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PHY 121 – Test 3 – Key
Spring 2011
Explanations are provided only for questions requiring quantitative answers.
1. It would be appropriate to measure temperature in ______ and heat in _____.
a)
degrees, calories
b)
Btu; Btu
c)
calories; calories
d)
calories; degrees
e)
both (b) and (c) are correct answers
2. Say that the volume of a container containing a gas is slowly increased. Is it possible to keep the pressure
constant inside?
a)
No, it’s not possible since the gas expands
b)
Yes, by increasing the volume really slowly
c)
Yes, by gradually increasing the temperature
Yes, by gradually decreasing the temperature
e)
Yes, by forcing the temperature to stay constant
3. An ideal gas at temperature
T
= 100 K has an internal energy of about 3110 Joules. Which of the numbers below
is the closest to the number of molecules in the gas??
a)
6.3×10
23
b)
6.3×10
24
c)
5.0×10
23
1.5×10
23
e)
1.5×10
24
4. The temperature of a volume of air is increased by 10 degrees Celsius. Therefore
a)
the temperature increased by 10 degrees Kelvin.
b)
the temperature increased by 18 degrees Fahrenheit.
c)
the temperature in Fahrenheit decreased.
None of the above.
e)
Both (a) and (b) are correct answers.
5. Say that you want to raise the temperature of two objects – one made of aluminum and one of iron – by 10
degrees Celsius. Knowing that the masses are equal, which object would require more heat, and by how much?
a)
Both objects require the same heat.
b)
The aluminum object requires two times more energy.
c)
The iron object requires two times more energy.
The aluminum object requires ten times more energy.
e)
Both (a) and (b) are correct answers.
6. Water has a higher specific heat than soil.
If you add equal amounts of thermal energy to equal masses of water
and soil, then
a)
the soil will undergo the greater temperature change.
b)
the water will undergo the greater temperature change.
c)
the temperature change for water and soil will be the same. Specific heat has nothing to do with temperature
changes.
it is impossible to tell which undergoes the greater temperature change. Specific heat has nothing to do with
temperature changes.
e)
the water conducts the heat away faster than the soil.
Explanation
: The internal energy
U
of an ideal gas of
N
molecules is given by the sum over
N
average kinetic energies
E
which is proportional to temperature. Therefore
24
3
2
1.5 10 molecules
UU
U
NE
N
E
kT
Explanation
: By definition, the Kelvin and respectively the Fahrenheit scales are given in terms of Celsius scale by
273
KC
and
9
5
32
FC
So, if the temperature increases by 10°C, it increases by the same amount in °K and by 9
10/5 °F = 18 °F.
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View Full DocumentPHY 121 – Test 3 – Key
Spring 2011
7. The first law of thermodynamics states
a) an engine must reject more heat than it takes in.
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 Fall '11
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