Deck 28: Magnetic Fields

Question 1

At any point the magnetic field lines are in the direction of:

A) the magnetic force on a moving positive charge
B) the magnetic force on a moving negative charge
C) the velocity of a moving positive charge
D) the velocity of a moving negative charge
E) none of the above

Answer 1

none of the above

Question 2

In the formula

A) must be perpendicular to but not necessarily to
B) must be perpendicular to but not necessarily to
C) must be perpendicular to but not necessarily to
D) all three vectors must be mutually perpendicular
E) must be perpendicular to both

Answer 2

must be perpendicular to both

Question 3

An electron moves in the negative x direction, through a uniform magnetic field that is in the negative y direction. The magnetic force on the electron is:

A) in the negative x direction
B) in the positive y direction
C) in the negative y direction
D) in the positive z direction
E) in the negative z direction

Answer 3

in the negative z direction

Question 4

A static magnetic field CANNOT:

A) exert a force on a charge
B) accelerate a charge
C) change the momentum of a charge
D) change the kinetic energy of a charge
E) exist

Question 5

A hydrogen atom that has lost its electron is moving east in a region where the magnetic field is directed from south to north. It will be deflected:

A) up
B) down
C) north
D) south
E) not at all

Question 6

An electron and a proton are both initially moving with the same speed and in the same direction at 90˚ to the same uniform magnetic field. They experience magnetic forces, which are initially:

A) identical
B) equal in magnitude but opposite in direction
C) in the same direction and differing in magnitude by a factor of 1840
D) in opposite directions and differing in magnitude by a factor of 1840
E) equal in magnitude but perpendicular to each other

Question 7

At one instant an electron (charge = -1.6 *10^-19C) is moving in the xy plane, the components of its velocity being v_x = 5.0 * 10⁵ m/s and v_y = 3.0 * 10⁵ m/s. A magnetic field of 0.80 T is in the positive x direction. At that instant the magnitude of the magnetic force on the electron is:

A) 0 N
B) 3.8 * 10^-14 N
C) 6.0 * 10^-14 N
D) 6.4 *10^-14 N
E) 1.0 *10^-14

Question 8

An electron enters a region of uniform perpendicular and fields. It is observed that the velocity of the electron is unaffected. A possible explanation is:

A) is parallel to and has magnitude E/B
B) is parallel to
C) is perpendicular to both and and has magnitude B/E
D) is perpendicular to both and and has magnitude E/B
E) the given situation is impossible

Question 9

At one instant an electron (charge = -1.6 *10^-19C) is moving in the xy plane, the components of its velocity being v_x = 5.0 *10⁵ m/s and v_y = 3.0 * 10⁵ m/s. A magnetic field of 0.80 T is in the positive y direction. At that instant the magnitude of the magnetic force on the electron is:

A) 0 N
B) 3.8 * 10^-14 N
C) 6.4 * 10^-14 N
D) 7.5 *10^-14 N
E) 1.0 * 10^-14

Question 10

A proton (charge e), traveling perpendicular to a magnetic field, experiences the same force as an alpha particle (charge 2e) which is also traveling perpendicular to the same field. The ratio of their speeds, v_proton/v_alpha is:

A) 0.5
B) 1
C) 2
D) 4
E) 8

Question 11

A beam of electrons is sent horizontally down the axis of a tube to strike a fluorescent screen at the end of the tube. On the way, the electrons encounter a magnetic field directed vertically downward. The spot on the screen will therefore be deflected:

A) upward
B) downward
C) to the right as seen from the electron source
D) to the left as seen from the electron source
E) not at all

Question 12

A charged particle is projected into a region of uniform, parallel, and fields. The force on the particle is:

A) zero
B) at some angle < 90° with the field lines
C) along the field lines
D) perpendicular to the field lines
E) unknown (need to know the sign of the charge)

Question 13

The direction of the magnetic field in a certain region of space is determined by firing a test charge into the region with its velocity in various directions in different trials. The field direction is:

A) one of the directions of the velocity when the magnetic force is zero
B) the direction of the velocity when the magnetic force is a maximum
C) the direction of the magnetic force
D) perpendicular to the velocity when the magnetic force is zero
E) none of the above

Question 14

An electron travels due north through a vacuum in a region of uniform magnetic field that is also directed due north. It will:

A) be unaffected by the field
B) speed up
C) slow down
D) follow a right-handed corkscrew path
E) follow a left-handed corkscrew path

