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Silver has a Fermi energy of \(5.48 \mathrm{eV}\). Calculate the electron contribution to the molar heat capacity at constant volume of silver, \(C_{V},\) at \(300 \mathrm{~K}\). Express your result (a) as a multiple of \(R\) and (b) as a fraction of the actual value for silver, \(C_{V}=25.3 \mathrm{~J} / \mathrm{mol} \cdot \mathrm{K}\). (c) Is the value of \(C_{V}\) due principally to the electrons? If not, to what is it due? (Hint: See Section \(18.4 .)\)

Short Answer

Expert verified
Using the steps above, we can calculate the heat capacity contribution, \(C_V\), of electrons in silver. Firstly, the Fermi energy is converted from eV to J. Then, this energy is used to calculate the heat capacity at constant volume. This value is compared to the known value for silver to determine the contribution of the electrons, as well as converted to multiples of R. Finally, if \(C_V\) value is much less than the actual value for silver, it indicates that the heat capacity is most likely due to other processes, such as vibrations of the atoms within the material, rather than the electrons.

Step by step solution

01

Calculating Fermi Energy in joules

First, the Fermi energy is given in eV, we need to convert it to joules. The conversion factor for eV to joules is \(1.6 × 10^{-19} \mathrm{J/eV}\). The Fermi energy \(E_f\) in joules would be: \(E_f = 5.48 \mathrm{eV} \times 1.6 × 10^{-19} \mathrm{J/eV} = 8.77 × 10^{-19} \mathrm{J}\)
02

Calculating the electron contribution to the molar heat capacity

The electron contribution to the molar heat capacity (\(c_v\)) can be approximately given by \(c_v \approx \pi^2 k T / 2 E_f\), where \(k\) is the Boltzmann constant \((1.38 x 10^{-23} J/K)\) and \(T\) is absolute temperature. Insert the values into formula: \(c_v \approx \pi^2 × 1.38 × 10^{-23} \mathrm{J/K} × 300 \mathrm{K} / 2 × 8.77 × 10^{-19} \mathrm{J}\)
03

Convert \(c_V\) to multiples of \(R\)

Next, convert the result to multiples of \(R\) (the molar gas constant), \(R = k \times Avogadro's \; number = 8.314 \mathrm{J/K/mol}\). Therefore, the molar heat capacity in terms of \(R\) would be \(c_v / R\)
04

Calculate the fraction of the actual value for silver

Now, we shall calculate the percentage of the actual silver (\(c_v = 25.5 \mathrm{J / mol K}\)). This can be done by dividing our calculated heat capacity by the given silver heat capacity and multiplying by 100: \((c_v / 25.5 \mathrm{J / mol K}) \times 100\)
05

Determine if \(c_V\) is due principally to the electrons

Compare the calculated heat capacity with that of silver. If they are comparable then the capacity can be attributed to the electrons, otherwise it may be due to other degrees of freedom such as vibrations of atoms (phonons).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Molar Heat Capacity
The molar heat capacity is an important concept in thermodynamics as it represents the amount of heat required to raise the temperature of one mole of a substance by one Kelvin at constant volume. In the context of the exercise, this specifically refers to the electron contribution within metallic silver, such as silver's unique conductive properties at a microscopic level.
Understanding this concept helps us explore how electrons, the primary carriers of charge in metals, contribute to a material's overall thermal capacity. Typically, molar heat capacity is measured in energy units per mole per Kelvin, making it easier for scientists to experiment with and compare different materials.
Electron Contribution
Electrons play a key role in the thermal properties of metals. When we calculate the electron contribution to heat capacity, we acknowledge that electrons are not the only contributors. However, they do provide crucial insights into the behavior of metals like silver.
In metals, electrons move freely, transporting both energy and charge. At lower temperatures, their contribution to heat capacity is minor compared to other factors. Yet, as temperature increases, electrons' interaction with thermal energy becomes more pronounced, allowing us to calculate their specific contribution. This is often modeled using Fermi energy, a fundamental concept in quantum mechanics.
Fermi Energy Conversion
Fermi energy is a vital concept that illustrates the highest energy level that electrons can occupy at absolute zero temperature. In metallic elements, it's crucial for understanding electron behavior.
In the exercise, the Fermi energy of silver is initially given in electron volts (eV) and needs conversion to joules for practical thermal calculations. This conversion is achieved by using the factor: \[1 \, \text{eV} = 1.6 \times 10^{-19} \, \text{J}\]Understanding this conversion makes it easier to further calculate contributions to properties like heat capacity, where precise energy measurements are essential.
Boltzmann Constant
The Boltzmann constant bridges microscopic and macroscopic physics by relating temperature to energy with the equation \( k = 1.38 \times 10^{-23} \, \text{J/K} \).
This constant is crucial in statistical mechanics, providing the necessary framework to calculate physical phenomena like heat capacity.
It appears in the equation to calculate the electron contribution to molar heat capacity, linking thermal energy (dependent on temperature) to characteristics like Fermi energy. Essentially, it helps us break down complex formulas into comprehensible physical phenomena.
Phonons
Phonons are quasiparticles representing quantized lattice vibrations in a crystal structure. They play a significant role in heat capacity, especially in materials like silver where atomic vibrations dominate thermal properties.
Unlike electron contributions, which result from charge carriers, phonons link to atomic vibrations, often overwhelmingly so at room temperature. As phonon interactions escalate, their contribution to thermal and electrical conductivity supersedes that of electrons. In contexts like the exercise, understanding these contributions explains why electron contribution might be overshadowed by phononic activities.
Constant Volume Heat Capacity
Constant volume heat capacity is the measure of how much heat energy is needed to change a system's temperature with no change in volume. This concept is crucial in scenarios where gases or substances remain enclosed, preventing expansion or contraction.
Within solid materials like silver, understanding this property helps quantify heat transfer dynamics within a material's structure. By isolating heat capacity calculations to constant volume, we negate external influences such as pressure variations, focusing solely on intrinsic thermal properties. This precision aids in detailed thermal assessments, crucial in material science and thermodynamics.

