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An electron is moving at a speed \(0.01 c\) on a circular orbit of radius \(10^{-8} \mathrm{~cm} .\) What is the strength of the resulting magnetic field at the center of the orbit? (The numbers given are typical, in order of magnitude, for an electron in an atom.)

Short Answer

Expert verified
The strength of the magnetic field at the center is approximately \(-4.8 \times 10^{-5}\) T.

Step by step solution

01

Understand the Context

The problem involves an electron moving in a circular orbit, which creates a current loop, thereby generating a magnetic field at the center of the orbit.
02

Recognize the Relevant Formula

The magnetic field at the center of a circular loop can be given by the Biot-Savart Law as \( B = \frac{{\mu_0 I}}{2r} \), where \( I \) is the current and \( r \) is the radius of the loop.
03

Calculate the Current

The current \( I \) caused by the electron can be calculated as \( I = \frac{q}{T} \), where \( q \) is the charge of the electron \( q = -1.6 \times 10^{-19} \) C and \( T \) is the period, with \( T = \frac{2\pi r}{v} \).
04

Substitute Known Values

Substitute the values in the formula for current: \( I = \frac{-1.6 \times 10^{-19}}{\frac{2\pi \times 10^{-10}}{0.01 \times 3 \times 10^8}} \).
05

Simplify the Current Formula

Calculate: \( I = \frac{-1.6 \times 10^{-19} \times 0.01 \times 3 \times 10^8}{\pi \times 2 \times 10^{-10}} = \frac{-4.8 \times 10^{-10}}{\pi \times 2 \times 10^{-10}} \approx -0.764 \times 10^{-3} \) A.
06

Use the Magnetic Field Formula

Insert \( I \) and \( r \) into the magnetic field equation: \( B = \frac{\mu_0 \times (-0.764 \times 10^{-3})}{2 \times 10^{-10}} \), where \( \mu_0 = 4\pi \times 10^{-7} \).
07

Calculate the Magnetic Field

Calculate \( B = \left(4\pi \times 10^{-7}\right) \times \frac{-0.764 \times 10^{-3}}{2 \times 10^{-10}} = -4.8 \times 10^{-5} \) T. The negative sign indicates the direction of the magnetic field.

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Key Concepts

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

Biot-Savart Law
The Biot-Savart Law is a fundamental principle used to calculate the magnetic field generated by a current-carrying conductor. It states that the magnetic field (\( B \)) at a point in space due to a small segment of current (\( I \)) is directly proportional to the current, inversely proportional to the square of the distance from the segment to the point, and depends on the angle between the current direction and the line connecting the segment to the point.

The equation is given by:\[B = \frac{{\mu_0 I}}{2r}\]where:
  • \( \mu_0 \) is the permeability of free space (\( 4\pi \times 10^{-7} \) T·m/A).
  • \( I \) is the current through the loop, and \( r \) is the radius of the loop.
In our exercise, this law helps determine the magnetic field created at the center of a circular loop by a moving electron. By effectively applying this principle, we can calculate the exact value of the magnetic field strength produced.
Electron Motion
The concept of electron motion is crucial in understanding magnetic fields produced by atomic structures. Electrons, when moving, create a tiny current, similar to a loop of wire carrying electricity. The speed and path of electron movement have a direct impact on the resulting magnetic field.

In a circular orbit, the electron's motion can be characterized by:
  • Speed: In the exercise, the electron moves at \(0.01 \times c\), where \(c\) is the speed of light (\(3 \times 10^8 \) m/s).
  • Radius: The path's radius is crucial, as it helps determine the period \(T\) and subsequently the current \(I\) resulting from this motion.
Ultimately, recognizing these factors allows us to calculate the magnetic field through the Biot-Savart Law.
Current Loop
A current loop results from the continual movement of electrons along a circular path, such as within atoms. Each electron that travels a complete orbit contributes to a small current that circles in a loop.