Question 15

An electron is moving north in a region where the magnetic field is south. The magnetic force exerted on the electron is:

A) zero
B) up
C) north
D) south
E) west

Question 16

J. J. Thomson's experiment, involving the motion of an electron beam in mutually perpendicular and fields, gave the value of:

A) the mass of an electron
B) the charge of an electron
C) the Earth's magnetic field
D) the charge/mass ratio for an electron
E) Avogadro's number

Question 17

The magnetic force on a charged particle is in the direction of its velocity if:

A) it is moving in the direction of the field
B) it is moving opposite to the direction of the field
C) it is moving perpendicular to the field
D) it is moving in some other direction
E) never

Question 18

Units of a magnetic field might be:

A) C.m/s
B) C.s/m
C) C/kg
D) kg/C.s
E) N/C.m

Question 19

A magnetic field exerts a force on a charged particle:

A) always
B) never
C) if the particle is moving across the field lines
D) if the particle is moving along the field lines
E) if the particle is at rest

Question 20

An electron (charge = -1.6 *10^-19C) is moving at 3.0 * 10⁵ m/s in the positive x direction. A magnetic field of 0.80 T is in the positive z direction. The magnetic force on the electron is:

A) 0 N
B) 4.5 * 10^-14 N in the positive z direction
C) 4.5 *10^-14 N in the negative z direction
D) 4.5 *10^-14 N in the positive y direction
E) 4.5 * 10^-14 N in the negative y direction

Question 21

An electron is launched with velocity in a uniform magnetic field . The angle $\theta$ between and is between 0 and 90^o. As a result, the electron follows a helical path. The pitch of the helix is:

A) the angle the helix makes with the magnetic field
B) the angle the helix makes with the electron's velocity vector
C) the radius of the circular motion
D) the distance between adjacent turns of the helix
E) the time it takes the electron to move from one turn of the helix to the next

Question 22

The diagram shows a straight wire carrying a flow of electrons into the page. The wire is between the poles of a permanent magnet. The direction of the magnetic force exerted on the wire is:

A) $\uparrow$
B) $\downarrow$
C) $\leftarrow$
D) $\rightarrow$
E) into the page

Question 23

An electron is launched with velocity in a uniform magnetic field . The angle $\theta$ between and is between 0 and 90^o. As a result, the electron follows a helix, its velocity vector returning to its initial value in a time interval of:

A) 2πm/eB
B) 2πmv/eB
C) 2πmv sin $\theta$ /eB
D) 2πmv cos $\theta$ /eB
E) none of these

Question 24

A cyclotron operates with a given magnetic field and at a given frequency. If R denotes the radius of the final orbit, the final particle energy is proportional to:

A) 1/R
B) R
C) R²
D) R³
E) R⁴

Question 25

A strip 1.2 mm wide is moving at a speed of 25 cm/s through a uniform magnetic field of 5.6 T. What is the maximum Hall voltage across the strip?

A) 1.7 mV
B) 8.5 mV
C) 27 mV
D) 1.2 V
E) 17 V

Question 26

The resonance condition in a cyclotron states that:

A) the time it takes the protons to make one cycle equals the natural frequency of the proton
B) the protons oscillate on a vertical axis once per cycle
C) the proton spin changes direction once per cycle
D) the frequency of the proton orbits equals the frequency of the electrical oscillator
E) the frequency of the proton orbits is an integer multiple of 60 Hz

Question 27

In a certain mass spectrograph, an ion beam passes through a velocity filter consisting of mutually perpendicular fields and . The beam then enters a region of another magnetic field perpendicular to the beam. The radius of curvature of the resulting ion beam is proportional to:

A) EB'/B
B) EB/B'
C) BB'/E
D) B/EB'
E) E/BB'

Question 28

Which is NOT one of the differences between a cyclotron and a synchrotron?

A) Orbits in a cyclotron are spirals, while in a synchrotron they are circles
B) Conventional cyclotrons fail above energies of about 50 MeV because the proton speeds get too close to the speed of light, while synchrotrons are designed to accommodate all proton energies
C) Large cyclotrons would require extremely large magnets, since they must cover all possible orbital radii, while synchrotrons only need a thin ring
D) In general, synchrotrons are much smaller than cyclotrons
E) Both cyclotrons and synchrotrons require electrical oscillators to accelerate the protons

Question 29

The Hall effect can be used to calculate the charge-carrier number density in a conductor. If a conductor carrying a current of 2.0 A is 0.5 mm thick, and the Hall effect voltage is 4.5 µV when it is in a uniform magnetic field of 1.2 T, what is the density of charge carriers in the conductor?