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Most popular questions from this chapter

The \(\mathrm{H}_{2}\) molecule has a moment of inertia of \(4.6 \times 10^{-48} \mathrm{~kg} \cdot \mathrm{m}^{2} .\) What is the wavelength \(\lambda\) of the photon absorbed when \(\mathrm{H}_{2}\) makes a transition from the \(l=3\) to the \(l=4\) rotational level?

(a) Suppose a piece of very pure germanium is to be used as a light detector by observing, through the absorption of photons, the increase in conductivity resulting from generation of electron-hole pairs. If each pair requires \(0.67 \mathrm{eV}\) of energy, what is the maximum wavelength that can be detected? In what portion of the spectrum does it lie? (b) What are the answers to part (a) if the material is silicon, with an energy requirement of \(1.12 \mathrm{eV}\) per pair, corresponding to the gap between valence and conduction bands in that element?

Consider a system of \(N\) free electrons within a volume \(V\). Even at absolute zero, such a system exerts a pressure \(p\) on its surroundings due to the motion of the electrons. To calculate this pressure, imagine that the volume increases by a small amount \(d V\). The electrons will do an amount of work \(p d V\) on their surroundings, which means that the total energy \(E_{\text {tot }}\) of the electrons will change by an amount \(d E_{\mathrm{tot}}=-p d V .\) Hence \(p=-d E_{\mathrm{tot}} / d V\) (a) Show that the pressure of the electrons at absolute zero is $$ p=\frac{3^{2 / 3} \pi^{4 / 3} \hbar^{2}}{5 m}\left(\frac{N}{V}\right)^{5 / 3} $$ (b) Evaluate this pressure for copper, which has a free-electron concentration of \(8.45 \times 10^{28} \mathrm{~m}^{-3} .\) Express your result in pascals and in atmospheres. (c) The pressure you found in part (b) is extremely high. Why, then, don't the electrons in a piece of copper simply explode out of the metal?

At a temperature of \(290 \mathrm{~K},\) a certain \(p-n\) junction has a saturation current \(I_{\mathrm{S}}=0.500 \mathrm{~mA}\). (a) Find the current at this temperature when the voltage is (i) \(1.00 \mathrm{mV}\), (ii) \(-1.00 \mathrm{mV}\), (iii) \(100 \mathrm{mV}\), and (iv) \(-100 \mathrm{mV}\). (b) Is there a region of applied voltage where the diode obeys Ohm's law?

(a) Calculate the electric potential energy for a \(\mathrm{K}^{+}\) ion and a \(\mathrm{Br}^{-}\) ion separated by a distance of \(0.29 \mathrm{nm},\) the equilibrium separation in the KBr molecule. Treat the ions as point charges. (b) The ionization energy of the potassium atom is \(4.3 \mathrm{eV}\). Atomic bromine has an electron affinity of \(3.5 \mathrm{eV}\). Use these data and the results of part (a) to estimate the binding energy of the KBr molecule. Do you expect the actual binding energy to be higher or lower than your estimate? Explain your reasoning.

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