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It is a well-known physiological phenomenon that some materials feel cooler to the touch than others. This is important in the design of instruments for the use of operators, especially in a space station. Experiments conducted at NASA have shown that different materials, e.g., aluminum and plastic, though in identical size and temperature feel very different to the touch. Consider two plates, one made of aluminum and the other of plastic, each at an initial temperature of 25°C. Determine which of the two will feel hotter to the touch of your finger if it is initially at 32°C. Assume that your finger has a density of 1000 kg/m3, specific heat of 4180 J/(kg K), and thermal conductivity of 0.625 W/(m K). Also, properties of aluminum are given in and plastic is considered to have a density
of 1990 kg/m3, specific heat of 1470 J/(kg K), and thermal conductivity of 0.21 W/(m K).
GIVEN
FIND
which of the aluminum or plastic feel hotter to touch to your finger.
ASSUMPTIONS
constant thermal properties
Negligible contact resistance
The finger and plate behave as semi infinite solid.
SKETCH
According to the theory that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from
A) nuclear fusion in the accretion disk surrounding the black hole. B) gravitational potential energy released by matter that is falling toward the black hole. C) matter-antimatter annihilation occurring just outside the event horizon of the black hole. D) jets emerging from the accretion disk.
A proton has a speed of 3.0 × 106 m/s in a direction perpendicular to a uniform magnetic field, and the proton moves in a circle of radius 0.20 m. What is the magnitude of the magnetic field?
A) 0.040 T B) 0.080 T C) 0.16 T D) 0.24 T E) 0.32 T
Sea breezes that occur near the shore are attributed to a difference between land and water with respect to what property?
a. mass density b. coefficient of volume expansion c. specific heat d. emissivity