University Physics with Modern Physics

Wolfgang Bauer, Gary D. Westfall

Chapter 27

Magnetism - all with Video Answers

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Chapter Questions

Problem 1

A magnetic field is oriented in a certain direction in a horizontal plane. An electron moves in a certain direction in the horizontal plane. For this situation, there
a) is one possible direction for the magnetic force on the electron.
b) are two possible directions for the magnetic force on the electron.
c) are infinite possible directions for the magnetic force on the electron.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 2

A particle with charge $q$ is at rest when a magnetic field is suddenly turned on. The field points in the $z$ -direction. What is the direction of the net force acting on the charged particle?
a) in the $x$ -direction
b) in the $y$ -direction
c) The net force is zero.
d) in the $z$ -direction

Ajay Singhal

Numerade Educator

Problem 3

Which of the following has the largest cyclotron frequency?
a) an electron with speed $v$ in a magnetic field with magnitude $B$
b) an electron with speed $2 v$ in a magnetic field with magnitude $B$
c) an electron with speed $v / 2$ in a magnetic field with magnitude $B$
d) an electron with speed $2 v$ in a magnetic field with magnitude $B / 2$
e) an electron with speed $v / 2$ in a magnetic field with magnitude $2 B$

Ajay Singhal

Numerade Educator

Problem 4

In the Hall effect, a potential difference produced across a conductor of finite thickness in a magnetic field by a current flowing through the conductor is given by
a) the product of the density of electrons, the charge of an electron, and the conductor's thickness divided by the product of the magnitudes of the current and the magnetic field.
b) the reciprocal of the expression described in part (a).
c) the product of the charge on an electron and the conductor's thickness divided by the product of the density of electrons and the magnitudes of the current and the magnetic field.
d) the reciprocal of the expression described in (c).
e) none of the above.

Ajay Singhal

Numerade Educator

Problem 5

An electron (with charge $-e$ and mass $m_{\mathrm{e}}$ ) moving in the positive $x$ -direction enters a velocity selector. The velocity selector consists of crossed electric and magnetic fields: $\vec{E}$ is directed in the positive $y$ -direction, and $\vec{B}$ is directed in the positive $z$ -direction. For a velocity $v$ (in the positive $x$ -direction $),$ the net force on the electron is zero, and the electron moves straight through the velocity selector. With what velocity will a proton (with charge $+e$ and mass $m_{\mathrm{p}}=1836 \mathrm{~m}_{\mathrm{e}}$ ) move straight through the velocity selector?
a) $v$
b) $-v$
c) $v / 1836$
d) $-v / 1836$

Ajay Singhal

Numerade Educator

Problem 6

In which direction does a magnetic force act on an electron that is moving in the positive $x$ -direction in a magnetic field pointing in the positive $z$ -direction?
a) the positive $y$ -direction
b) the negative $y$ -direction
c) the negative $x$ -direction
d) any direction in the $x y$ -plane

Ajay Singhal

Numerade Educator

Problem 7

A charged particle is moving in a constant magnetic field. State whether each of the following statements concerning the magnetic force exerted on the particle is true or false? (Assume that the magnetic field is not parallel or antiparallel to the velocity.)
a) It does no work on the particle.
b) It may increase the speed of the particle.
c) It may change the velocity of the particle.
d) It can act only on the particle while the particle is in motion.
e) It does not change the kinetic energy of the particle.

Suhas Katkar

Numerade Educator

Problem 8

An electron moves in a circular trajectory with radius $r_{\mathrm{i}}$ in a constant magnetic field. What is the final radius of the trajectory when the magnetic field is doubled?
a) $\frac{r_{i}}{4}$
b) $\frac{r_{i}}{2}$
c) $r_{i}$
d) $2 r_{i}$
e) $4 r_{\mathrm{i}}$

Ajay Singhal

Numerade Educator

Problem 9

Draw on the $x y z$ coordinate system and specify (in terms of the unit vectors $\hat{x}, \hat{y},$ and $\hat{z}$ ) the direction of the magnetic force on each of the moving particles shown in the figures. Note: The positive $y$ -axis is toward the right, the positive $z$ -axis is toward the top of the page, and the positive $x$ -axis is directed out of the page.

Ajay Singhal

Numerade Educator

Problem 10

A particle with mass $m$, charge $q$, and velocity $v$ enters a magnetic field of magnitude $B$ and with direction perpendicular to the initial velocity of the particle. What is the work done by the magnetic field on the particle? How does this affect the particle's motion?

