Question 1

A segment of wire carries a current of 25 A along the x axis from x = −2.0 m to x = 0 and then along the z axis from z = 0 to z = 3.0m. In this region of space, the magnetic field is equal to 40 mT in the positive z direction. What is the magnitude of the force on this segment of wire?

Accepted Answer

A)  1.0 N 
B)  5.0 N 
C)  2.0 N 
D)  3.6 N 
E)  3.0 N

Question 2

A particle (q = −4.0 μC, m = 5.0 mg) moves in a uniform magnetic field with a velocity having a magnitude of 2.0 km/s and a direction that is 50° away from that of the magnetic field. The particle is observed to have an acceleration with a magnitude of 5.8 m/s2. What is the magnitude of the magnetic field?

Accepted Answer

A)  5.3 mT 
B)  4.9 mT 
C)  5.1 mT 
D)  4.7 mT 
E)  3.6 mT

Question 3

What is the magnetic force on a 2.0-m length of (straight) wire carrying a current of 30 A in a region where a uniform magnetic field has a magnitude of 55 mT and is directed at an angle of 20° away from the wire?

Accepted Answer

A)  1.5 N 
B)  1.3 N 
C)  1.1 N 
D)  1.7 N 
E)  3.1 N

Question 4

A particle (mass = 2.0 mg, charge = −6.0 μC) moves in the positive direction along the x axis with a velocity of 3.0 km/s. It enters a magnetic field of (  ) mT. What is the acceleration of the particle?

Accepted Answer

A)  (36
 − 27
 ) m/s2 
B)  (−36
 + 27
 ) m/s2 
C)  (−24
 + 18
 ) m/s2 
D)  (24
 − 18
 ) m/s2 
E)  (24
 − 27
 ) m/s2

Question 5

The figure shows the orientation of a flat circular loop consisting of 50 closely wrapped turns each carrying a current I. The magnetic field in the region is directed in the positive z direction and has a magnitude of 50 mT. The loop can turn about the y axis. If θ = 20°, R = 0.50 m, and I = 12A, what is the magnitude of the torque exerted on the loop?

Accepted Answer

A)  8.1 N ⋅ m 
B)  24 N ⋅ m 
C)  22 N ⋅ m 
D)  13 N ⋅ m 
E)  16 N ⋅ m

Question 6

A straight 10-cm wire bent at its midpoint so as to form an angle of 90° carries a current of 10 A. It lies in the xy plane in a region where the magnetic field is in the positive z direction and has a constant magnitude of 3.0 mT. What is the magnitude of the magnetic force on this wire?

Accepted Answer

A)  3.2 mN 
B)  2.1 mN 
C)  5.3 mN 
D)  4.2 mN 
E)  6.0 mN

Question 7

What is the kinetic energy of an electron that passes undeviated through perpendicular electric and magnetic fields if E = 4.0 kV/m and B = 8.0 mT?

Accepted Answer

A)  0.65 eV 
B)  0.71 eV 
C)  0.84 eV 
D)  0.54 eV 
E)  1.4 eV

Question 8

A proton moves around a circular path (radius = 2.0 mm) in a uniform 0.25-T magnetic field. What total distance does this proton travel during a 1.0-s time interval? (m = 1.67 × 10−27 kg, q = 1.6 × 10−19 C)

Accepted Answer

A)  82 km 
B)  59 km 
C)  71 km 
D)  48 km 
E)  7.5 km

Question 9

A magnetic field of 2.00 T is applied to a bubble chamber to make the tracks of protons and other charged particles identifiable by the radius of the circles they move in. If a high-energy proton moves along an arc of a 3.30-m circle, what is the momentum of the proton? [q = 1.60 × 10−¹⁹ C, m = 1.67 × 10−²⁷ kg]

Accepted Answer

The answer of A magnetic field of 2.00 T is...

Question 10

Bert says that a charged particle in a vacuum can travel in a helix only if a uniform electric field and a uniform magnetic field are both present and both parallel to the axis of the helix. Stuart says that only a magnetic field with a component parallel to the axis of the helix is needed. Which one, if either, is correct, and why?

Accepted Answer

A)  Bert, because the charged particle's velocity can have a vertical component only if an electric field in the vertical direction is present. 
B)  Stuart, because a component of velocity in the vertical direction is not changed by a vertical component of a magnetic field. 
C)  Bert, because a component of velocity in the vertical direction is changed by a vertical component of a magnetic field. 
D)  Stuart, because an electric field in the vertical direction would cause the particle to come to a complete stop. 
E)  Neither, because particles cannot move in helical paths in the presence of magnetic and electric fields.