In this exercise:
  • The electron's charge creates a current as it orbits, which can be quantified with \( I = \frac{q}{T} \), where \( q \) is the charge (\(-1.6 \times 10^{-19} \) C)
  • The period \( T \) of electron motion depends on its speed and orbital radius, calculated as \(T = \frac{2\pi r}{v} \).
Understanding how electrons form current loops is vital because it forms the basis for analyzing and predicting the behavior of magnetic phenomena in materials.
Physics Problem-Solving
Physics problem-solving involves systematically analyzing a question to arrive at a logical solution. This process can be clearly seen in our exercise through a few crucial steps.

  • Understanding Context: Recognizing that a moving electron forms a current loop allows us to see how it affects magnetic field generation.
  • Using Equations: Being able to identify and apply the Biot-Savart Law aids in translating physical phenomena into mathematical expressions.
  • Calculating Values: Determining quantities like current and integrating them into the equation provides the desired solution.
  • Interpreting Results: A negative sign in magnetic field calculation, for instance, indicates directional properties worth noting.
By following systematic procedures, physics problems can be solved effectively, helping us to understand complex systems more holistically.

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Most popular questions from this chapter

Suppose we had a situation in which the component of the magnetic field parallel to the plane of a sheet had the same magnitude on both sides, but changed direction by \(90^{\circ}\) in going through the sheet. What is going on here? Would there be a force on the sheet? Should our formula for the force on a current sheet apply to cases like this?

A wire carrying current \(I\) runs down the \(y\) axis to the origin, thence out to infinity along the positive \(x\) axis. Show that the magnetic field in the quadrant \(x>0, y>0\) of the \(x y\) plane is given by $$ B_{z}=\frac{I}{c}\left(\frac{1}{x}+\frac{1}{y}+\frac{x}{y \sqrt{x^{2}+y^{2}}}+\frac{y}{x \sqrt{x^{2}+y^{2}}}\right) $$

For some purposes it is useful to accelerate negative hydrogen ions in a cyclotron. A negative hydrogen ion, \(\mathrm{H}^{-}\), is a hydrogen atom to which an extra electron has become attached. The attachment is fairly weak; an electric field of only \(1.5 \times 10^{4}\) statvolts \(/ \mathrm{cm}\) (a rather small field by atomic standards) will pull an electron loose, leaving a hydrogen atom. If we want to accelerate \(\mathrm{H}^{-}\) ions up to a kinetic energy of 1 Gev \(\left(10^{9}\right.\) ev), what is the highest magnetic field we dare use to keep them on a circular orbit up to final encrgy? (To find \(\gamma\) for this problem you only need the rest mass of the \(\mathrm{H}^{-}\) ion, which is of course practically the same as that of the proton, approximately 1 Gev.)

A Hall probe for measuring magnetic fields is made from arsenic-doped silicon which has \(2 \times 10^{15}\) conduction electrons per \(\mathrm{cm}^{3}\) and a resistivity of \(1.6 \mathrm{ohm}-\mathrm{cm}\). The Hall voltage is measured across a ribbon of this \(n\) -type silicon which is \(0.2 \mathrm{~cm}\) wide, \(0.005 \mathrm{~cm}\) thick, and \(0.5 \mathrm{~cm}\) long between thicker ends at which it is connected into a 1-volt battery circuit. What voltage will be measured across the \(0.2\) \(\mathrm{cm}\) dimension of the ribbon when the probe is inserted into a field of 1 kilogauss?

Consider two solenoids, one of which is a tenth-scale model of the other. The larger solenoid is 2 meters long, and 1 meter in diameter and is wound with 1 -cm-diameter copper wire. When the coil is connected to a 120 -volt direct- current generator, the magnetic field at its center is 1000 gauss. The scaled- down model is exactly one-tenth the size in every linear dimension, including the diameter of the wire. The number of turns is the same, and it is designed to provide the same central field. (a) Show that the voltage required is the same, namely, 120 volts. (b) Compare the coils with respect to the power dissipated and the difficulty of removing this heat by some cooling means.

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