/*! 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 75 Average of six points Let \(\p... [FREE SOLUTION] | 91Ó°ÊÓ

91Ó°ÊÓ

Average of six points Let \(\phi(x, y, z)\) be any function that can be expanded in a power series around a point \(\left(x_{0}, y_{0}, z_{0}\right)\). Write a Taylor series expansion for the value of \(\phi\) at each of the six points \(\left(x_{0}+\delta, y_{0}, z_{0}\right),\left(x_{0}-\right.\) \(\left.\delta, y_{0}, z_{0}\right),\left(x_{0}, y_{0}+\delta, z_{0}\right),\left(x_{0}, y_{0}-\delta, z_{0}\right),\left(x_{0}, y_{0}, z_{0}+\delta\right),\left(x_{0}, y_{0},\right.\), \(z_{0}-\delta\) ), which symmetrically surround the point \(\left(x_{0}, y_{0}, z_{0}\right)\) at a distance \(\delta\). Show that, if \(\phi\) satisfies Laplace's equation, the average of these six values is equal to \(\left(x_{0}, y_{0}, z_{0}\right)\) through terms of the third order in \(\delta\).

Short Answer

Expert verified
\(\phi(x_0, y_0, z_0)\) is equal to the average of the function \(\phi\) valued at the six points lying symmetrically around \((x_0, y_0, z_0)\) with distance \(\delta\), up to terms of the third order in delta as per the given condition that \(\phi\) satisfies Laplace's equation.

Step by step solution

01

Write Out the Taylor Series Expansion for each point

By applying the Taylor series expansion formula, the value of \(\phi\) at each of the indicated six points can be represented as follows: \(\phi(x_0+\delta, y_0, z_0)=\phi(x_0, y_0, z_0)+\delta\phi'_x(x_0, y_0, z_0)+(1/ 2)\delta^2\phi''_{xx}(x_0, y_0, z_0)\). Similarly, write out the corresponding series for the other five points.
02

Calculate the Average of the Six Series Expansions

Following from step 1, proceed by adding all six series expansions from each of the points and divide by six to get the average. This will yield an expression in terms of \(\phi(x_0, y_0, z_0)\) and its derivatives, multiplied by factors of \(\delta\).
03

Apply Laplace's Equation

Laplace’s equation states: \(\phi''_{xx}+\phi''_{yy}+\phi''_{zz}=0\). Applying this, many terms will cancel each other. This results in an expression with terms of \(\delta\) up to the third order.
04

Compare with \(\phi(x_0, y_0, z_0)\)

Finally, compare the resulting expression with \(\phi(x_0, y_0, z_0)\) as per the problem context. We can safely ignore higher order terms of \(\delta\) to conclude that the average of the values at the six points equals \(\phi(x_0, y_0, z_0)\) up to the third order in \(\delta\).

Unlock Step-by-Step Solutions & Ace Your Exams!

  • Full Textbook Solutions

    Get detailed explanations and key concepts

  • Unlimited Al creation

    Al flashcards, explanations, exams and more...

  • Ads-free access

    To over 500 millions flashcards

  • Money-back guarantee

    We refund you if you fail your exam.

Over 30 million students worldwide already upgrade their learning with 91Ó°ÊÓ!

Key Concepts

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

Laplace's Equation
Laplace's equation is a second-order partial differential equation often encountered in physics and engineering, particularly in the fields of electrostatics and fluid dynamics. It is given by the formula \[ abla^2 \phi = \frac{\partial^2 \phi}{\partial x^2} + \frac{\partial^2 \phi}{\partial y^2} + \frac{\partial^2 \phi}{\partial z^2} = 0 \], where \( \phi \) represents a scalar potential, and \( abla^2 \) denotes the Laplacian operator.

In the context of electrostatics, Laplace's equation describes the behavior of the electric potential in a charge-free region of space. A fundamental property is that solutions to Laplace's equation are harmonic functions, meaning they're infinitely differentiable and have a Taylor series expansion. This harmonicity ensures that, under certain conditions, the value of the potential at any point within a region is the average of the potential around that point on any small surrounding surface.

Power Series Expansion
Power series expansion is a mathematical technique used to express a function as an infinite sum of terms powered by the function's variables. In other words, it is a way of approximating functions using polynomials of increasing degree. The general form of a power series centered at a point \( a \) is given by \[ f(x) = \sum_{n=0}^{\infty} c_n (x-a)^n \], where \( c_n \) are the coefficients.

When applied to Taylor series, each coefficient is determined by the function's derivatives at point \( a \). The power series expansion has extensive applications in physics, especially when exploring complex problems such as the behavior of electrostatic potential. It allows the approximation of potentials and fields in situations where exact solutions are difficult to find.

