white (cw32974) – HW6 - Magnetism – fabi – (54328) 1

This print-out should have 27 questions.Multiple-choice questions may continue onthe next column or page – find all choicesbefore answering.

001 10.0 points

An electron is in a uniform magnetic fieldB that is directed into the plane of the page,as shown.

ve−

B

B

B

B

When the electron is moving in the planeof the page in the direction indicated by thearrow, the force on the electron is directed

1. into the page.

2. toward the left

3. toward the right

4. toward the bottom of the page.

5. out of the page.

6. toward the top of the page.

002 10.0 pointsAn electron in a vacuum is first accelerated

by a voltage of 31200 V and then enters aregion in which there is a uniform magneticfield of 0.607 T at right angles to the directionof the electron’s motion.The mass of the electron is 9.11× 10−31 kg

and its charge is 1.60218× 10−19 C.What is the magnitude of the force on the

electron due to the magnetic field?Answer in units of N

003 10.0 pointsAn alpha particle (the nucleus of a helium

atom, carrying a charge of 3.2× 10−19 Cmoves at 5.8× 107 m/s at a right angle toa magnetic field.If the particle experiences a force of

4.7× 10−14 N due to the magnetic field, whatis the magnitude of the magnetic field? Thecharge on a proton is 3.2× 10−19 C.Answer in units of T

004 10.0 pointsThe figure represents two long, straight, par-allel wires extending in a direction perpendic-ular to the page. The current in the right wireruns into the page and the current in the leftruns out of the page.

a b c

What is the direction of the magnetic fieldcreated by these wires at location a, b and c?(b is midway between the wires.)

1. up, down, up

2. up, zero, down

3. up, up, down

4. down, down, up

5. down, up, down

6. down, zero, up

005 10.0 pointsA wire carrying a current 40 A has a length0.1 m between the pole faces of a magnet atan angle 60 ◦ (see the figure). The magneticfield is approximately uniform at 0.5 T. Weignore the field beyond the pole pieces.

white (cw32974) – HW6 - Magnetism – fabi – (54328) 2

ℓθ

I

B

What is the force on the wire?Answer in units of N

006 10.0 pointsThe two wires of length 2 m are 3 mm apartand carry a current of 10 A dc.Calculate the force between these wires.Answer in units of N

007 10.0 pointsA thin 2.39 m long copper rod in a uniformmagnetic field has a mass of 57.9 g. Whenthe rod carries a current of 0.269 A directedperpendicular to the magnetic field, it floatsin the magnetic field.The acceleration of gravity is 9.81 m/s2 .What is the field strength of the magnetic

field?Answer in units of T

008 10.0 pointsA small rectangular coil composed of 76 turnsof wire has an area of 16 cm2 and carries acurrent of 1.3 A. When the plane of the coilmakes an angle of 43◦ with a uniform magneticfield, the torque on the coil is 0.16 Nm.What is the magnitude of the magnetic

field?Answer in units of T

009 (part 1 of 3) 10.0 pointsAn alpha particle has a mass of 5.7× 10−27 kgand bears a double elementary positivecharge. Such a particle is observed to movethrough a 4.3 T magnetic field along a circularpath of radius 0.18 m.The charge on a proton is 1.60218×10−19 C.What speed does it have?Answer in units of m/s

010 (part 2 of 3) 10.0 pointsWhat is its kinetic energy?Answer in units of J

011 (part 3 of 3) 10.0 pointsWhat potential difference in MV would berequired to give it this kinetic energy?Answer in units of MV

012 (part 1 of 2) 10.0 points

A particle of mass 1.1952 × 10−25 kg andcharge of 4.8×10−19 C is accelerated from restin the plane of the page through a potentialdifference of 437 V between two parallel platesas shown. The particle is injected through ahole in the right-hand plate into a region ofspace containing a uniform magnetic field ofmagnitude 0.0802 T. The particle curves in asemicircular path and strikes a detector.

q

m

Region ofMagnetic

Field B

E

hole

Which way does the magnetic field point?

1. toward the lower left corner of the page

2. toward the lower right corner of the page

3. to the left

4. toward the upper left corner of the page

5. to the right

6. into the page

7. toward the bottom of the page

8. toward the upper right corner of the page

9. toward the top of the page

white (cw32974) – HW6 - Magnetism – fabi – (54328) 3

10. out of the page

013 (part 2 of 2) 10.0 pointsWhat is the magnitude of the force exerted onthe charged particle as it enters the region ofthe magnetic field ~B ?Answer in units of N

014 (part 1 of 2) 10.0 pointsConsider the passage of a horizontal electronbeam through a velocity selector, where theelectric field ~E points downward and the mag-netic field ~B points into the paper.

v

E

B

Based on the Lorentz force law, what isthe critical speed vc (the speed at which thehorizontal electron beam is not deflected)?