Question 11

A coaxial cable has an inner cylindrical conductor surrounded by cylindrical insulation and an outer cylindrical conducting shell. The outer shell carries the same current but in the opposite direction from that in the inner conductor as shown. If the coaxial cable sits in a uniform magnetic field directed upwards with respect to the cable, the effect of the field on the cable is

Accepted Answer

A)  a net force to the left. 
B)  a net force to the right. 
C)  a net force upwards. 
D)  no net force but a slight shift of the inner conductor to the left and the outer conductor to the right. 
E)  no net force but a slight shift of the inner conductor to the right and the outer conductor to the left.

Question 12

A positively charged particle has a velocity in the negative z direction at point P. The magnetic force on the particle at this point is in the negative y direction. Which one of the following statements about the magnetic field at point P can be determined from this data?

Accepted Answer

A)  Bx is positive. 
B)  Bz is positive. 
C)  By is negative. 
D)  By is positive. 
E)  Bx is negative.

Question 13

An electron moves in a region where the magnetic field is uniform and has a magnitude of 80 μT. The electron follows a helical path which has a pitch of 9.0 mm and a radius of 2.0 mm. What is the speed of this electron as it moves in this region?

Accepted Answer

A)  48 km/s 
B)  28 km/s 
C)  20 km/s 
D)  35 km/s 
E)  8.0 km/s

Question 14

An unusual lightning strike has a vertical portion with a current of −400 A downwards. The Earth's magnetic field at that location is parallel to the ground and has a magnitude of 30 μT. In N, the force exerted by the Earth's magnetic field on the 25 m-long current is

Accepted Answer

A)  0. 
B)  0.012 N, East. 
C)  0.012 N, West. 
D)  0.30 N, West. 
E)  300 N, East.

Question 15

A velocity selector uses a fixed electric field of magnitude E and the magnetic field is varied to select particles of various energies. If a magnetic field of magnitude B is used to select a particle of a certain energy and mass, what magnitude of magnetic field is needed to select a particle of equal mass but twice the energy?

Accepted Answer

A)  0.50 B 
B)  1.4 B 
C)  2.0 B 
D)  0.71 B 
E)  1.7 B

Question 16

The point P lies along the perpendicular bisector of the line connecting two long straight wires S and T perpendicular to the page. A set of directions A through H is shown next to the diagram. When the two equal currents in the wires are directed up out of the page, the direction of the magnetic field at P is closest to the direction of

Accepted Answer

A)  E. 
B)  F. 
C)  G. 
D)  H. 
E)  A.

Question 17

An electron moving in the positive x direction experiences a magnetic force in the positive z direction. If Bx = 0, what is the direction of the magnetic field?

Accepted Answer

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

Question 18

A 2.0-C charge moves with a velocity of (  ) m/s and experiences a magnetic force of (  ) N. The x component of the magnetic field is equal to zero. Determine the z component of the magnetic field.

Accepted Answer

A)  −3.0 T 
B)  +3.0 T 
C)  +5.0 T 
D)  −5.0 T 
E)  +6.0 T

Question 19

One reason why we know that magnetic fields are not the same as electric fields is because the force exerted on a charge +q

Accepted Answer

A)  is in opposite directions in electric and magnetic fields. 
B)  is in the same direction in electric and magnetic fields. 
C)  is parallel to a magnetic field and perpendicular to an electric field. 
D)  is parallel to an electric field and perpendicular to a magnetic field. 
E)  is zero in both if the charge is not moving.

Question 20

What current must be maintained in a square loop (50 cm on a side) to create a torque of 1.0 N ⋅ m about an axis through its center and parallel to one of its sides when a magnetic field of magnitude 70 mT is directed at 40° to the plane of the loop?

Accepted Answer

A)  66 A 
B)  89 A 
C)  61 A 
D)  75 A 
E)  37 A

Exam 29: Magnetic Fields

A segment of wire carries a current of 25 A along the x axis from x = −2.0 m to x = 0 and then along the z axis from z = 0 to z = 3.0m. In this region of space, the magnetic field is equal to 40 mT in the positive z direction. What is the magnitude of the force on this segment of wire?

What is the magnetic force on a 2.0-m length of (straight) wire carrying a current of 30 A in a region where a uniform magnetic field has a magnitude of 55 mT and is directed at an angle of 20° away from the wire?

A particle (mass = 2.0 mg, charge = −6.0 μC) moves in the positive direction along the x axis with a velocity of 3.0 km/s. It enters a magnetic field of ( ) mT. What is the acceleration of the particle?