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A charged particle of mass \(m\) and charge \(q\) moves in a circle due to a uniform magnetic field of magnitude \(B\), which points perpendicular to the plane of the circle. a. Assume the particle is positively charged. Make a sketch showing the direction of motion and the direction of the field, and show that the resulting force is in the right direction to produce circular motion. b. Find the radius, \(r\), of the circle, in terms of \(m\), \(q\), \(v\), and \(B\).(answer check available at lightandmatter.com) c. Show that your result from part b has the right units. d. Discuss all four variables occurring on the right-hand side of your answer from part b. Do they make sense? For instance, what should happen to the radius when the magnetic field is made stronger? Does your equation behave this way? e. Restate your result so that it gives the particle's angular frequency, \(\omega\), in terms of the other variables, and show that \(v\) drops out.(answer check available at lightandmatter.com)

Short Answer

Expert verified
The radius is \(r = \frac{mv}{qB}\), and the angular frequency is \(\omega = \frac{qB}{m}\). The forces act correctly for circular motion, and the units are consistent.

Step by step solution

01

Direction of Motion and Magnetic Field

For a positively charged particle moving in a circle under a magnetic field, the direction of the Lorentz force (and hence the motion) can be determined using the right-hand rule. If the magnetic field, \(B\), points out of the page, and the velocity \(v\) of the particle is tangential to the circle, the force will be directed towards the center, perpendicular to both \(v\) and \(B\). This centripetal force causes circular motion.
02

Calculate the Radius of the Circle

To find the radius \(r\), equate the magnetic force to the centripetal force required for circular motion.The magnetic (Lorentz) force is given by:\[ F_m = qvB \]The centripetal force is:\[ F_c = \frac{mv^2}{r} \]Setting these forces equal gives:\[ qvB = \frac{mv^2}{r} \]Solving for \(r\):\[ r = \frac{mv}{qB} \]
03

Verify the Units

The units for \(r\) must be in meters. Each component's units are:- Velocity \(v\) in \(\text{m/s}\)- Mass \(m\) in kg- Charge \(q\) in C (Coulombs)- Magnetic field \(B\) in T (Tesla = \(\frac{kg \, s^{-2}}{C}\))Substituting these into the equation \(r = \frac{mv}{qB}\) gives units:\[ \text{m} = \frac{\text{kg} \times \text{m/s}}{\text{C} \times \text{kg/s}^2/\text{C}} \]Simplifying this, we indeed find meters \((\text{m})\).
04

Analyze the Variables

The radius \(r = \frac{mv}{qB}\) is proportional to the mass \(m\) and velocity \(v\), and inversely proportional to the charge \(q\) and magnetic field \(B\). This means:- Increase in mass or speed enlarges the radius.- Increase in charge or magnetic field strength decreases the radius.This behavior is consistent because stronger fields or increased charge cause tighter curvature of the path.
05

Derive the Particle's Angular Frequency \(\omega\)

The angular frequency \(\omega\) is defined as \(\omega = \frac{v}{r}\). Substituting our expression for \(r\), we find:\[ \omega = \frac{v}{\frac{mv}{qB}} = \frac{qB}{m} \]Notice that velocity \(v\) is not present in this expression, meaning angular frequency depends only on \(q\), \(B\), and \(m\).

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Key Concepts

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

Centripetal Force
Centripetal force is a crucial concept for understanding motion in a circle. It's the force that pulls an object moving in a straight-line fashion into a curved path. Without this force, the object would continue in a linear motion, in accordance with Newton's First Law of Motion. In the scenario of a charged particle moving in a magnetic field, this force is what keeps the particle in a circular path.

Centripetal force acts towards the center of the circle, ensuring circular motion. In mathematical terms, it's described by the formula:- \( F_c = \frac{mv^2}{r} \) Where:- \( m \) is the mass of the particle,- \( v \) is the velocity,- \( r \) is the radius of the circular path.

The relationship shows that an increase in velocity or mass results in a larger force being required to maintain the circle. Conversely, if the radius is larger, less force is needed. Understanding the balance of these quantities is essential for grasping how charges move in magnetic fields.
Lorentz Force
The Lorentz force plays a central role when charged particles move through a magnetic field. It's the force exerted on a charged particle by an electromagnetic field, and it is described by the equation:- \( F_m = qvB \) Where:- \( F_m \) is the magnetic force,- \( q \) is the charge of the particle,- \( v \) is the velocity of the particle, - \( B \) is the magnetic field.