Ajay Singhal

Numerade Educator

Problem 11

An electron is moving with a constant velocity. When it enters an electric field that is perpendicular to its velocity, the electron will follow a ________ trajectory. When the electron enters a magnetic field that is perpendicular to its velocity, it will follow a ________ trajectory.

Ajay Singhal

Numerade Educator

Problem 12

A proton, moving in negative $y$ -direction in a magnetic field, experiences a force of magnitude $F$, acting in the negative $x$ -direction.
a) What is the direction of the magnetic field producing this force?
b) Does your answer change if the word "proton" in the statement is replaced by “electron"?

Ajay Singhal

Numerade Educator

Problem 13

It would be mathematically possible, for a region with zero current density, to define a scalar magnetic potential analogous to the electrostatic potential: $V_{B}(\vec{r})=-\int_{\vec{r}_{0}}^{\vec{r}} \vec{B} \cdot d \vec{s},$ or $\vec{B}(\vec{r})=-\nabla V_{B}(\vec{r}) .$ However, this has not been done. Explain why not.

Eduard Sanchez

Numerade Educator

Problem 14

A current-carrying wire is positioned within a large, uniform magnetic field, $\vec{B}$. However, the wire experiences no force. Explain how this might be possible.

Ajay Singhal

Numerade Educator

Problem 15

A charged particle moves under the influence of an electric field only. Is it possible for the particle to move with a constant speed? What if the electric field is replaced with a magnetic field?

Ajay Singhal

Numerade Educator

Problem 16

A charged particle travels with speed $v$, at an angle $\theta$ with respect to the $z$ -axis. It enters at time $t=0$ a region of space where there is a magnetic field of magnitude $B$ in the positive $z$ -direction. When does it emerge from this region of space?

Ajay Singhal

Numerade Educator

Problem 17

An electron is traveling horizontally from the northwest toward the southeast in a region of space where the Earth's magnetic field is directed horizontally toward the north. What is the direction of the magnetic force on the electron?

Ajay Singhal

Numerade Educator

Problem 18

At the Earth's surface, there is an electric field that points approximately straight down and has magnitude $150 \mathrm{~N} / \mathrm{m}$. Suppose you had a tuneable electron gun (you can release electrons with whatever kinetic energy you like) and a detector to determine the direction of motion of the electrons when they leave the gun. Explain how you could use the gun to find the direction toward the north magnetic pole. Specifically, what kinetic energy would the electrons need to have? (Hint: It might be easier to think about finding which direction is east or west.)

Suhas Katkar

Numerade Educator

Problem 19

The work done by the magnetic field on a charged particle in motion in a cyclotron is zero. How, then, can a cyclotron be used as a particle accelerator, and what essential feature of the particle's motion makes it possible? A blue problem number indicates a worked-out solution is available in the Student Solutions Manual. One $\bullet$ and two $\bullet$ indicate increasing level of problem difficulty.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 20

A proton moving with a speed of $4.0 \cdot 10^{5} \mathrm{~m} / \mathrm{s}$ in the positive $y$ -direction enters a uniform magnetic field of $0.40 \mathrm{~T}$ pointing in the positive $x$ -direction. Calculate the magnitude of the force on the proton.

Suhas Katkar

Numerade Educator

Problem 21

The magnitude of the magnetic force on a particle with charge $-2 e$ moving with speed $v=1.0 \cdot 10^{5} \mathrm{~m} / \mathrm{s}$ is $3.0 \cdot 10^{-18} \mathrm{~N}$. What is the magnitude of the magnetic field component perpendicular to the direction of motion of the particle?

Ajay Singhal

Numerade Educator

Problem 22

A particle with a charge of $+10.0 \mu \mathrm{C}$ is moving at $300 \cdot \mathrm{m} / \mathrm{s}$ in the positive $z$ -direction.
a) Find the minimum magnetic field required to keep it moving in a straight line at constant speed if there is a uniform electric field of magnitude $100 . \mathrm{V} / \mathrm{m}$ pointing in the positive $y$ -direction.
b) Find the minimum magnetic field required to keep the particle moving in a straight line at constant speed if there is a uniform electric field of magnitude $100 . \mathrm{V} / \mathrm{m}$ pointing in the positive $z$ -direction.

Ajay Singhal

Numerade Educator

Problem 23

A particle with a charge of $20.0 \mu \mathrm{C}$ moves along the $x$ -axis with a speed of $50.0 \mathrm{~m} / \mathrm{s}$. It enters a magnetic field given by $\vec{B}=0.300 \hat{y}+0.700 \hat{z},$ in teslas. Determine the magnitude and the direction of the magnetic force on the particle.