1. vc = B2E

2. vc =B

E2

3. vc =1

BE

4. vc =E

B

5. vc =E

B2

6. vc = BE

7. vc =B2

E

8. vc =B

E

9. vc = BE2

10. vc =E2

B

015 (part 2 of 2) 10.0 pointsIf the speed of the electrons is greater than thecritical speed, the trajectory of the electron

beam will be

1. deflected downward.

2. deflected out of the paper.

3. deflected downward and out of the pa-per.

4. deflected upward and into the paper.

5. deflected upward and out of the paper.

6. deflected downward and into the paper.

7. undeflected.

8. deflected upward.

9. deflected into the paper.

016 (part 1 of 2) 10.0 pointsAn electron in an electron beam experiencesa downward force of 1.5× 10−14 N while trav-eling in a magnetic field of 8.9× 10−2 T west.The charge on a proton is 1.60×10−19.a) What is the magnitude of the velocity?Answer in units of m/s

017 (part 2 of 2) 10.0 pointsb) What is its direction?

1. West

2. North

3. None of these

4. South

5. East

018 10.0 pointsCan an electron at rest in a magnetic field beset into motion by the magnetic field? Whatif it were at rest in an electric field?

1. no; yes

white (cw32974) – HW6 - Magnetism – fabi – (54328) 4

2. None of these

3. It depends on the intensity of the fields,which is not provided in the problem.

4. yes; no

5. no for both

6. yes for both

019 10.0 pointsA charged particle is projected with its initialvelocity parallel to a uniform magnetic field.What is the resulting path?

1. circular arc.

2. spiral.

3. parabolic arc.

4. straight line parallel to the field.

5. straight line perpendicular to the field.

020 10.0 pointsA friend tells you that a refrigerator door,beneath its layer of white painted plastic, ismade of aluminum.How could you check to see if this is true

(without any scraping)?

1. Apply a small magnet to the door. If itsticks, your friend is wrong because aluminumis not magnetic. If it doesn’t stick, your friendmight be right, (but not necessarily).

2. Burn the refrigerator door and then an-alyze the remains with chemical reagents tofind if aluminum is present.

3. Apply a small magnet to the door. Ifit sticks, your friend might be right becausealuminum is magnetic. If it doesn’t stick,your friend might be wrong.

4. It cannot be checked without scraping thepainted plastic.

021 10.0 pointsThe direction of the magnetic field in a certainregion of space is determined by firing a testcharge into the region with its velocity invarious directions in different trials.The field direction is

1. the direction of the velocity when themagnetic force is a maximum.

2. None of these

3. the direction of the magnetic force.

4. one of the direction of the velocity whenthe magnetic field is zero.

5. perpendicular to the velocity when themagnetic force is zero.

022 10.0 pointsA positively charged particle passes through alaboratory traveling in an easterly direction.There are both electric and magnetic fieldin the room and their effects on the chargedparticle cancel.If the electric field points upward, what

must be the direction of the magnetic field?

1. upward

2. north

3. south

4. downward

5. east

6. west

023 (part 1 of 2) 10.0 pointsA uniform 1.3 T magnetic field points north.If an electron moves vertically downward (to-ward the ground) with a speed of 4.8 × 107

m/s through this field.The charge on a proton is 1.60×10−19.a)What is the magnitude of the force acting

white (cw32974) – HW6 - Magnetism – fabi – (54328) 5

on it?Answer in units of N

024 (part 2 of 2) 10.0 pointsb) What is the direction of the force?

1. South

2. None of these

3. East

4. West

5. North

025 (part 1 of 2) 10.0 pointsIn a nuclear research laboratory, a protonmoves in a particle accelerator through a mag-netic field of intensity 0.187 T at a speed of4.25× 107 m/s.The charge of a proton is

1.60218× 10−19 C.If the proton is moving perpendicular to the

field, what force acts on it?Answer in units of N

026 (part 2 of 2) 10.0 pointsIf the proton of mass 1.67262× 10−27 kg con-tinues to move in a direction that is consis-tently perpendicular to the field, what is theradius of curvature of its path?.Answer in units of m

027 10.0 pointsA circular coil consisting of a single loop ofwire has a radius of 26.1 cm and carries acurrent of 34.5 A. It is placed in an externalmagnetic field of 0.457 T.Find the magnitude of the torque on the

wire when the plane of the coil makes anangle of 53.1◦ with the direction of the field.Answer in units of N ·m