___ the Motion of a Charged Particle in the Presence of a (Nearly Uniform) Magnetic Field

Physics 202Test Bank IIIFall 1999

___ The motion of a charged particle in the presence of a (nearly uniform) magnetic field tends to be

(A)helical (i.e. spiral), circulating about the magnetic field lines.

(B)straight line, except when the charges bounce off of a magnetic field line as in magnetic mirrors.

(C)rapidly slowing as the frictional effect of the magnetic force quickly uses up the particles kinetic energy.

(D) rapidly increasing in speed as the particle is accelerated by the magnetic force.

(E)completely unaffected by the magnetic field.

___ A negative charge moves south through a magnetic field directed north. The particle will be deflected

(A)North.

(B)Up.

(C)Down.

(D) East.

(E)not at all.

___ In order to use magnetic forces to levitate (i.e. magnetic force is upward against the force ofgravity) a horizontal wire carrying a current towards the east, the magnetic field must be directed

(A) Up.

(B) Down.

(D) East.

(E) South.

___ The force on an electron traveling eastward through a northward directed magnetic field is

(A) south.

(B) north.

(D) down.

(E) up.

___ A negative charge moves south through a magnetic field directed north. The particle will be deflected

(A)North.

(B)Up.

(C)Down.

(D) East.

(E)not at all.

___ The features (a) and (b) of the magnetization versus applied magnetic field plot at right are
(A) hysteresis and permanent magnetization, respectively.
(B) saturation and permanent magnetization, respectively.
(C) hysteresis and saturation, respectively.
(D) permanent magnetization and hysteresis, respectively.
(E) none of the above. /

___ The magnetic field of a long straight wire carrying a current into the page has field lines given by

(A) / (B)
(C) / (D)

(E) There is no magnetic field unless the current is changing.

___ A material which will weaken the net magnetic field (as compared to the same situation without the material) when an external field is applied is

(A) paramagnetic.

(B) ferromagnetic.

(D) quasimagnetic.

(E) paranormal.

___ A jet with a wingspan of 20m travels west at 1000 m/s through a region near the north magnetic pole where the magnetic field is 40x10-6 T downward. The magnitude of the motional emf induced across the wingspan is

(A) 0 V.

(B) 2x10-9 V.

(D) .8 V.

(E) none of the above.

___ Two long parallel wires are separated by .05 m and both carry a current of 10 A in the opposite directions. The force exerted on a 1m section of one wire is

(A) 4x10-3 N, towards the other wire.

(B) 4x10-3 N, away from the other wire.

(D) 4x10-4 N, along the wire, in the opposite direction of the current.

(E) none of the above.

___ The equation: is useful for a finite length of current carrying wire when

(A) when the distance from the wire is much larger than the distances from the ends of the wire.

(B) when the distance from the wire is much smaller than the distances from the ends of the wire.

(D) when the wire is carrying very small currents.

(E) when ever the instructor needs it to be.

___ A bar magnet is passed through a coil of wire. The induced current is greatest when
(A) the magnet moves quickly, so that it is inside the coil for a short time.
(B) the magnet moves slowly, so that it is inside the coil for a long time.
(C) the north pole enters the coil first.
(D) the south pole enters the coil first.
(E) never (no current is induced since the coil is not moving). /

___ Which property is associated with ferromagnetic materials?

(A)Strong increase of magnetic field within the material.

(B)Hysteresis.

(C)Saturation.

(D) Permanent magnetization.

(E)all of the above.

___ The equation:

(A) indicates that there is no magnetic field (B = 0).

(B) indicates that there is no magnetic charge (no isolated poles).

(D) indicates that Gauss's law does not work for magnetic fields.

___ A circular loop of wire is in a region of magnetic field which is uniform and increasing in strength, directed out of the page.
(A) There will be an induced current, which circulates clockwise.
(B) There will be an induced current, which circulates counter clockwise.
(C) There is insufficient information to determine the direction of the induced current flow.
(D) There will not be any induced current. /
___ A circular loop of wire is in a region of magnetic field which is uniform, directed into the page and increasing in strength with time,.
(A) There will be an induced current which circulates clockwise.
(B) There will be an induced current which circulates counter clockwise.
(C) There is insufficient information to determine the direction of the induced current flow.
(D) There will not be any induced current. /

[The same set of answers applies to the following 3 questions]

___ Which of the following is Ampere's law, which allows us to calculate magnetic fields in situations with a great deal of symmetry (such as around a long straight wire) ?

___ A displacement current (i.e. a changing electric field) creates a magnetic field :

___ How to create electric fields from changing magnetic flux:

(A) .

(B).

(D) .

(E) .