This force is fundamental for making the charged particle travel in a circular path. By acting perpendicular to both the velocity and the magnetic field, the Lorentz force is effectively what provides the centripetal force required for circular motion. The direction of this force can be determined using the right-hand rule: Point your thumb in the direction of the particle's velocity, and your fingers in the direction of the magnetic field. Your palm will then face the direction of the force acting on a positively charged particle.

Understanding the Lorentz force is essential to predicting and describing the movement patterns of charged particles under magnetic influences.
Magnetic Field Influence
The magnetic field's influence on a charged particle is not only critical but also fascinating. This field, a region around a magnet where the force of magnetism acts, causes charged particles to deflect in specific ways. When a charged particle enters this magnetic area, it experiences a force due to its charge and velocity.

The strength of the influence is directly related to the particle's velocity and its charge, as seen in the Lorentz force equation. Increasing the field strength, in turn, tightens the curvature of the particle's path. This means that stronger fields cause the particle to spiral or loop more sharply.
The direction of the magnetic field typically determines whether a particle moves towards or away from its initial path. This deflection results in various practical applications such as cyclotrons and other particle accelerators, where particles are made to travel in controlled, circular paths for innovative research.
Angular Frequency
Angular frequency is a measure of how quickly an object travels around a circular path. It's important for understanding periodic motion, such as that seen in charged particles circulating in a magnetic field. Defined by the following relationship:- \( \omega = \frac{v}{r} \) where \( v \) is velocity and \( r \) is the radius.

This formula can be restructured using previously derived equations to express angular frequency entirely in terms of fixed parameters:- \( \omega = \frac{qB}{m} \) The outcome, \( \omega \), depends on the charge \( q \), magnetic field strength \( B \), and mass \( m \) of the particle, rather than its velocity.
This result reveals why angular frequency remains constant for a given set of conditions—highlighting why different particles have distinctive frequencies based on their unique characteristics. This principle is central to fields like spectroscopy, where material's internal movements are analyzed by their angular frequency responses to magnetic fields.

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Most popular questions from this chapter

(a) A line charge, with charge per unit length \(\lambda\), moves at velocity \(v\) along its own length. How much charge passes a given point in time \(d t\) ? What is the resulting current? Nwans\\{hwans:linechargecurrent \(\\}\) (b) Show that the units of your answer in part a work out correctly. This constitutes a physical model of an electric current, and it would be a physically realistic model of a beam of particles moving in a vacuum, such as the electron beam in a television tube. It is not a physically realistic model of the motion of the electrons in a current-carrying wire, or of the ions in your nervous system; the motion of the charge carriers in these systems is much more complicated and chaotic, and there are charges of both signs, so that the total charge is zero. But even when the model is physically unrealistic, it still gives the right answers when you use it to compute magnetic effects. This is a remarkable fact, which we will not prove. The interested reader is referred to E.M. Purcell, Electricity and Magnetism, McGraw Hill, \(1963 .\)

A charged particle is released from rest. We see it start to move, and as it gets going, we notice that its path starts to curve. Can we tell whether this region of space has \(\mathbf{E} \neq 0\), or \(\mathbf{B} \neq 0\), or both? Assume that no other forces are present besides the possible electrical and magnetic ones, and that the fields, if they are present, are uniform.

If you put four times more current through a solenoid, how many times more energy is stored in its magnetic field?(answer check available at lightandmatter.com)

Two long, parallel strips of thin metal foil form a configuration like a long, narrow sandwich. The air gap between them has height \(h\), the width of each strip is \(w\), and their length is \(\ell\). Each strip carries current \(I\), and we assume for concreteness that the currents are in opposite directions, so that the magnetic force, \(F\), between the strips is repulsive. (a) Find the force in the limit of \(w \gg h\).(answer check available at lightandmatter.com) (b) Find the force in the limit of \(w \ll h\), which is like two ordinary wires. (c) Discuss the relationship between the two results.

(a) A long, skinny solenoid consists of \(N\) turns of wire wrapped uniformly around a hollow cylinder of length \(\ell\) and crosssectional area \(A\). Find its inductance.(answer check available at lightandmatter.com) (b) Show that your answer has the right units to be an inductance.

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