four resistors are connected across an ideal dc source of V = 8.0 V, as shown in the figure. Assume all resistances shown are accurate to two significant figures. What is the current through the 9.0-? resistor?

A. 1.0 A B. 0.67 A C. 0.50 A D. 0.90 A E. 2.0 A

Physics & Space Science

Water enters a double pipe heat-exchanger at 60°C. The water flows on the inside through a copper tube 2.54 cm ID at a velocity of 2 cm/s. Steam flows in the annulus and condenses on the outside of the copper tube at a temperature of 80°C. Calculate the outlet temperature of the water if the heat exchanger is 3-m-long.

GIVEN
• Water flow through a tube in a double pipe heat-exchanger
• Water entrance temperature (Tb,in.) = 60°C
• Inside tube diameter (D) = 2.54 cm = 0.0254 m
• Water velocity (V) = 2 cm/s = 0.02 m/s
• Steam condenses at (Ts) = 0.80°C on the outside of the pipe
• Length of heat exchanger (L) = 3 m
FIND
• Outlet temperature of the water (Tb,out)
ASSUMPTIONS
• Steady state
• Thermal resistance of the copper pipe is negligible
• Pressure in the annulus is uniform therefore, Ts is uniform
• Heat transfer coefficient of the condensing steam is large (see Table 1.4) so its thermal resistance can be neglected
• Outside surface of the heat exchanger is insulated
SKETCH

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