Ajay Singhal

Numerade Educator

Problem 24

The magnetic field in a region in space (where $x>0$ and $y>0$ ) is given by $B=(x-a z) \hat{y}+(x y-b) \hat{z},$ where $a$ and $b$ are positive constants. An electron moving with a constant velocity, $\vec{v}=v_{0} \hat{x},$ enters this region. What are the coordinates of the points at which the net force acting on the electron is zero?

Ajay Singhal

Numerade Educator

Problem 25

A proton is accelerated from rest by a potential difference of $400 .$ V. The proton enters a uniform magnetic field and follows a circular path of radius $20.0 \mathrm{~cm} .$ Determine the magnitude of the magnetic field.

Ajay Singhal

Numerade Educator

Problem 26

An electron with a speed of $4.0 \cdot 10^{5} \mathrm{~m} / \mathrm{s}$ enters a uniform magnetic field of magnitude $0.040 \mathrm{~T}$ at an angle of $35^{\circ}$ to the magnetic field lines. The electron will follow a helical path.
a) Determine the radius of the helical path.
b) How far forward will the electron have moved after completing one circle?

Suhas Katkar

Numerade Educator

Problem 27

A particle with mass $m$ and charge $q$ is moving within both an electric field and a magnetic field, $\vec{E}$ and $\vec{B}$. The particle has velocity $\vec{v},$ momentum $\vec{p}$, and kinetic energy, $K$. Find general expressions for $d \vec{p} / d t$ and $d K / d t$, in terms of these seven quantities.

Ajay Singhal

Numerade Educator

Problem 28

The Earth is showered with particles from space known as muons. They have a charge identical to that of an electron but are many times heavier $\left(m=1.88 \cdot 10^{-28} \mathrm{~kg}\right)$ Suppose a strong magnetic field is established in a lab $(B=0.50 \mathrm{~T})$ and a muon enters this field with a velocity of $3.0 \cdot 10^{6} \mathrm{~m} / \mathrm{s}$ at a right angle to the field. What will be the radius of the resulting orbit of the muon?

Suhas Katkar

Numerade Educator

Problem 29

An electron in a magnetic field moves counterclockwise on a circle in the $x y$ -plane, with a cyclotron frequency of $\omega=1.2 \cdot 10^{12} \mathrm{~Hz}$. What is the magnetic field, $\vec{B}$ ?

Suhas Katkar

Numerade Educator

Problem 30

An electron with energy equal to $4.00 \cdot 10^{2} \mathrm{eV}$ and an electron with energy equal to $2.00 \cdot 10^{2} \mathrm{eV}$ are trapped in a uniform magnetic field and move in circular paths in a plane perpendicular to the magnetic field. What is the ratio of the radii of their orbits?

Ajay Singhal

Numerade Educator

Problem 31

A proton with an initial velocity given by $(1.0 \hat{x}+$$2.0 \hat{y}+3.0 \hat{z})\left(10^{5} \mathrm{~m} / \mathrm{s}\right)$ enters a magnetic field given by $(0.50 \mathrm{~T}) \hat{z}$. Describe the motion of the proton

Suhas Katkar

Numerade Educator

Problem 32

Initially at rest, a small copper sphere with a mass of $3.00 \cdot 10^{-6} \mathrm{~kg}$ and a charge of $5.00 \cdot 10^{-4} \mathrm{C}$ is accelerated through a $7000 .-\mathrm{V}$ potential difference before entering a magnetic field of magnitude $4.00 \mathrm{~T}$, directed perpendicular to its velocity. What is the radius of curvature of the sphere's motion in the magnetic field?

Ajay Singhal

Numerade Educator

Problem 33

Two particles with masses $m_{1}$ and $m_{2}$ and charges $q$ and $2 q$ travel with the same velocity, $v$, and enter a magnetic field of strength $B$ at the same point, as shown in the figure. In the magnetic field, they move in semicircles with radii $R$ and $2 R$. What is the ratio of their masses? Is it possible to apply an electric field that would cause the particles to move in a straight line in the magnetic field? If yes, what would be the magnitude and direction of the field?

