Ch20; Heat and the First Law of Thermodynamics

Why can you get "burned" by biting into the hot CHEESE on a steaming pizza far easier than by biting into the CRUST of the pizza?

Homework Assignment

From the fifth edition(!)

Ch20; Questions: 2, 3, 4, 6, 20, *

Ch20; Problems: 1, 3, *, **, ***, ****, 11, (*5), 27, 39, (*6), (*7), (*8), (*9), 51, 45, (*10)

[Yes, I know I got carried away. Just remember, I have to solve all these, too!

Do not wait and just watch me solve these. Work them out yourself!]

20.Q2 Give one reason why coastal regions tend to have a more moderate climagte than inland regions.

20.Q3 A small meta crucible is taken from a 200oC oven and immersed in a tub full of water at room temperature (this process is often referred to as quenching). What is the approximate final equilibrium temperature?

20.Q4 What is the major problem that arises in measuring specific heats if a sample with a temperature greater than 100oC is placed in water?

20.Q6 The pioneers found that a large tub of water placed in a storage cellar would prevent their food from freezing on really cold nights. Explain why.

20.Q20 In an experimental house, Styrofoam beads were pumped into the air space between the double windows at night in the winter and pumped out to holding bins during the day. How would this assist in conserving heat energy in the house?

20.Q* Why can you get a more severe burn from steam at 100oC than from water at 100oC?

20.1 Water at the top of Niagra Falls has a temperature of 10oC. If it falls through a distance of 50 m and all of its potential energy goes into theating the water, calculate the temperature of the water at the bottom of the falls.

20.3 The temperature of a silver bar rises by 10.0oC when it absorbs 1.23 kJ of heat. The mass of the bar is 525 g. Determine the specific heat of silver.

20.* If 100 g of water at 100oC is poured into a 20-g aluminum cup containing 50 g of water at 20oC, what is the equilibrium temperature of the system?

20.** What is the final equilibrium temperature when 10 g of milk at 10oC is added to 160 g of coffee at 90oC? (Assume the heat capacities of the two liquids are the same as that of water, and neglect the heat capacity of the container).

20.*** (a) A calorimeter contains 500 ml of water at 30oC and 25 g of ice at 0oC. Determine the final temperature of the system.

(b) Repeat part (a) if 250 g of ice is initially present at 0oC.

20.**** The air temperature above coastal areas is profoundly influenced by the large specific heat of water. One reason is that the heat released when 1 cubic meter of water cools by 1.0 Co will raise the temperature of an enormously larger volume of air by 1.0 Co. Estimate this volume of air.

The specific heat of air is approximately 1.0 kJ/kg-Co. Take the density of air to be 1.25 kg/m3.

20.11 A water heater is operated by solar power. If the solar collector has an area of 6.0 m2 and the power delivered by sunlight is 550 W/m2, how long does it take to increase the temperature of 1.0 m3 of water from 20oC to 60oC?

20.(*5) If 90.0 g of molten lead at 327.3oC is poured into a 300.0-g casting made of iron initially at 20.0oC, what is the final temperature of the sytem? (Assume there are no heat losses.)

20.27 A gas expands from I to F along three possible paths as indicated by Figure P20.27. Calculate the work in joules done by the gas along the paths IAF, IF, and IBF.

20.39 An ideal gas undergoes a thermodynamic process that consists of two isobaric and two isothermal steps as shown in Figure P20.32. Show that the et work done during the four steps is

Wnet = P1 (V2 - V2) ln (P2/P1)

20.(6*) A sample of ideal gas is expanded to twice its original volume of 1.0 m3 in a quasi-static process for which P = a V2, with a = 5.0 atm/m6, as shown in Figure P20.(6*). How much work was done by the expanding gas?

20.(*7) A gas is compressed at a constant pressure of 0.80 m from 9.0 L to 2.0 L. In the process, 400 J of thermal energy leaves the gas.

(a) What is the work done by the gas?

(b) What is the change in its internal energy?

20.41 Five moles of an ideal gas expands isothermally at 127oC to four times its intiial volume. Find --

(a) the work done by the gas and

(b) the thermal energy transferred to the system;

put both in units of joules.

20.(*8) An ideal gas initially at 300 K undergoes an isobaric expansion at 2.50 kPa. If the volume increases from 1.00 m3 to 3.00 m3 and 12.5 kJ of thermal energy is transferred to the gas, find

(a) the change in its internal energy and

(b) its final temperature.

20.(*9) Helium with an initial volume of 1.00 liter and an initial pressure of 10.0 atm expands to a final volume of 1.00 m3. The relationship between pressure and volume during the expansion is PV = constant. Determine

(a) the value of the constant,

(b) the final pressure, and

(c) the work done by the helium during the expansion.

20.45 A bar of gold is in thermal constanct with a bar of silver of the same length and area (as shown in Figure P20.55). One end of the compound bar is maintained at 80.0oC while the opposite end is at 30.0oC. When the heat flow reaches steady state, find the temperature at the junction.

20.(*10) The brick wall (k = 0.8 W/m-Co) of a building has dimensions of 4.0 m x 10.0 m and is 15 cm (0.15 m) thick. How much heat (in joules) flows through the wall in a 12-h period when the average inside and outside temperatures are, 20oC and 5oC, respectively?

Ch20 ToC

Heat & Internal Energy

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