The first law of thermodynamics is written $Q=\Delta U+W$. What does $\Delta U$ represent?
The change in internal energy of the system
The heat lost to the surroundings
The work done on the system
The entropy change
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Why$\Delta U$ is the change in internal energy of the system; $Q$ is heat added and $W$ is work done by the gas.
2Multiple choice · Medium
For an isothermal process involving an ideal gas, the change in internal energy is:
Zero
Equal to the heat supplied
Negative
Equal to the work done
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WhyInternal energy of an ideal gas depends only on temperature. At constant temperature, $\Delta U=0$, so $Q=W$.
3Multiple choice · Hard
A Carnot engine operates between a hot reservoir at $T_{H}=500\,\text{K}$ and a cold reservoir at $T_{C}=300\,\text{K}$. What is its maximum efficiency?
The second law of thermodynamics states that the total of an isolated system never decreases.
Answer:
entropy
WhyEntropy, a measure of disorder, tends to increase for any real (irreversible) process in an isolated system.
5Multiple choice · Hard
75 J of heat is added to an ideal gas at constant pressure 1.0 x 10^5 Pa while its volume rises from 2.7 x 10^-3 to 3.0 x 10^-3 m cubed. What is the change in internal energy?
45 J
105 J
-45 J
-105 J
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WhyWork done by gas = P delta V = 1.0e5 x 0.3e-3 = 30 J; delta U = Q - W = 75 - 30 = 45 J.
6Multiple choice · Medium
The first law of thermodynamics states that the change in internal energy equals
heat added minus work done by the gas
work done by the gas minus heat added
heat added plus work done by the gas
pressure times volume only
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Whythe heat added to the system minus the work done by the system.
7Fill in the blank · Medium
The work done by a gas expanding at constant pressure is P times delta .
Answer:
V / volume
WhyIt is P delta V (change in volume).
8Fill in the blank · Medium
For an ideal gas, the internal energy depends only on its .
Answer:
temperature
WhyIt depends only on temperature.
9Multiple choice · Hard
An ideal gas is compressed at constant temperature. How do the entropies of the gas and its surroundings change?
Gas entropy decreases, surroundings entropy increases
Gas entropy increases, surroundings entropy decreases
Both increase
Both decrease
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WhyCompression reduces the gas volume and disorder so its entropy decreases; the heat expelled raises the entropy of the surroundings.
10Multiple choice · Medium
For any real (irreversible) process, the total entropy of the system plus surroundings
increases
decreases
stays exactly constant
becomes zero
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Whyincreases.
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