Partial Derivatives in Physics
Partial derivatives play a crucial role in physics when dealing with functions that depend on multiple variables. They measure how a function changes as one of the variables changes, holding other variables constant. The notation \( \frac{\partial \phi}{\partial x} \) represents the partial derivative of \( \phi \) with respect to \( x \).

In the realm of electrodynamics, partial derivatives can help determine how the electrostatic potential changes with position. They are used to compute gradients, divergences, and curls of vector fields, all of which are essential in understanding electric and magnetic field behaviors. Partial derivatives are also integral components of the Taylor series expansions used to approximate the values of physical quantities near a point of interest.

Electrostatic Potential
Electrostatic potential, represented as \( \phi \), describes the potential energy per unit charge due to a static electric field. It is a scalar quantity that provides insightful information about the electric field structure. The electrostatic potential at a point in space can be calculated by integrating the electric field along a path to that point.

Understandably, a charged particle moving in an electric field will experience a change in its electrostatic potential energy. Regions where the potential is constant are called equipotential surfaces. By leveraging the power series expansion, one can approximate the potential in the vicinity of a point, allowing for an understanding of the electric field in that region without solving the entire field distribution.

One App. One Place for Learning.

All the tools & learning materials you need for study success - in one app.

Get started for free

Most popular questions from this chapter

Maximum field from a passing charge In a colliding beam storage ring an antiproton going east passes a proton going west, the distance of closest approach being \(10^{-10} \mathrm{~m}\). The kinetic energy of each particle in the lab frame is \(93 \mathrm{GeV}\), corresponding to \(\gamma=100\). In the rest frame of the proton, what is the maximum intensity of the electric field at the proton due to the charge on the antiproton? For about how long, approximately, does the field exceed half its maximum intensity?

Maximum energy storage between cylinders ** We want to design a cylindrical vacuum capacitor, with a given radius \(a\) for the outer cylindrical shell, that will be able to store the greatest amount of electrical energy per unit length, subject to the constraint that the electric field strength at the surface of the inner cylinder may not exceed \(E_{0}\). What radius \(b\) should be chosen for the inner cylindrical conductor, and how much energy can be stored per unit length?

This exercise is more of a math problem than a physics problem, so maybe it doesn't belong in this book. But it's a fun one. Consider a series of events that happen at independent random times, such as the collisions in Section \(4.4\) that led to the issue discussed in Footnote \(8 .\) Such a process can be completely characterized by the probability per unit time (call it \(p\) ) of an event happening. The definition of \(p\) is that the probability of an event happening in an infinitesimal \(^{16}\) time \(d t\) equals \(p d t\). (a) Show that starting at any particular time (not necessarily the time of an event), the probability that the next event happens between \(t\) and \(t+d t\) later equals \(e^{-p t} p d t\). You can do this by breaking the time interval \(t\) into a large number of tiny intervals, and demanding that the event does not happen in any of them, but that it does happen in the following \(d t\). (You will need to use the fact that \((1-x / N)^{N}=e^{-x}\) in the \(N \rightarrow \infty\) limit.) Verify that the integral of \(e^{-p t} p d t\) correctly equals 1 . (b) Show that starting at any particular time (not necessarily the time of an event), the average waiting time (also called the expectation value of the waiting time) to the next event equals \(1 / p\). Explain why this is also the average time between events. (c) Pick a random point in time, and look at the length of the time interval (between successive events) that it belongs to. Explain, using the above results, why the average length of this interval is \(2 / p\), and not \(1 / p\). (d) We have found that the average time between events is \(1 / p\), and also that the average length of the interval surrounding a randomly chosen point in time is \(2 / p .\) Someone might think that these two results should be the same. Explain intuitively why they are not. (e) Using the above probability distribution \(e^{-p t} p d t\) properly, show mathematically why \(2 / p\) is the correct result for the average length of the interval surrounding a randomly chosen point in time.

Maximum power * Show that if a battery of fixed emf \(\mathcal{E}\) and internal resistance \(R_{i}\) is connected to a variable external resistance \(R\), the maximum power is delivered to the external resistor when \(R=R_{i}\).

Electron beam A beam of \(9.5\) megaelectron-volt (MeV) electrons \((\gamma \approx 20)\), amounting as current to \(0.05 \mu \mathrm{A}\), is traveling through vacuum. The transverse dimensions of the beam are less than \(1 \mathrm{~mm}\), and there are no positive charges in or near it. (a) In the lab frame, what is the average distance between an, electron and the next one ahead of it, measured parallel to the beam? What approximately is the average electric field strength 1 cm away from the beam? (b) Answer the same questions for the electron rest frame.

See all solutions

Recommended explanations on Physics Textbooks

View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Study anywhere. Anytime. Across all devices.