Ajay Singhal

Numerade Educator

Problem 34

The figure shows a schematic diagram of a simple mass spectrometer, consisting of a velocity selector and a particle detector and being used to separate singly ionized atoms $\left(q=+e=1.60 \cdot 10^{-19} \mathrm{C}\right)$ of gold $(\mathrm{Au})$ and molybdenum (Mo). The electric field inside the velocity selector has magnitude $E=1.789 \cdot 10^{4} \mathrm{~V} / \mathrm{m}$ and points toward the top of the page, and the magnetic field has magnitude $B_{1}=1.00 \mathrm{~T}$ and points out of the page.
a) Draw the electric force vector, $\vec{F}_{E},$ and the magnetic force vector, $\vec{F}_{B},$ acting on the ions inside the velocity selector.
b) Calculate the velocity, $v_{0}$, of the ions that make it through the velocity selector (those that travel in a straight line). Does $v_{0}$ depend on the type of ion (gold versus molybdenum), or is it the same for both types of ions?
c) Write the equation for the radius of the semicircular path of an ion in the particle detector: $R=R\left(m, v_{0}, q, B_{2}\right)$.
d) The gold ions (represented by the black circles) exit the particle detector at a distance $d_{2}=40.00 \mathrm{~cm}$ from the entrance slit, while the molybdenum ions (represented by the gray circles) exit the particle detector at a distance $d_{1}=19.81 \mathrm{~cm}$ from the entrance slit. The mass of a gold ion is $m_{\text {gold }}=$ $3.27 \cdot 10^{-25}$ kg. Calculate the mass of a molybdenum ion.

Suhas Katkar

Numerade Educator

Problem 35

A small particle accelerator for accelerating ${ }^{3} \mathrm{He}^{+}$ ions is shown in the figure. The ${ }^{3} \mathrm{He}^{+}$ ions exit the ion source with a kinetic energy of $4.00 \mathrm{keV}$. Regions 1 and 2 contain magnetic fields directed into the page, and region 3 contains an electric field directed from left to right. The ${ }^{3} \mathrm{He}^{+}$ ion beam exits the accelerator from a hole on the right that is $7.00 \mathrm{~cm}$ below the ion source, as shown in the figure.
a) If $B_{1}=1.00 \mathrm{~T}$ and region 3 is $50.0 \mathrm{~cm}$ long with $E=60.0 \mathrm{kV} / \mathrm{m},$ what value should $B_{2}$ have to cause the ions to move straight through the exit hole after being accelerated twice in region $3 ?$
b) What minimum width $X$ should region 1 have?
c) What is the velocity of the ions when they leave the accelerator?

Keshav Singh

Numerade Educator

Problem 36

A straight wire of length $2.00 \mathrm{~m}$ carries a current of $24.0 \mathrm{~A} .$ It is placed on a horizontal tabletop in a uniform horizontal magnetic field. The wire makes an angle of $30.0^{\circ}$ with the magnetic field lines. If the magnitude of the force on the wire is $0.500 \mathrm{~N}$, what is the magnitude of the magnetic field?

Ajay Singhal

Numerade Educator

Problem 37

As shown in the figure, a straight conductor parallel to the $x$ -axis can slide without friction on top of two horizontal conducting rails that are parallel to the $y$ -axis and a distance of $L=0.200 \mathrm{~m}$ apart, in a vertical magnetic field of 1.00 T. A 20.0 - A current is maintained through the conductor. If a string is connected exactly at the center of the conductor and passes over a frictionless pulley, what mass $m$ suspended from the string allows the conductor to be at rest?

Ajay Singhal

Numerade Educator

Problem 38

A copper wire of radius $0.500 \mathrm{~mm}$ is carrying a current at the Earth's Equator. Assuming that the magnetic field of the Earth has magnitude $0.500 \mathrm{G}$ at the Equator and is parallel to the surface of the Earth and that the current in the wire flows toward the east, what current is required to allow the wire to levitate?

Ajay Singhal

Numerade Educator

Problem 39

A copper sheet with length $1.0 \mathrm{~m}$, width $0.50 \mathrm{~m},$ and thickness $1.0 \mathrm{~mm}$ is oriented so that its largest surface area is perpendicular to a magnetic field of strength $5.0 \mathrm{~T}$. The sheet carries a current of 3.0 A across its length. What is the magnitude of the force on this sheet? How does this magnitude compare to that of the force on a thin copper wire carrying the same current and oriented perpendicularly to the same magnetic field?

Eduard Sanchez

Numerade Educator

Problem 40

A conducting rod of length $L$ slides freely down an inclined plane, as shown in the figure. The plane is inclined at an angle $\theta$ from the horizontal. A uniform magnetic field of strength $B$ acts in the positive $y$ -direction. Determine the magnitude and the direction of the current that would have to be passed through the rod to hold it in position on the inclined plane.

