If the current density in a wire is given by j = kr, 0 < r < R, where R is the radius of the wire, the current in the wire is
A)
2?kR3/3.
B)
3?kR3/2.
C)
kR3/3.
D)
kR2.
E)
kR2/2.
A
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Consider Newton's second law states, often written as force = mass × acceleration. If a known force is applied to an object with a known mass, what does this law predict for the object's acceleration?
A) acceleration = mass × force B) acceleration = mass / force C) acceleration = force / mass D) Newton's second law is irrelevant for solving this problem.
Two objects are dropped from a bridge, an interval of 1.0 s apart. Neglect air. During the time that both objects continue to fall, their separation
A) increases. B) decreases. C) stays constant. D) increases at first, but then stays constant. E) decreases at first, but then stays constant.
Which statements must be true about the surface of a charged conductor in which no charge is moving? (There could be more than one correct choice.)
A) The electric field is zero at the surface. B) The electric potential of the surface is zero. C) The electric field is constant at the surface. D) The electric potential is constant over the surface. E) The electric field is perpendicular to the surface.
Water flowing at a rate of 10 kg/s through 50 tubes double-pass shell and tube heat exchanger heats air that flows through the shell side. The length of the brass tubes is 6.7 m and they have an outside diameter of 2.6 cm and an inside diameter of 2.3 cm. The heat transfer coefficient of the water and air are 470 W/(m2 K) and 210 W/(m2 K), respectively. The air enters the shell at a temperature of 15°C and a flow rate of 16 kg/s. The temperature of the water as it enters the tubes is 75°C. Calculate (a) the heat exchanger effectiveness, (b) the heat transfer rate to the air, and (c) the outlet temperature of the air and water.
GIVEN
• Shell-and-tube heat exchanger - one shell, two tube passes
• Water in brass tubes, air in shell
• Water flow rate ( m w)= 10 kg/s
• Number of double passes (N) = 50
• Tube length (L) = 6.7 m
• Tube diameters
? Do = 2.6 cm = 0.026 m
? Di = 2.3 cm = 0.023 m
• Heat transfer coefficient
? Water ( )ih = 470 W/(m2 K)
? Air ( )oh = 210 W/(m2 K)
• Air inlet temperature (Ta,in) = 15°C
• Air flow rate ( m a)= 16 kg/s
• Water inlet temperature (Tw,in) = 75°C
FIND
(a) Effectiveness (e)
(b) The heat transfer rate (q)
(c) Outlet temperatures (Ta,out, Tw,out)
ASSUMPTIONS
• Tube length includes both passes
SKETCH
OPERTIES AND CONSTANTS
the specific heat of water at 75°C (cpw) = 4190 J/(kg K)
the specific heat of air at 15°C (cpa) = 1012 J/(kg K)
the thermal conductivity of brass (kb) = 111 W/(m K)