A 15-turn rectangular loop of wire of width 10 cm and length 20 cm has a current of 2.5 A flowing through it. Two sides of the loop are oriented parallel to a uniform magnetic field of strength 0.037 T,
and the other two sides are perpendicular to the magnetic field.
(a) What is the magnitude of the magnetic moment of the loop?
(b) What torque does the magnetic field exert on the loop?
(a) 0.75 A ? m2 (b) 0.028 N ? m
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An electronic device that internally generates 600 mW of heat has a maximum permissible operating temperature of 70°C. It is to be cooled in 25°C air by attaching aluminum fins with a total surface area of 12 cm2 . The convection heat transfer coefficient between the fins and the air is 20 W/(m2 K). Estimate the operating temperature when the fins are attached in such a way that: (a) there exists a contact resistance between the surface of the device and the fin array of approximately 50 K/W, and (b) there is no contact resistance but the construction of the device is more expensive. Comment on the design options.
GIVEN
FIND
Operating temperature (To) for
(a) contact resistance (Ri) = 50 K/W
(b) no contact resistance
ASSUMPTIONS
-One dimensional heat transfer
-Steady state has been reached
-The temperature of the device is uniform
-The temperature of the aluminum fins is uniform (the thermal resistance of the aluminum is
negligible)
-The heat loss from the edges and back of the device is negligible
SKETCH
If electricity costs 6.00¢/kWh (kilowatt-hour), how much would it cost you to run a 120 W stereo system 4.0 hours per day for 4.0 weeks?
A) $0.81 B) $0.12 C) $1.38 D) $2.27
A child is standing at the rim of a freely rotating disk holding a rock. The disk rotates without friction. The rock is dropped at the instant shown. Which of the following statements is true about the process of dropping the rock?
Which body glows with electromagnetic waves?
A) the Sun B) the Earth C) you and your classmates D) all of the above