Ajay Singhal

Numerade Educator

Problem 41

A square loop of wire, with side length $d=8.0 \mathrm{~cm},$ carries a current of $\mathrm{mag}$ nitude $i=0.15 \mathrm{~A}$ and is free to rotate. It is placed between the poles of an electromagnet that produce a uniform magnetic field of $1.0 \mathrm{~T}$. The loop is initially placed so that its normal vector, $\hat{n}$, is at a $35.0^{\circ}$ angle relative to the direction of the magnetic field vector, with the angle $\theta$ defined as shown in the figure. The wire is copper (with a density of $\left.\rho=8960 \mathrm{~kg} / \mathrm{m}^{3}\right)$, and its diameter is $0.50 \mathrm{~mm}$. What is the magnitude of the initial angular acceleration of the loop when it is released?

Gregory Devenport

Gregory Devenport

Numerade Educator

Problem 42

A rail gun accelerates a projectile from rest by using the magnetic force on a current-carrying wire. The wire has radius $r=5.1 \cdot 10^{-4} \mathrm{~m}$ and is made of copper having a density of $\rho=8960 \mathrm{~kg} / \mathrm{m}^{3}$. The gun consists of rails of length $L=1.0 \mathrm{~m}$ in a constant magnetic field of magnitude $B=2.0 \mathrm{~T}$ oriented perpendicular to the plane defined by the rails. The wire forms an electrical connection across the rails at one end of the rails. When triggered, a current of $1.00 \cdot 10^{4}$ A flows through the wire, which accelerates the wire along the rails. Calculate the final speed of the wire as it leaves the rails. (Neglect friction.)

Suhas Katkar

Numerade Educator

Problem 43

A square loop of wire of side length $\ell$ lies in the $x y$ -plane, with its center at the origin and its sides parallel to the $x$ - and $y$ -axes. It carries a current, $i$, in the counterclockwise direction, as viewed looking down the $z$ -axis from the positive direction. The loop is in a magnetic field given by $\vec{B}=\left(B_{0} / a\right)(z \hat{x}+x \hat{z}),$ where $B_{0}$ is a constant field strength, $a$ is a constant with the dimension of length, and $\hat{x}$ and $\hat{z}$ are unit vectors in the positive $x$ -direction and positive $z$ -direction. Calculate the net force on the loop.

Suhas Katkar

Numerade Educator

Problem 44

A rectangular coil with 20 windings carries a current of 2.00 mA flowing in the counterclockwise direction. It has two sides that are parallel to the $y$ -axis and have length $8.00 \mathrm{~cm}$ and two sides that are parallel to the $x$ -axis and have length $6.00 \mathrm{~cm} .$ A uniform magnetic field of $50.0 \mu \mathrm{T}$ acts in the positive $x$ -direction. What torque must be applied to the loop to hold it steady?

Ajay Singhal

Numerade Educator

Problem 45

A coil consists of 120 circular loops of wire of radius $4.8 \mathrm{~cm} .$ A current of 0.49 A runs through the coil, which is oriented vertically and is free to rotate about a vertical axis (parallel to the $z$ -axis). It experiences a uniform horizontal magnetic field in the positive $x$ -direction. When the coil is oriented parallel to the $x$ -axis, a force of $1.2 \mathrm{~N}$ applied to the edge of the coil in the positive $y$ -direction can keep it from rotating. Calculate the strength of the magnetic field.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 46

Twenty loops of wire are tightly wound around a round pencil that has a diameter of $6.00 \mathrm{~mm} .$ The pencil is then placed in a uniform 5.00 -T magnetic field, as shown in the figure. If a 3.00 - A current is present in the coil of wire, what is the magnitude of the torque on the pencil?

Ajay Singhal

Numerade Educator

Problem 47

A copper wire with density $\rho=8960 \mathrm{~kg} / \mathrm{m}^{3}$ is formed into a circular loop of radius $50.0 \mathrm{~cm} .$ The cross-sectional area of the wire is $1.00 \cdot 10^{-5} \mathrm{~m}^{2},$ and a potential difference of $0.012 \mathrm{~V}$ is applied to the wire. What is the maximum angular acceleration of the loop when it is placed in a magnetic field of magnitude $0.25 \mathrm{~T}$ ? The loop rotates about an axis through a diameter.

