Two identical coils of wire are placed on a single horizontal wooden broom handle. The coils are separated by a few centimeters. We now run identical currents in the same direction through each coil. The coils experience

A. a repulsive magnetic force.
B. a repulsive electrostatic force because both coils are identically charged.
C. an attractive electrostatic force because one coil is negatively charged and the other positively charged.
D. no forces because the currents are identical.
E. an attractive magnetic force.


Answer: E

Physics & Space Science

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Consider a heat treatment process in which steel rods with a 10 cm diameter at an initial temperature of 600°C are inserted into an oil bath at 25°C. Assuming a convection coefficient of 400 W/(m2 K) between the oil and the rod, estimate how long it takes for the centerline of the rod to cool to 60°C. If the steel rod, which is 1 m long, is withdrawn from the bath at this point in the process, i.e., when the centerline temperature has reached 60°C, estimate the rate at which heat must be extracted from the oil to maintain its temperature constant during the process of treating 25 rods per hour.

GIVEN
• Heat treatment of steel rod inserted in oil bath
• Rod outside diameter = 0.1 m
• Initial temperature (T0) = 600°C
• Oil bath temperature (T?) = 25°C
• The average heat transfer coefficient ch = 400 W/(m2 K)
FIND
• Time it takes for centerline of the rod to cool to 600C.
• Rate at which heat must be extracted from oil to maintain its temperature constant during the process of treating 25 rods per hour.
ASSUMPTIONS
• Radial conduction only in billet
• Uniform and constant properties
SKETCH

Physics & Space Science

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Physics & Space Science

As shown in the diagram, you've just shined a flash light toward Al, so from your point of view the light is moving to the right at speed c. Al is moving away from you to the right at speed 0.9c. According to Al, what is the light doing?

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Physics & Space Science

How do astronomers think Jupiter generates its internal heat?

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Physics & Space Science