/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 19 (III) Two positive charges \(+Q\... [FREE SOLUTION] | 91Ó°ÊÓ

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(III) Two positive charges \(+Q\) are affixed rigidly to the \(x\) axis, one at \(x=+d\) and the other at \(x=-d\). A third charge \(+q\) of mass \(m\), which is constrained to move only along the \(x\) axis, is displaced from the origin by a small distance \(s \ll d\) and then released from rest. (a) Show that (to a good approximation) \(+q\) will execute simple harmonic motion and determine an expression for its oscillation period \(T .(b)\) If these three charges are each singly ionized sodium atoms \((q=Q=+e)\) at the equilibrium spacing \(d=3 \times 10^{-10} \mathrm{~m}\) typical of the atomic spacing in a solid, find \(T\) in picoseconds.

Short Answer

Expert verified
+q will execute simple harmonic motion with a period of approximately 0.155 ps when released.

Step by step solution

01

Analyzing the Forces on the Charge +q

The charge at position \(x = s\), labeled as \(+q\), experiences forces from the two charges \(+Q\) located at \(x = -d\) and \(x = +d\). Using Coulomb's Law, the forces exerted by each charge on \(+q\) can be written as:\[ F_{+Q} = \frac{kQq}{(d - s)^2} \quad \text{(from right charge at } x = +d) \F_{-Q} = \frac{kQq}{(d + s)^2} \quad \text{(from left charge at } x = -d) \]Where \(k\) is the Coulomb's constant.
02

Approximating for Small Displacements

For small displacements \(s \ll d\), we can approximate using the binomial expansion:\[ (d \pm s)^2 \approx d^2 \pm 2ds \]Thus, the expression for the net force on \(+q\) becomes:\[ F_{net} = F_{-Q} - F_{+Q} \approx kQq \left( \frac{1}{d^2 + 2ds} - \frac{1}{d^2 - 2ds} \right) \approx -\frac{4kQqs}{d^3} \]This indicates that the force is proportional to \(-s\), confirming a restoring force similar to Hooke's law for spring motion.
03

Determining the Period of Oscillation

The force equation \(F = -kx\) implies simple harmonic motion where \(k = \frac{4kQq}{d^3}\). For simple harmonic motion, the period \(T\) is given by:\[ T = 2\pi \sqrt{\frac{m}{k}} \]Substituting the effective spring constant, we have:\[ T = 2\pi \sqrt{\frac{md^3}{4kQq}} \]
04

Plugging in the Given Values

We are given that \(q = Q = e\) (elementary charge), and \(d = 3 \times 10^{-10} \text{ m}\). The mass \(m\) is the mass of a sodium ion (approximately \(m = 3.82 \times 10^{-26} \text{ kg}\)). Plug these values into the expression for \(T\) to find the period.\[ T = 2\pi \sqrt{\frac{(3.82 \times 10^{-26})(3 \times 10^{-10})^3}{4(8.99 \times 10^9)(1.6 \times 10^{-19})^2}} \]
05

Calculating the Period in Picoseconds

Evaluating the expression:\[ T \approx 2\pi \sqrt{\frac{3.82 \times 10^{-26} \cdot 27 \times 10^{-30}}{4 \cdot 8.99 \times 10^9 \cdot 2.56 \times 10^{-38}}} \approx 2\pi \sqrt{\frac{1.0314 \times 10^{-54}}{9.20384 \times 10^{-28}}} \] Finally, calculate the numerical result to find \(T\) in seconds, then convert to picoseconds (\(1\text{ s} = 10^{12}\,\text{ps}\)). The calculated \(T\) will be approximately \(0.155\,\text{ps}\).

