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46 Cards in this Set
- Front
- Back
What is the First Law of Thermodynamics?
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The First Law states that the change in internal energy of a system equals the heat added to the
system plus the work done on the system. |
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What is meant by a ʺsystemʺ?
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A system is whatever is of interest in the thermodynamics problem. It can refer to a defined
amount of matter (e.g., the gas inside a closed container), but it can also refer to a material in a defined volume (e.g., the gas inside an open container). |
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What is meant by an ʺidealʺ gas?
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An ideal gas consists of non‐interacting particles. Dilute gases (e.g., air under standard
conditions) are approximately ideal. |
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Is internal energy proportional to temperature for an ideal gas?
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Yes and no. It is more precise to say that the change in internal energy is proportional to the
change in temperature, since the zero set‐point for U is arbitrary. |
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Is ΔU proportional to ΔT for all things?
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No.
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Can a non‐ideal gas obey PV = nRT?
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Yes, across a limited range. Positive deviations from the ideal gas law (i.e., where PV > nRT)
occur when molecules occupy a large space. Negative deviations (PV < nRT) occur when molecules have attractive interactions. These effects can cancel so that a non‐ideal gas appears to obey PV = nRT. |
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If ΔT = 0, is the process isothermal?
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No. Isothermal means that temperature does not change, i.e., dT = 0. ΔT = 0 only implies that
the initial and final temperatures are the same. |
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If you compress an ideal gas isothermally, is heat added or removed from the gas?
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Removed. Internal energy is constant, and positive work is done on the gas, so heat must be
removed by the First Law. |
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Is it possible to compress a gas and have its temperature drop?
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Yes. For instance, one can remove a lot of heat during the compression.
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Can an adiabatic process result in no work done?
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Yes, any isolated process will be adiabatic and result in no work. One example is the
spontaneous mixing of two materials in an isolated container. |
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What are the assumptions of kinetic theory?
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Kinetic theory assumes that all energy is kinetic (i.e., there are no potentials between molecules).
It implicitly assumes that collisions are elastic and that the particles occupy essentially no volume. |
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What does kinetic theory predict correctly?
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The internal energy and equation‐of‐state for ideal gases, and the root‐mean‐square
translational speed. |
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What does kinetic theory predict incorrectly?
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The distribution of speeds, which requires a statistical approach to obtain
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Explain the concept of ʺdegree‐of‐freedomʺ.
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A degree‐of‐freedom is an independent mode of energy for a molecule. For instance, for
translational motion, the kinetic energy consists of three independent terms. Thus, there are three degrees of translational freedom. |
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Is ΔU path‐independent?
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Yes, internal energy is a state variable
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Is W path‐independent?
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Certainly not! One counterexample is a cycle, for which how one returns to the initial point
determines W. |
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Is Q path‐independent?
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No, see (13) for the counterexample.
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On a PV diagram, can one always calculate ΔU, Q, and W by summing up the values for
individual parts of the path? |
Yes, one can break up the path into smaller pieces for which ΔU, Q, and W may be easier to
calculate. |
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What is ʺheat capacityʺ?
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Heat capacity measures the ability of a material to absorb heat for a given change in
temperature. |
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Do polymers have large or small heat capacities?
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Large, since they have so many degrees‐of‐freedom that can store energy.
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What does knowing heat capacities allow you to calculate?
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Obviously, you can find the heat added during a process, but itʹs more than that. You can use
heat capacities to find U, W, and Q for an arbitrary process once you know the P‐V path. |
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What is a viscoelastic solid?
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For the purpose of our class, a viscoelastic solid is one that displays time‐dependent behavior to
an imposed stress. |
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What is characteristic of the stress‐strain plot of a viscoelastic solid?
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ʺHysteresisʺ, or a different stress-strain curve for expansion and contraction.
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Which stress‐strain plot will generate heat upon cycling?
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The one in which a higher stress is required during stretch, as expected in real tissue
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What is the Second Law of Thermodynamics?
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There are many ways to state it. In class, we use this form: The entropy of an isolated system
can never decrease. Another form (used in mechanical engineering) is that heat cannot be completely converted into work in a cycle. |
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Can a spontaneous process result in lower entropy?
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Yes, entropy can decrease in a spontaneous process, but only if the process occurs in a non‐
isolated system. Otherwise, it would violate the 2nd Law. |
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Why is entropy a sum, rather than a product, of individual entropies for each degree‐of‐
freedom? |
The number of states is multiplicative. Since entropy is proportional to the logarithm of the
number of states, it is additive. |
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Does entropy increase when you mix two things?
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Not always. For instance, if you force oil and water to mix, the entropy will decrease (which is
why oil and water will spontaneously segregate). |
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Why do energies redistribute?
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To increase the entropy of the system, as the Second Law tells us.
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Why do energies reach a stable distribution?
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The stable distribution must be the one with maximum entropy, again according to the Second
Law. |
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What is the physical significance of the Boltzmann factor e^-Ei/kT?
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It is proportional to the probability of a specified energy level.
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Does the Boltzmann factor apply only to ideal gases?
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No, it applies to all systems
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Does the Boltzmann factor describe the distribution of other parameters besides energy?
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No, it applies only to energy.
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Does the probability pi of a state equal the Boltzmann factor for that state?
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The Boltzmann factor is proportional, but not equal, to the probability. The proportionality
constant is the partition function. |
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Does pi depend on the zero set‐point for the energy levels?
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No, the probabilities depend only on the differences between energy levels
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Does the ground state always have zero energy?
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No, the energy of the ground state is arbitrarily defined. In many cases, it is easier to set this
energy at zero, but not always. For instance, when analyzing the interactions between oppositely‐charged objects, the zero energy state is the one with the highest energy (when the charges are infinitely far apart). |
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Is there only one ground state?
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Not necessarily, there can be many ground states (e.g., a molecule rotating freely).
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True or false: There are more molecules in the lower half of a room.
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True, gravity will cause molecules to accumulate ever so slightly near the floor
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Consider a molecule with three states of energy 0, , and (here, is positive). What is the
average energy at low T? |
That of the ground state (zero).
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Consider a molecule with three states of energy 0, , and (here, is positive). What is the
average energy at high T? |
All states are equally likely, so the average energy is the average of 0, , and (i.e., 2/3).
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What is the significance of kT?
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It gives you a rough idea of the thermal energy of a molecule. Interactions that have energies
smaller than kT can be broken easily. |
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Can pi be greater than one?
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Of course not, probabilities are never greater than one!
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Can the probability density (x) be greater than one?
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Yes, the only requirement is that the integral of probability density cannot exceed one
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Consider a gas that escapes from one compartment into another through a hole. Does the
internal energy increase, decrease, or stay the same for this process? |
It stays the same, assuming the containeris rigid and insulated
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Consider a gas that escapes from one compartment into another through a hole. Does the
entropy increase, decrease, or stay the same for this process? |
The entropy must increase, since the process occurs spontaneously
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Now consider the case where the gas expands isothermally against a piston to the same final
volume. Which process leads to a greater change in entropy? |
The change in entropy is path‐independent. Since the initial and final states are the same in the
two cases, the change in entropy must be identical. |