# Thermodynamics Homework

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27 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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Thermodynamics

Homework

1.

Which of the following is characterized by the actual movement of mass?

a.

conduction

b.

c.

convection

d.

reflection

2.

Why is it important to protect water pipes from freezing?

The water expands

as it freezes and can bust the pipes
.

3.

Why do you not burn your finger when it is held to the side of a burning candle flame, yet it does
burn when you place it over the flame?

The heat rises above the flame. That is where it is hot. Heat expands and
rises.

4.

Does cold flow from an ice cube to your hand, or does heat flow from your hand to the ice
cube…or both? Explain.

Heat flows from your hand leaving your hand cooler. You feel the cold.

5.

Ice at 0
°
C is more effective than water at the same temperature in cooling down a drink.

This is
because

a.

i
ce floats and cools from the top down

b.

m
elting ice requires energy

c.

i
ce has a higher specific heat capacity than water

d.

t
his is not really true, it is an old
wife’s tale

6.

An engine takes in 400 J of heat and does 40 J of work.

How much heat is expelled?

a.

40 J

b.

400 J

c.

360 J

d.

440 J

7.

A system at 500 Kelvin has 6000 J of energy inserted and does 2000 J of work, but does not
lose any energy to the outside, what is the in
crease in internal energy?

a.

500 K

b.

4000 J

c.

2000 J

d.

8000 J

K
EY

8.

Alcohol has about ½ the specific heat capacity of water.

If equal masses of alcohol and water in
separate beakers at the same temperature are supplied with the same amount of energy, which
will have
the higher final temperature?

The alcohol will have a larger temperature change

9.

Even Texas home owners are encouraged to water tender plants left outside when a freeze is
expected. What is the virtue of this practice?

For a harder freeze, plants are covered with a
blanket.

Why might this help?

The (some) plants are not harmed until they get below the freezing temperature. The
water protects because while there is water, the temperature is freezing or above. The
blanket

reflects the radiated heat energy back to the plants
-

just like your blanket in bed.

10.

A salt solution is placed in a bowl and the water is allowed to evaporate, leaving behind highly
ordered salt crystals.

This is a more ordered state than the randomly
dispersed ions in the
solution.

Does this violate the 2nd Law of Thermodynamics?

No. The entropy of the salt is less, but it took an external source to do the ordering. The
entropy of the external source is much larger. The total entropy of the universe
is larger.

11.

What would be the
equilibrium

temperature if you mixed 1 liter of 40
°
C water with 2 liters of
20
°
C water?

(1)(40) + (2)(20) = 80

80 / 3 = 26.7°C

12.

20g of ice at 0°C is mixed with 20g of water at 100°C. Assuming no heat is lost or gained fro
m
outside the mixture, what will be the equilibrium temperature reached when all the ice melts?

Energy of hot water:

(20)(100) = 2000

Energy to melt ice:

Q = L m = (80)(20) = 1600

After ice melted, 400 cal∙g remaining (2000

1600)

400 / 40 = 10°C