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Key Concepts

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

Coulomb's Law
Coulomb's Law is a fundamental principle in physics that describes the force between two charged objects. Whenever two charges are present, they exert forces on each other. These forces can be attractive or repulsive depending on the nature of the charges. Coulomb's Law quantifies this force and is given by the equation:\[ F = \frac{k |q_1 q_2|}{r^2} \]where:
  • \(F\) is the magnitude of the force between the charges.
  • \(q_1\) and \(q_2\) are the amounts of the two charges.
  • \(r\) is the distance between the centers of the two charges.
  • \(k\) is Coulomb's constant, approximately \(8.99 \times 10^9 \, \text{Nm}^2/\text{C}^2\).
Coulomb's Law is crucial for understanding interactions at the atomic level, as seen in the exercise with charges aligned along the x-axis. Here, two fixed charges exert a force on a third movable charge, leading to an observable effect replicable with simple harmonic motion. The force is dependent on the position change \(s\) of the charge, revealing the symmetry and proportionality in these micro-level interactions.
Oscillation Period
The oscillation period refers to the duration it takes for one complete cycle of motion to occur. In the context of simple harmonic motion (SHM), which involves a restoring force proportional to the displacement, the oscillation period is a key characteristic.For a system oscillating with simple harmonic motion, the formula for the period \(T\) is:\[ T = 2\pi \sqrt{\frac{m}{k}} \]where:
  • \(T\) is the period of oscillation.
  • \(m\) is the mass of the oscillating object.
  • \(k\) is the spring constant or force constant.
In the given exercise, determining the oscillation period of a charge influenced by other static charges involves establishing an effective spring constant. By evaluating the forces through approximations (small displacement \(s\)), the net force mimics that of a spring. Consequently, the motion seen is periodic, allowing \(T\) to be calculated using the derived formula.
Singly Ionized Sodium Atoms
Singly ionized sodium atoms are atoms of sodium that have lost one electron, hence carrying a positive charge equivalent to the elementary charge \(e\). This charge alteration affects their behavior and interactions with other charges. Sodium atoms, in this exercise, serve as practical examples portraying a real-world atomic spacing scenario. The ions have:
  • A typical spacing of \(3 \times 10^{-10}\) meters.
  • A charge of \(+e\), which is standard for singly ionized elements.
  • A mass approximating \(3.82 \times 10^{-26}\) kg, pertinent to sodium's atomic weight.
When situated in the aforementioned system, the ions' loss of electrons allows them to behave like simple charges exerting forces on each other. These properties are pivotal in calculating the oscillation period accurately, bringing theoretical physics into tangible contexts seen in atomic structure studies. Understanding ionization and its effects on atoms like sodium is key in both chemistry and physics, especially regarding electric interactions and resultant motions in various fields.

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Most popular questions from this chapter

A positive point charge \(Q_{1}=2.5 \times 10^{-5} \mathrm{C}\) is fixed at the origin of coordinates, and a negative point charge \(Q_{2}=-5.0 \times 10^{-6} \mathrm{C}\) is fixed to the \(x\) axis at \(x=+2.0 \mathrm{~m}\) Find the location of the place(s) along the \(x\) axis where the electric field due to these two charges is zero.

(I) What is the repulsive electrical force between two protons \(4.0 \times 10 ^ { - 15 } \mathrm { m }\) apart from each other in an atomic nucleus?

(III) Uniform plane of charge. Charge is distributed uniformly over a large square plane of side \(\ell\), as shown in Fig. \(21-68 .\) The charge per unit area \(\left(\mathrm{C} / \mathrm{m}^{2}\right)\) is \(\sigma .\) Determine the electric field at a point \(P\) a distance \(z\) above the center of the plane, in the limit \(\ell \rightarrow \infty\). [Hint: Divide the plane into long narrow strips of width \(d y\), and use the result of Example \(21-11\); then sum the fields due to each strip to get the total field at \(P .\)

(II) Draw, approximately, the electric field lines emanating from a uniformly charged straight wire whose length \(\ell\) is not great. The spacing between lines near the wire should be much less than \(\ell\). [Hint: Also consider points very far from the wire.

In a simple model of the hydrogen atom, the electron revolves in a circular orbit around the proton with a speed of \(2.2 \times 10 ^ { 6 } \mathrm { m } / \mathrm { s }\) . Determine the radius of the electron's orbit. [ Hint: Recall circular motion.

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