Keshav Singh

Numerade Educator

Problem 48

A simple galvanometer is made from a coil that consists of $N$ loops of wire of area $A .$ The coil is attached to a mass, $M$, by a light rigid rod of length $L$. With no current in the coil, the mass hangs straight down, and the coil lies in a horizontal plane. The coil is in a uniform magnetic field of magnitude $B$ that is oriented horizontally. Calculate the angle from the vertical of the rigid rod as a function of the current, $i$, in the coil.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 49

Show that the magnetic dipole moment of an electron orbiting in a hydrogen atom is proportional to its angular momentum, $L: \mu=-e L / 2 m,$ where $-e$ is the charge of the electron and $m$ is its mass.

Ajay Singhal

Numerade Educator

Problem 50

The figure shows a top view of a current-carrying ring of wire having a diameter $d=8.00 \mathrm{~cm},$ which is suspended from the ceiling via a thin string. A 1.00 -A current flows in the ring in the direction indicated in the figure. The ring is connected to one end of a spring with a spring constant of $100 . \mathrm{N} / \mathrm{m}$. When the ring is in the position shown in the figure, the spring is at its equilibrium length, $\ell$. Determine the extension of the spring when a magnetic field of magnitude $B=2.00 \mathrm{~T}$ is applied parallel to the plane of the ring as shown in the figure.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 51

A coil of wire consisting of 40 rectangular loops, with width $16.0 \mathrm{~cm}$ and height $30.0 \mathrm{~cm}$, is placed in a constant magnetic field given by $\vec{B}=0.065 \mathrm{~T} \hat{x}+0.250 \mathrm{~T} \hat{z}$. The coil is hinged to a fixed thin rod along the $y$ -axis (along segment $d a$ in the figure) and is originally located in the $x y$ plane. A current of 0.200 A runs through the wire.
a) What are the magnitude and the direction of the force, $\vec{F}_{a b},$ that $\vec{B}$ exerts on segment $a b$ of the coil?
b) What are the magnitude and the direction of force, $\vec{F}_{b c}$, that $\vec{B}$ exerts on segment $b c$ of the coil?
c) What is the magnitude of the net force, $F_{\text {net }}$, that $\vec{B}$ exerts on the coil?
d) What are the magnitude and the direction of the torque, $\vec{\tau},$ that $\vec{B}$ exerts on the coil?
e) In what direction, if any, will the coil rotate about the $y$ -axis (viewed from above and looking down that axis)?

Eduard Sanchez

Numerade Educator

Problem 52

A high electron mobility transistor (HEMT) controls large currents by applying a small voltage to a thin sheet of electrons. The density and mobility of the electrons in the sheet are critical for the operation of the HEMT. HEMTs consisting of AlGaN/GaN/Si are being studied because they promise better performance at higher powers, temperatures, and frequencies than conventional silicon HEMTs can achieve. In one study, the Hall effect was used to measure the density of electrons in one of these new HEMTs. When a current of $10.0 \mu$ A flows through the length of the electron sheet, which is $1.00 \mathrm{~mm}$ long, $0.300 \mathrm{~mm}$ wide, and $10.0 \mathrm{nm}$ thick, a magnetic field of $1.00 \mathrm{~T}$ perpendicular to the sheet produces a voltage of $0.680 \mathrm{mV}$ across the width of the sheet. What is the density of electrons in the sheet?

Eduard Sanchez

Numerade Educator

Problem 53

The figure shows schematically a setup for a Hall effect measurement using a thin film of zinc oxide of thickness $1.50 \mu \mathrm{m}$. The current, $i$, across the thin film is $12.3 \mathrm{~mA}$ and the Hall potential, $V_{\mathrm{H}}$, is $-20.1 \mathrm{mV}$ when the magnetic field of magnitude $B=0.90 \mathrm{~T}$ is applied perpendicular to the current flow.
a) What are the charge carriers in the thin film? [Hint: They can be either electrons with charge $-e$ or electron holes (missing electrons) with charge $+e$.
b) Calculate the density of charge carriers in the thin film.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 54

A cyclotron in a magnetic field of $9 \mathrm{~T}$ is used to accelerate protons to $50 \%$ of the speed of light. What is the cyclotron frequency of these protons? What is the radius of their trajectory in the cyclotron? What is the cyclotron frequency and trajectory radius of the same protons in the Earth's magnetic field? Assume that the Earth's magnetic field is about $0.5 \mathrm{G}$.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 55

A straight wire carrying a current of 3.41 A is placed at an angle of $10.0^{\circ}$ to the horizontal between the pole tips of a magnet producing a field of $0.220 \mathrm{~T}$ upward. The poles tips each have a $10.0 \mathrm{~cm}$ diameter. The magnetic force causes the wire to move out of the space between the poles. What is the magnitude of that force?