A) 1.0 x 10²⁸/m³
B) 6.7 x 10²⁷/m³
C) 4.6 x 10²⁷/m³
D) 1.7 x 10²⁷/m³
E) 1.2 x 10²⁷/m³

Question 30

An electron and a proton both each travel with equal speeds around circular orbits in the same uniform magnetic field, as shown in the diagram (not to scale). The field is into the page on the diagram. Because the electron is less massive than the proton and because the electron is negatively charged and the proton is positively charged:

A) the electron travels clockwise around the smaller circle and the proton travels counterclockwise around the larger circle.
B) the electron travels counterclockwise around the smaller circle and the proton travels clockwise around the larger circle
C) the electron travels clockwise around the larger circle and the proton travels counterclockwise around the smaller circle
D) the electron travels counterclockwise around the larger circle and the proton travels clockwise around the smaller circle
E) the electron travels counterclockwise around the smaller circle and the proton travels counterclockwise around the larger circle

Question 31

The diagram shows a straight wire carrying current i in a uniform magnetic field. The magnetic force on the wire is indicated by an arrow but the magnetic field is not shown. Of the following possibilities, the direction of the magnetic field is:

A) to the right
B) opposite the direction of
C) in the direction of
D) into the page
E) out of the page

Question 32

A uniform magnetic field is in the positive z direction. A positively charged particle is moving in the positive x direction through the field. The net force on the particle can be made zero by applying an electric field in what direction?

A) Positive y
B) Negative y
C) Positive x
D) Negative x
E) Positive z

Question 33

A conducting strip of width 1.5 mm is in a magnetic field. As a result, there is a potential difference of 4.3 mV across the width of the strip. What is the magnitude of the electric field in the strip?

A) 0.35 V/m
B) 1.2 V/m
C) 1.9 V/m
D) 2.9 V/m
E) 6.4 V/m

Question 34

The figure shows the motion of electrons in a wire which is near the N pole of a magnet. The wire will be pushed:

A) toward the magnet
B) away from the magnet
C) downward
D) upward
E) along its length

Question 35

The figure shows a uniform magnetic field directed to the left and a wire carrying a current into the page. The magnetic force acting on the wire is:

A) toward the top of the page
B) toward the bottom of the page
C) toward the left
D) toward the right
E) zero

Question 36

An electron is travelling in the positive x direction. A uniform electric field is in the negative y direction. If a uniform magnetic field with the appropriate magnitude and direction also exists in the region, the total force on the electron will be zero. The appropriate direction for the magnetic field is:

A) the positive y direction
B) the negative y direction
C) into the page
D) out of the page
E) the negative x direction

Question 37

At one instant an electron is moving in the positive x direction along the x axis in a region where there is a uniform magnetic field in the positive z direction. When viewed from a point on the positive z axis, it subsequent motion is:

A) straight ahead
B) counterclockwise around a circle in the xy plane
C) clockwise around a circle in the xy plane
D) in the positive z direction
E) in the negative z direction

Question 38

An ion with a charge of +3.2 *10⁻¹⁹ C is in region where a uniform electric field of 5* 10^4. V/m is perpendicular to a uniform magnetic field of 0.8 T. If its acceleration is zero then its speed must be:

A) 0 m/s
B) 1.6 * 10^-5 m/s
C) 4.0 *10⁵ m/s
D) 6.3 *10⁵ m/s
E) any value but 0 m/s

Question 39

The current is from left to right in the conductor shown. The magnetic field is into the page and point S is at a higher potential than point T. The charge carriers are:

A) positive
B) negative
C) neutral
D) absent
E) moving near the speed of light

Question 40

A uniform magnetic field is directed into the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius as shown. A reasonable explanation is:

A) the charge is positive and slowing down
B) the charge is negative and slowing down
C) the charge is positive and speeding up
D) the charge is negative and speeding up
E) none of the above

Question 41

The diagrams show five possible orientations of a magnetic dipole in a uniform magnetic field . For which of these does the magnetic torque on the dipole have the greatest magnitude?

