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(II) The neutrons in a neutron star can be treated as a Fermi gas with neutrons in place of the electrons in our model of an electron gas. Determine the Fermi energy for a neutron star of radius 12 \(\mathrm{km}\) and mass 2.5 times that of our Sun. Assume that the star is made entirely of neutrons and is of uniform density.

Short Answer

Expert verified
The Fermi energy is calculated using the properties of the neutron star and constants.

Step by step solution

01

Gather Constants and Parameters

First, gather the necessary constants for this problem. The mass of the Sun, denoted as \(M_\odot\), is approximately \(2 \times 10^{30} \text{ kg}\). The radius of the neutron star is given as 12 km, which is \(12,000 \text{ m}\). The mass of the neutron star is \(2.5 \times M_\odot\).
02

Calculate the Volume of the Neutron Star

Calculate the volume \(V\) of the neutron star assuming it is spherical. Use the formula for the volume of a sphere: \[ V = \frac{4}{3} \pi R^3 \] where \(R = 12,000 \) m. Substitute to find \(V\).
03

Determine the Density of the Neutron Star

Next, determine the density \(\rho\) of the neutron star using the formula: \[ \rho = \frac{\text{mass of the neutron star}}{\text{volume of the neutron star}} \] where the mass of the neutron star is \(2.5 \times 2 \times 10^{30} \text{ kg}\).
04

Find the Number of Neutrons

To find the number of neutrons \(N\), use the formula:\[ N = \frac{\text{mass of the neutron star}}{m_n} \] where \(m_n\) is the mass of a neutron, approximately \(1.675 \times 10^{-27} \text{ kg}\).
05

Determine the Fermi Energy

Use the formula for the Fermi energy \(E_F\) for a completely degenerate Fermi gas:\[ E_F = \frac{\hbar^2}{2m_n} \left( 3\pi^2 \frac{N}{V} \right)^{\frac{2}{3}} \] where \(\hbar\) is the reduced Planck's constant, approximately \(1.054 \times 10^{-34} \text{ J}\cdot\text{s}\). Substitute the values of \(N\), \(V\), and \(m_n\) to find \(E_F\).

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Key Concepts

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

Neutron Star Physics
Neutron stars are fascinating celestial objects formed from the collapsed core of massive stars after a supernova explosion. They are incredibly dense, with masses up to that of several Suns compressed into a sphere of just around 12 kilometers in diameter.
A significant aspect of neutron stars is that they are predominantly composed of neutrons, as the extreme pressure forces electrons and protons to combine into neutrons. This dense composition gives rise to their immeasurable gravitational pull and compactness.
Observing and studying neutron stars helps scientists understand the extreme realms of physics. The intense gravitational fields allow the study of general relativity in ways that are impossible on Earth. Additionally, neutron stars may play a role in the creation of heavy elements in the universe, which is still an area of active research.
Fermi Gas Model
The Fermi gas model is an important concept in understanding the behavior of particles such as neutrons within neutron stars. In this model, particles are treated as a gas but without interactions between them, acting according to Fermi-Dirac statistics.
When dealing with a neutron star, we replace electrons, typically used in a classical electron gas model, with neutrons. At extremely low temperatures, neutrons settle into the lowest available energy states, filling these states up to the energy called the Fermi energy, denoted as \(E_F\).
The Fermi energy is a critical parameter, representing the energy of the highest occupied state at zero temperature. It gives insights into the quantum mechanical properties of the gas and helps predict how the neutron star would behave under various conditions. By calculating the Fermi energy, scientists can comprehend the stability and structure of such stars under extreme pressures and densities.
Uniform Density Assumption
The uniform density assumption is key when simplifying the complex calculations associated with objects like neutron stars. This assumption posits that the density of the particle distribution is constant throughout the object.
In the case of neutron stars, assuming a uniform density facilitates the computation of the volume and mass distribution, enabling easier application of mathematical models. With this approximation, we can assess properties like the Fermi energy by treating the neutron star as a single spherical body.
Although real neutron stars may not be perfectly uniform in density due to variations and instabilities, this simplification provides a good starting point for calculations. It helps focus on understanding other influencing factors without the complexities of variable density, thus making it a valuable tool in theoretical astrophysics research.

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Most popular questions from this chapter

(II) An amplifier has a voltage gain of 65 and a \(25-\mathrm{k} \Omega\) load (output) resistance. What is the peak output current through the load resistor if the input voltage is an ac signal with a peak of 0.080 \(\mathrm{V} ?\)

(1I) For a one-dimensional potential well of width \(\ell,\) start with \(E=n^{2} \frac{h^{2}}{8 m \ell^{2}}, \quad n=1,2,3, \cdots\) and show that the number of states per unit energy interval for an electron gas is given by \(g_{\ell}(E)=\sqrt{\frac{8 m \ell^{2}}{h^{2} E}}\) Remember that there can be two electrons (spin up and spin down) for each value of \(n .[\) Hint. Write the quantum number \(n\) in terms of \(E .\) Then \(g_{t}(E)=2 d n / d E\) where \(d n\) is the number of energy levels between \(E\) and \(E+d E . ]\)

(I) What is the reduced mass of the molecules \((a) \mathrm{KCl}\) ; (b) \(\mathrm{O}_{2} ;(c) \mathrm{HCl}\) ?

(II) A very simple model of a "one-dimensional" metal consists of \(N\) electrons confined to a rigid box of width \(\ell\) We neglect the Coulomb interaction between the electrons. (a) Calculate the Fermi energy for this one-dimensional metal \(\left(E_{\mathrm{F}}=\) the energy of the most energetic electron at \right. \(T=0 \mathrm{K}\) , taking into account the Pauli exclusion principle. You can assume for simplicity that \(N\) is even. (b) What is the smallest amount of energy, \(\Delta E,\) that this \(1-\mathrm{D}\) metal can absorb? (c) Find the limit of \(\Delta E / E_{\mathrm{F}}\) for large \(N .\) What does this result say about how well metals can conduct?

At what wavelength will an LED radiate if made from a material with an energy gap \(E_{\mathrm{g}}=1.6 \mathrm{eV} ?\)

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