Deck 12: Magnetic Fields

Question 1

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 1

must be perpendicular to both

Question 2

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

Answer 2

if the particle is moving across the field lines

Question 3

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) down
D) east
E) west

Answer 3

zero

Question 4

A 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 enters a region of uniform perpendicular 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 has magnitude B/E
D) is perpendicular to both and has magnitude E/B
E) the given situation is impossible

Question 7

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 8

An electron is launched with velocity in a uniform magnetic field The angle $\theta$ between 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) 2 $\pi$ m/eB
B) 2 $\pi$ mv/eB
C) 2 $\pi$ mv sin $\theta$ /eB
D) 2 $\pi$ mv cos $\theta$ /eB
E) none of these

Question 9

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 10

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 11

The diagrms 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