A) I
B) II
C) III
D) IV
E) V

Question 42

You are facing a loop of wire which carries a clockwise current of 3.0 A and which surrounds an area of 5.8 x 10⁻²m². The magnetic dipole moment of the loop is:

A) 3.0 A.m², into the page
B) 3.0 A.m², out of the page
C) 0.17 A.m², into the page
D) 0.17 A.m², out of the page
E) 0.17 A.m², left to right

Question 43

A current is clockwise around the outside edge of this page and a uniform magnetic field is directed parallel to the page, from left to right. If the magnetic force is the only force acting on the page, the page will rotate so the right edge:

A) moves toward you
B) moves away from you
C) moves to your right
D) moves to your left
E) does not move

Question 44

For a loop of current-carrying wire in a uniform magnetic field the potential energy is a minimum if the magnetic dipole moment of the loop is:

A) in the same direction as the field
B) in the direction opposite to that of the field
C) perpendicular to the field
D) at an angle of 45 $\degree$ to the field
E) none of the above

Question 45

The units of magnetic dipole moment are:

A) ampere
B) ampere.meter
C) ampere .meter²
D) ampere/meter
E) ampere/meter²

Question 46

A loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal sides, each 15 cm long. A 0.7 T uniform magnetic field is in the plane of the triangle and is perpendicular to the hypotenuse. The resultant magnetic force on the two equal sides has a magnitude of:

A) 0 N
B) 0.21 N
C) 0.30 N
D) 0.41 N
E) 0.51 N

Question 47

The diagrams show five possible orientations of a magnetic dipole in a uniform magnetic field . For which of these is the potential energy the greatest?

A) I
B) II
C) III
D) IV
E) V

Question 48

A loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal sides, each 15 cm long. A 0.7 T uniform magnetic field is parallel to the hypotenuse. The total magnetic force on the two equal sides has a magnitude of:

A) 0 N
B) 0.21 N
C) 0.30 N
D) 0.41 N
E) 0.51 N

Question 49

A coil of 1000 turns of wire has a radius of 12 cm and carries a counterclockwise current of 15A. If it is lying flat on the ground, and the Earth's magnetic field points due north, has a magnitude of 5.8 x 10^-5 T, and makes a downward angle of 25° with the vertical, what is the torque on the loop?

A) 1.7 x 10^-2 N·m west
B) 3.6 x 10^-2 N·m west
C) 1.7 x 10^-2 N·m east
D) 3.6 x 10^-2 N·m east
E) 3.6 x 10^-2 N·m south

Question 50

A loop of current-carrying wire has a magnetic dipole moment of 5.0 * 10^-4 A.m². If the dipole moment makes an angle of 57° with a magnetic field of 0.35 T, what is its potential energy?

A) -9.5 x 10^-5 J
B) -1.5 x 10^-4 J
C) -1.8 x 10^-4 J
D) +1.5 x 10^-4 J
E) +9.5 x 10^-5 J

Question 51

A circular loop of wire with a radius of 20 cm lies in the xy plane and carries a current of 2 A, counterclockwise when viewed from a point on the positive z axis. Its magnetic dipole moment is:

A) 0.25 A.m², in the positive z direction
B) 0.25 AF.m², in the negative z direction
C) 2.5 A.m², in the positive z direction
D) 2.5 A.m², in the negative z direction
E) 0.25 A.m², in the xy plane

Question 52

The magnetic dipole moment of a current-carrying loop of wire is in the positive z direction. If a uniform magnetic field is in the positive x direction the magnetic torque on the loop is:

A) zero
B) in the positive y direction
C) in the negative y direction
D) in the positive z direction
E) in the negative z direction

Question 53

A loop of current-carrying wire has a magnetic dipole moment of 5.0 * 10^-4 A.m². The moment initially is aligned with a 0.50-T magnetic field. To rotate the loop so its dipole moment is perpendicular to the field and hold it in that orientation, you must do work of:

A) 0 J
B) 2.5 * 10^-4 J
C) -2.5 * 10^-4 J
D) 1.0 *10^-3 J
E) -1.0 * 10^-3 J

Question 54

The magnetic torque exerted on a flat current-carrying loop of wire by a uniform magnetic field is:

A) maximum when the plane of the loop is perpendicular to
B) maximum when the plane of the loop is parallel to
C) dependent on the shape of the loop for a fixed loop area
D) independent of the orientation of the loop
E) such as to rotate the loop around the magnetic field lines

Question 55

A square loop of wire lies in the plane of the page and carries a current I as shown. There is a uniform magnetic field directed towards the top of the page, as indicated. The loop will tend to rotate:

A) about PQ with KL coming out of the page
B) about PQ with KL going into the page
C) about RS with MK coming out of the page
D) about RS with MK going into the page
E) about an axis perpendicular to the page