Magnetism HW-1 (“The Magnetic Force”): due______________

Read pp. 714 – 725 Concept Q’s (p. 742): 1 – 15 Prob’s (p. 744 ): 1, 3, 5, 8, 9, 10, 13, 14, 15, 17, 18, 19

Conceptual Questions 1. Two charged particles move at right angles to a magnetic field and deflect in opposite directions. Can one

conclude that the particles have opposite charges?

2. An electron moves with constant velocity through a region of space that is free of magnetic fields. Can one

conclude that the electric field is zero in this region? Explain.

3. An electron moves with constant velocity through a region of space that is free of electric fields. Can one

conclude that the magnetic field is zero in this region? Explain.

4. An electron moves west to east in the continental United States. What is the direction of the magnetic force

exerted on the electron by the Earth’s magnetic field?

5. Proton 1 moves with a speed v from the east coast to the west coast in the continental United States; proton 2

moves with the same speed from the southern United States toward Canada. Is the magnetic force experienced

by proton 2 greater than, equal to, or less than the force experienced by proton 1? Explain.

6. At a point near the equator, the Earth’s magnetic field is horizontal and points to the north. If an electron is

moving vertically upward at this point, what is the direction of the magnetic force acting on it?

7. Consider the paths of particles 1 and 2 in Figure 22–31. If these particles have the same charge and mass, which

is moving more rapidly?

Figure 22–31

8. Referring to Figure 22–31, give the sign of the charge for particles that move along paths 1, 2, and 3.

9. The particles in Figure Figure 22–31 have the same mass and speed. List the particles in order of increasing

magnitude of charge.

10. A proton is to orbit the Earth at the equator using the Earth’s magnetic field to supply part of the necessary

centripetal force. Should the proton move eastward or westward?

11. An electron moving in the positive x direction, at right angles to a magnetic field, experiences a magnetic force

in the positive y direction. What is the direction of the magnetic field?

12. Describe how the motion of a charged particle can be used to distinguish between an electric and a magnetic

field.

13. Explain how a charged particle moving in a circle of small radius can take the same amount of time to complete

an orbit as an identical particle orbiting in a circle of large radius.

14. A velocity selector is to be constructed using a magnetic field in the positive y direction. If charged particles

move through the selector in the positive z direction, what must be the direction of the electric field? Does your

answer depend on the sign of the charge on the particles? Explain.

15. The photograph (p. 743) shows an electron beam whose initial direction of motion is horizontal (to the left in the

plane of the page). A magnetic field deflects the beam downward. What is the direction of the magnetic field?

Problems Section 22–2 The Magnetic Force on Moving Charges

1. • What is the acceleration of a proton moving with a speed of 9.5 m/s at right angles to a magnetic field of 1.2

T?

3. • A negatively charged ion moves due north with a speed of 15 106. /× m s at the Earth’s equator. What is the

magnetic force exerted on this ion?

5. •• A 0 32. - Cµ particle moves with a speed of 16 m/s through a region where the magnetic field has a strength

of 0.95 T. At what angle to the field is the particle moving if the force exerted on it is (a) 4 8 10 6. ,× − N (b)

3 0 10 6. ,× − N or (c) 10 10 7. ?× − N

8. •• An electron moving with a speed of 91 105. /× m s in the positive x direction experiences zero magnetic

force. When it moves in the positive y direction, it experiences a force of 2 0 10 13. × − N that points in the

negative z direction. What is the direction and magnitude of the magnetic field?

9. •• I P Two charged particles with different speeds move one at a time through a region of uniform magnetic

field. The particles move in the same direction and experience equal magnetic forces. (a) If particle 1 has four

times the charge of particle 2, which particle has the greater speed? Explain. (b) Find the ratio of the speeds,

v v1 2/ .

10. •• A 6 60. - Cµ particle moves through a region of space where an electric field of magnitude 1250 N/C points

in the positive x direction, and a magnetic field of magnitude 1.02 T points in the positive z direction. If the net

force acting on the particle is 6 23 10 3. × − N in the positive x direction, find the magnitude and direction of the

particle’s velocity. Assume the particle’s velocity is in the x-y plane.

Section 22–3 The Motion of Charged Particles in a Magnetic Field

13. • Find the radius of a proton’s orbit when it moves perpendicular to a magnetic field of 0.55 T with a speed of

6 27 105. / .× m s 14. • Charged particles pass through a velocity selector with electric and magnetic fields at right angles to each

other, as shown in Figure 22–36. If the electric field has a magnitude of 450 N/C and the magnetic field has a

magnitude of 0.12 T, what speed must the particles have to pass through the selector undeflected?

15. • The velocity selector in Figure 22–37 is designed to allow charged particles with a speed of 4 5 103. /× m s to

pass through undeflected. Find the direction and magnitude of the required electric field, given that the

magnetic field has a magnitude of 0.96 T.

17. •• An electron accelerated from rest through a voltage of 410 V enters a region of constant magnetic field. If

the electron follows a circular path with a radius of 17 cm, what is the magnitude of the magnetic field?

18. •• A 12 5. - Cµ particle with a mass of 2 80 10 5. × − kg moves perpendicular to a 1.01-T magnetic field in a

circular path of radius 26.8 m. (a) How fast is the particle moving? (b) How long will it take the particle to

complete one orbit?

19. •• I P When a charged particle enters a region of uniform magnetic field it follows a circular path, as

indicated in Figure 22–38. (a) Is this particle positively or negatively charged? Explain. (b) Suppose that the

magnetic field has a magnitude of 0.180 T, the particle’s speed is 6 0 106. / ,× m s and the radius of its path is

52.0 cm. Find the mass of the particle, given that its charge has a magnitude of 160 10 19. .× − C Give your result

in atomic mass units, u, where 1 167 10 27 u kg= × −. .

Figure 22–36 Figure 22–37 Figure 22–38

Answers 1. a = 1.1 x 10

9 m/s

2

3. FB = 0

5. (a) θ = 81° (b) θ = 38° (c) θ = 1.2°

8. B = 1.4 T in the negative x-direction

9. (a) Particle 2 must have a greater speed, because…explain. (b) 1

2

1

4

v

v=

10. v = 300. m/s in the negative y-direction

13. r = 1.2 cm

14. v = 3.8 km/s

15. E = 4.3 x 103 N/C in the negative y-direction

17. B = 0.40 mT

18. (a) v = 12.1 m/s (b) t = 13.9 sec

19. (a) Negatively charged…explain (b) m = 1.49 u (m = 2.50 x 10−27

kg)