___ All magnetic fields have their origin in

(A) iron atoms.

(B) permanent magnets.

(D) moving electric charges.

(E)The origin of magnetic fields cannot be characterized in any simple manner.

___ The magnetic field at the center of the circular loop (shown at right) carrying a current clockwise will be directed
(A) into the page.
(B) out of the page.
(C) clockwise.
(D) counterclockwise.
(E) there is no magnetic field unless the current is changing. /

___ A circular loop of wire is in a region of magnetic field, which is uniform and constant, directed into the page.

(A) There will be an induced current which circulates clockwise.
(B) There will be an induced current which circulates counter clockwise.
(C) There is insufficient information to determine the direction of the induced current flow.
(D) There will not be any induced current. /

___ Lenz's Law, which characterizes induced currents in terms of a resistance to change in magnetic flux, was characterized by Dr. Gallis as

(A) electromagnetic friction.

(B) electromagnetic inertia.

(D) electromagnetic hocus pocus.

Part II Problems

A strip of potassium 2.0 cm wide and 1mm thick carrying a current of 100 A produces a Hall emf with magnitude 223 V in a magnetic field of 5.00 T.

(a) What is the density n of free electrons in potassium?
(b) What is the magnitude of the drift velocity of the electrons?
If the magnetic field is then decreased to 2.50 T,
(c) What is the Hall emf?
(d) What is the density n of free electrons in potassium? /

Charges are accelerated by an accelerating potential of 80 kV (of appropriate polarity for positive or negative charges) into a region of uniform magnetic field (directed out of the page).

If the charges are electrons,
(A) What is the speed of the electrons as they enter the magnetic field?
(B) Indicate the path of the electrons on the diagram at right (label the path e).
(C) What is the radius of the circular path taken by the electrons as they travel in the uniform magnetic field?
If the charges are protons:
(D) What is the speed of the protons as they enter the magnetic field?
(E) Indicate the path of the protons on the diagram at right (label the path p).
(F) What is the radius of the circular path taken by the protons as they travel in the uniform magnetic field? /

A mass spectrometer is constructed as shown by allowing particles to enter a velocity selector (with crossed Electric and Magnetic fields and then entering a region of uniform magnetic field only. The electric field within the velocity selector is 1x106 V/m and the magnetic field with both the velocity selector and mass spectrometer is .2T, directed out of the page (see diagram). The ions are deflected by the magnetic field, and traverse a semicircle of radius R, at the end of which they are detected

(a) In the figure at right, sketch in the trajectory of the ions within the magnetic field.
(b) For both C12 and C14 (two isotopes of carbon), calculate the speed v of the ions as they leave the accelerating potential and the radius R of the semicircular trajectories. The masses of C12 and C14 are 12u and 14u, respectively, where 1u = 1.66x10-27 Kg.
KEEP AT LEAST 3 SIGNIFICANT FIGURES THROUGH OUT THESE CALCULATIONS.
(c) How far apart are the endpoints of the semicircular trajectories? /

The figure is an end view of two long parallel wires perpendicular to the xy plane, each carrying a current I, the top is coming out of the page, the bottom is going into the page.

(a) On the diagram, show the contributions of to B from each wire, and the resultant B at the point P

(b) Derive an expression for the magnitude of the resultant B for any point on the x-axis in terms of the x-coordinate of the point, the y-coordinate of the wire a, and the current I.

The long straight wire in the figure shown carries a current of 20.0 A. The rectangular loop whose long edges are parallel to the wire carries a current of 8.00 A. The loop is 10 cm long, 4 cm wide and the left side of the loop is located 2 cm from the long straight wire. Find the magnitude and direction of the net magnetic force exerted on the loop by the magnetic field of the wire.

A conducting bar moves on conducting rails as shown. There is a uniform magnetic with magnitude .4tesla directed into the page. The bar is pushed to the right at a constant speed of 25 m/s. The resistance (which completes the loop) is 2 .

a) What is the EMF?
b) What is the size and direction (clockwise or counter clockwise) of induced current?
c) What is the power dissipated in the EMF?
d) What is the force on the current due to the magnetic field?
e) The (mechanical) force which must be applied to keep the bar moving is equal in size (opposite direction) to the magnetic force. Using this information, calculate the mechanical power which must be delivered to keep the bar moving. (recall from physics 201 that P =Fv) /

A square loop of wire with resistance R is moved at a constant speed v across a uniform magnetic field confined to a square region whose sides are twice the length of the square loop. (a) In the space provided, graph the magnetic flux through the loop as a function of the position of the loop (referenced to the front of the loop). The maximum flux has been determined for you. (b) In the second space provided make a qualitative graph the induced current as a function of position.