Ajay Singhal

Numerade Educator

Problem 56

An electron is moving at $v=6.00 \cdot 10^{7} \mathrm{~m} / \mathrm{s}$ perpendicular to the Earth's magnetic field. If the field strength is $0.500 \cdot 10^{-4} \mathrm{~T}$, what is the radius of the electron's circular path?

Ajay Singhal

Numerade Educator

Problem 57

A straight wire with a constant current running through it is in Earth's magnetic field, at a location where the magnitude is $0.43 \mathrm{G}$. What is the minimum current that must flow through the wire for a 10.0 -cm length of it to experience a force of $1.0 \mathrm{~N} ?$

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 58

A small aluminum ball with a mass of $5.00 \mathrm{~g}$ and a charge of $15.0 \mathrm{C}$ is moving northward at $3000 \mathrm{~m} / \mathrm{s}$. You want the ball to travel in a horizontal circle with a radius of $2.00 \mathrm{~m}$, in a clockwise sense when viewed from above. Ignoring gravity, what is the magnitude and the direction of the magnetic field that must be applied to the aluminum ball to cause it to have this motion?

Mohammad Mehran

Mohammad Mehran

Numerade Educator

Problem 59

The velocity selector described in Solved Problem 27.2 is used in a variety of devices to produce a beam of charged particles of uniform velocity. Suppose the fields in such a selector are given by $\vec{E}=\left(1.00 \cdot 10^{4} \mathrm{~V} / \mathrm{m}\right) \hat{x}$ and $\vec{B}=(50.0 \mathrm{mT}) \hat{y} .$ Find the velocity in the $z$ -direction with which a charged particle can travel through the selector without being deflected.

Mohammad Mehran

Mohammad Mehran

Numerade Educator

Problem 60

A circular coil with a radius of $10.0 \mathrm{~cm}$ has 100 turns of wire and carries a current, $i=100 . \mathrm{mA} .$ It is free to rotate in a region with a constant horizontal magnetic field given by $\vec{B}=(0.0100 \mathrm{~T}) \hat{x}$. If the unit normal vector to the plane of the coil makes an angle of $30.0^{\circ}$ with the horizontal, what is the magnitude of the net magnetic torque acting on the coil?
27.61 At $t=0$ an electron crosses the positive $y$ -axis (so $x=0$ ) at $60.0 \mathrm{~cm}$ from the origin with velocity $2.00 \cdot 10^{5} \mathrm{~m} / \mathrm{s}$ in the positive $x$ -direction. It is in a uniform magnetic field.
a) Find the magnitude and the direction of the magnetic field that will cause the electron to cross the $x$ -axis at $x=60.0 \mathrm{~cm}$.
b) What work is done on the electron during this motion?
c) How long will the trip take from $y$ -axis to $x$ -axis?

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 62

A $12.0-\mathrm{V}$ battery is connected to a $3.00-\Omega$ resistor in a rigid rectangular loop of wire measuring $3.00 \mathrm{~m}$ by $1.00 \mathrm{~m}$. As shown in the figure, a length $\ell=1.00 \mathrm{~m}$ of wire at the end of the loop extends into a $2.00 \mathrm{~m}$ by $2.00 \mathrm{~m}$ region with a magnetic field of magnitude $5.00 \mathrm{~T}$, directed into the page. What is the net force on the loop?

Ajay Singhal

Numerade Educator

Problem 63

An alpha particle $\left(m=6.6 \cdot 10^{-27} \mathrm{~kg}, q=+2 e\right)$ is accelerated by a potential difference of $2700 \mathrm{~V}$ and moves in a plane perpendicular to a constant magnetic field of magnitude $0.340 \mathrm{~T}$, which curves the trajectory of the alpha particle. Determine the radius of curvature and the period of revolution.

Christopher Dzorkpata

Christopher Dzorkpata

Numerade Educator

Problem 64

In a certain area, the electric field near the surface of the Earth is given by $\vec{E}=(-150 . \mathrm{N} / \mathrm{C}) \hat{z},$ and the Earth's magnetic field is given by $\vec{B}=(50.0 \mu \mathrm{T}) \hat{r}_{\mathrm{N}}-(20.0 \mu \mathrm{T}) \hat{z}$, where $\hat{z}$ is a unit vector pointing vertically upward and $\hat{r}_{\mathrm{N}}$ is a horizontal unit vector pointing due north. What velocity, $\vec{v},$ will allow an electron in this region to move in a straight line at constant speed?

