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PHYSICS

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Summer 201

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Homework #7 Due Friday
August 2, 2013

1. (a) At what temperature is the reading on the Centigrade scale the same as the reading on the Fahrenheit
scale?

(b) What is this temperature on the Kelvin scale?

2. A steel rod is 3.000 cm in diameter at 2

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.0 ◦C. A brass ring has an interior diameter of 2.992 cm at
25.0 ◦C. At what common temperature will the ring just slide onto the rod? (Use coefficients of linear
expansion from the textbook).3.000 cm

3. What is the volume of an aluminum ball at 30.0 ◦C if its volume at 60.0 ◦C is exactly 30.0 cm3. (Use
data from the text book).

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PHYSICS 221
Summer 2013

Homework #7 Due Friday
August 2, 2013

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. A copper beaker of mass 0.20 kg contains 0.10 kg of water at 20.0 ◦C. A block of silver of mass 100 g at
90.0 ◦C is added to the beaker of water. The entire system is thermally insulated from the surroundings.
(specific heats- copper: 390 J/kg K, silver: 234 J/kg K, water: 4190 J/kg K)

(a) What is the final temperature of the system?

(b) What is the heat lost by the silver?

(c) What is the heat gained by the water? What is the heat gain of the copper?

5. 5.00 g of ice at −10.0 ◦C are added to 20.0 ml of hot coffee at 60.0 ◦C, inside a thermos that is then
tightly closed. What is the final temperature of the coffee? (This is similar to the in class example).
(specific heat of ice: 2100 J/kg K, latent heat of ice: 334 KJ/kg)

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PHYSICS 221
Summer 2013
Homework #7 Due Friday
August 2, 2013

6. (a) What is the heat flow through a glass window of dimensions 0.50 m × 0.50 m and thickness 4.0 mm,
when the inside temperature is 25 ◦C and the outside temperature is −15 ◦C. (thermal conductivity
of glass is 0.8 W/mK and of air is 0.024 W/mK)

(b) What is the heat flow when a thicker glass window of 8.0 mm is used?

(c) Estimate the heat flow when a double pane of glass is used with a 4.0 mm air gap between the panes.
Each individual glass sheet is 4.0 mm thick. (Refer to in class problems from the lectures).

7. The emissivity of tungsten is 0.35. A tungsten sphere of radius 1.5 cm is suspended within a large
evacuated enclosure whose walls are at 300 K. What power input is required to maintain the sphere at a
temperature of 3000 K. Assume that heat conduction along the supports can be neglected.

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PHYSICS 221
Summer 2013
Homework #7 Due Friday
August 2, 2013

8. Oxygen gas having a volume of 1.00 liter at 40.0 ◦C and a pressure of 1.00 atmosphere expands until its
volume is 1.50 liters and the pressure is 1.05 atmospheres.

(a) Find the number of moles of oxygen

(b) What is the final temperature of the gas?

9. Three moles of an ideal gas are contained in a 75 L container at a pressure of 1.6 atm.

(a) What is the temperature of the gas?

(b) What is the total average translational kinetic energy of all the molecules of the gas?

(c) If the gas is H2 with a molar mass of 2.0 g/mole, what is the rms speed of a H2 molecule?

(d) If the volume of the container containing the gas was decreased by half while keeping pressure at the
same value, by what factor will the average rms speed of the molecules change?

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PHYSICS 221
Summer 2013
Homework #7 Due Friday
August 2, 2013

10. You keep your coffee in a sealable thermal mug which has thermal conductivity k ∼ 0.036 W/m k. The
mug is about 15 cm high and its average radius is about 3.0 cm and has wall thickness of 5.0 mm. The
coffee can be considered to have the same properties of water. If the mug is filled with coffee and place
in room temperature (25 ◦C), Starting from boiling temperature, how long (in minutes) will it take the
coffee to cool down to: (a) 75 ◦C, (b) 50 ◦C, and (c) 30 ◦C,

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