Download 53 - Angelfire

6E/S&J
20.17. A 3.00 g lead bullet at 30C is fired at a speed of 240 m/s into a large block of ice
at 0C, in which it becomes embedded. What quantity of ice melts?
6E/S&J
20.21. In an insulated vessel, 250 g of ice at 0C is added to 600 g of water at 18C. (a)
What is the final temperature of the system? (b) How much ice remains when the system
reaches equilibrium?
6E/S&J
20.23. A sample of ideal gas is expanded to twice its original volume of 1.00 m3 in a
quasi-static process for which P  V 2 , with  = 5.00 atm/m6, as shown in Figure
P20.23. How much work is done on the expanding gas?
6E/S&J
20.27. One mole of an ideal gas is heated slowly so that it goes from the PV state (Pi, Vi)
to (3Pi, 3Vi) in such a way that the pressure is directly proportional to the volume. (a)
How much work is done on the gas in the process? (b) How is the temperature of the gas
related to its volume during this process?
6E/S&J
20.30. A gas is taken through a cyclic process described in Figure P20.30. (a) Find the
net energy transferred to the system by heat during one complete cycle. (b) If the cycle is
reversed – that is, if the process follows the path ACBA – what is the net energy input per
cycle by heat?
6E/S&J
20.31. Consider the cyclic process depicted in Figure P20.30. If Q is negative for the
process BC, and if Eint is negative for the process CA, what are the signs of Q, W, and
Eint that are associated with each process?
6E/S&J
20.32. A sample of an ideal gas goes through the process shown in Figure P20.32. From
A to B, the process is adiabatic; from B to C, it is isobaric, with 100 kJ of energy entering
the system by heat. From C to D, the process is isothermal; from D to A, it is isobaric,
with 150 kJ of energy leaving the system by heat. Determine the difference in internal
energy Eint,B – Eint,A.
6E/S&J
20.33. A sample of an ideal gas is in a vertical cylinder fitted with a piston. As 5.79 kJ of
energy is transferred to the gas by heat to raise the temperature, the weight on the piston
is adjusted so that the state of the gas changes from point A to point B along the
semicircle shown in Figure P20.33. Find the change in the internal energy of the gas.
6E/S&J
20.36. A 1.00-kg block of aluminum is heated at atmospheric pressure such that its
temperature increased from 22.0C to 40.0C. Find (a) the work done on the aluminum,
(b) the energy added to it by heat, and (c) the change in its internal energy.
6E/S&J
20.39. A 2.00-mol sample of helium gas initially at 300 K and 0.400 atm is compressed
isothermally to 1.20 atm. Noting that the helium behaves as an ideal gas, find (a) the final
volume of the gas, (b) the work done on the gas, and (c) the energy transferred by heat.
6E/S&J
20.40. In Figure P20.40, the change in internal energy of a gas that is taken from A to C is
+800 J. The work done on the gas along the path ABC is –500 J. (a) How much energy
must be added to the system by heat as it goes from A through B to C? (b) If the pressure
at point A is five times that of point C, what is the work done on the system in going from
C to D? (c) What is the energy exchanged with the surroundings by heat as the cycle goes
from C to A along the green path? (d) If the change in internal energy in going from point
D to point A is +500 J, how much energy must be added to the system by heat as it goes
from point C to point D?
6E/S&J
20.42. A glass window pane has an area of 3.00 m2 and a thickness of 0.600 cm. If the
temperature difference between its faces is 25.0C, what is the rate of energy transfer by
conduction through the window?
6E/S&J
20.43. A bar of gold is in thermal contact with a bar of silver of the same length and area
(Fig. P20.43). One end of the compound bar is maintained at 80.0C, while the opposite
end is at 30.0C. When the rate of energy transfer reaches steady state, what is the
temperature at the junction?
6E/S&J
20.49. The tungsten filament of a certain 100-W light bulb radiates 2.00 W of light (the
other 98 W is carried away by convection and conduction). The filament has a surface
area of 0.250 mm2 and an emissivity of 0.950. Find the filament’s temperature (note that
the melting point of tungsten is 3683 K).
6E/S&J
20.51. The intensity of solar radiation reaching the top of the Earth’s atmosphere is
1340 W/m2. The temperature of the Earth is affected by the so-called “greenhouse effect”
of the atmosphere. That effect makes our planet’s emissivity for visible light higher than
its emissivity for infrared light. For comparison, consider a spherical object with no
atmosphere, at the same distance from the Sun as the Earth. Assume that its emissivity is
the same for all kinds of electromagnetic waves and that its temperature is uniform over
its surface. Identify the projected area over which it absorbs sunlight and the surface area
over which it radiates. Compute its equilibrium temperature. Chilly, isn’t it? Your
calculation applies to (a) the average temperature of the Moon, (b) astronauts in mortal
danger aboard the crippled Apollo 13 spacecraft, and (c) global catastrophe on the Earth
if widespread fires should cause a layer of soot to accumulate throughout the upper
atmosphere, so that most of the radiation from the Sun were absorbed there rather than at
the surface below the atmosphere.