Eduard Sanchez

Numerade Educator

Problem 65

A helium leak detector uses a mass spectrometer to detect tiny leaks in a vacuum chamber. The chamber is evacuated with a vacuum pump and then sprayed with helium gas on the outside. If there is any leak, the helium molecules pass through the leak and into the chamber, whose volume is sampled by the leak detector. In the spectrometer, helium ions are accelerated and released into a tube, where their motion is perpendicular to an applied magnetic field, $\vec{B},$ and they follow a circular orbit of radius $r$ and then hit a detector. Estimate the velocity required if the orbital radius of the ions is to be no more than $5 \mathrm{~cm},$ the magnetic field is $0.15 \mathrm{~T}$ and the mass of a helium- 4 atom is about $6.6 \cdot 10^{-27} \mathrm{~kg}$. Assume that each ion is singly ionized (has one electron less than the neutral atom). By what factor does the required velocity change if helium- 3 atoms, which have about $\frac{3}{4}$ as much mass as helium- 4 atoms, are used?

Keshav Singh

Numerade Educator

Problem 66

In your laboratory, you set up an experiment with an electron gun that emits electrons with energy of $7.50 \mathrm{keV}$ toward an atomic target. What deflection (magnitude and direction) would Earth's magnetic field $(0.300 \mathrm{G})$ produce in the beam of electrons if the beam is initially directed due east and covers a distance of $1.00 \mathrm{~m}$ from the gun to the target? (Hint: First calculate the radius of curvature, and then determine how far away from a straight line the electron beam has deviated after $1.00 \mathrm{~m}$.)

Eduard Sanchez

Numerade Educator

Problem 67

A proton enters the region between the two plates shown in the figure moving in the $x$ -direction with a speed $v=1.35 \cdot 10^{6} \mathrm{~m} / \mathrm{s}$. The potential of the top plate is $200 . \mathrm{V},$ and the potential of the bottom plate is $0 \mathrm{~V}$. What is the direction and the magnitude of the magnetic field, $\vec{B}$, that is required between the plates for the proton to continue traveling in a straight line along the $x$ -direction?

Eduard Sanchez

Numerade Educator

Problem 68

An electron moving at a constant velocity, $\vec{v}=v_{0} \hat{x}$, enters a region in space where a magnetic field is present. The magnetic field, $\vec{B}$, is constant and points in the $z$ -direction. What is the magnitude and direction of the magnetic force acting on the electron? If the width of the region where the magnetic field is present is $d$, what is the minimum velocity the electron must have in order to escape this region?

Ajay Singhal

Numerade Educator

Problem 69

A 30 -turn square coil with a mass of $0.250 \mathrm{~kg}$ and a side length of $0.200 \mathrm{~m}$ is hinged along a horizontal side and carries a 5.00 -A current. It is placed in a magnetic field pointing vertically downward and having a magnitude of $0.00500 \mathrm{~T}$. Determine the angle that the plane of the coil makes with the vertical when the coil is in equilibrium. Use $g=9.81 \mathrm{~m} / \mathrm{s}^{2}$.

Ajay Singhal

Numerade Educator

Problem 70

A semicircular loop of wire of radius $R$ is in the $x y$ -plane, centered about the origin. The wire carries a current, $i$, counterclockwise around the semicircle, from $x=-R$ to $x=+R$ on the $x$ -axis. A magnetic field, $\vec{B}$, is pointing out of the plane, in the positive $z$ -direction. Calculate the net force on the semicircular loop.

Gregory Devenport

Gregory Devenport

Numerade Educator

Problem 71

A proton moving at speed $v=1.00 \cdot 10^{6} \mathrm{~m} / \mathrm{s}$ enters a region in space where a magnetic field given by $\vec{B}=$ $(-0.500 \mathrm{~T}) \hat{z}$ exists. The velocity vector of the proton is at an angle $\theta=60.0^{\circ}$ with respect to the positive $z$ -axis.
a) Analyze the motion of the proton and describe its trajectory (in qualitative terms only).
b) Calculate the radius, $r$, of the trajectory projected onto a plane perpendicular to the magnetic field (in the $x y$ -plane).
c) Calculate the period, $T,$ and frequency, $f$, of the motion in that plane.
d) Calculate the pitch of the motion (the distance traveled by the proton in the direction of the magnetic field in 1 period).

Gregory Devenport

Gregory Devenport

Numerade Educator