The identities of dark energy and dark matter are
A) unknown at this time.
B) subjects of intense astronomical interest.
C) both of these
D) neither of these
Answer: C
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A section of a composite wall with the dimensions shown below has uniform temperatures of 200°C and 50°C over the left and right surfaces, respectively. If the thermal conductivities of the wall materials are: kA = 70 W/(m K), kB = 60 W/(m K), kc = 40 W/(m K) and kD = 20 W/(m K), determine the rate of heat transfer through this section of the wall and the temperatures at the interfaces. including a contact resistance of 0.1 K/W at each of the interfaces.
GIVEN
• Composite wall
• Thermal conductivities:
? kA = 70 W/(m K)
? kB = 60 W/(m K)
? kC = 40 W/(m K)
? kD = 20 W/(m K)
• Surface temperatures
? Left side (TAs) = 200°C
? Right side (TDs) = 50°C
• Contact resistance at each interface (Ri) = 0.1 K/W FIND
(a) Rate of heat transfer through the wall (q) (b) Temperatures at the interfaces
ASSUMPTIONS
• One dimensional conduction
• The system is in steady state
SKETCH
A body oscillates with simple harmonic motion along the x axis. Its displacement varies with time according to the equation x = 5.0 sin (?t + ?/3). The velocity (in m/s) of the body at t = 1.0 s is
a. +7.9 b. ?7.9 c. ?14 d. +14 e. ?5.0
If the current in an inductor is doubled, by what factor is the stored energy multiplied?
1.4 2.2 3.1 4.1/2 5.1/4
Give two reasons why summer days are warmer than winter days
What will be an ideal response?