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Exam I, vers. 0001 - Physics 1120 - Fall, 2007

NAME (print please)__________________________________________

Student ID #__________________________________________________

TA’s Name(Circle one!): Edwin Widjonarko, Craig Hogle, Mike Mullan, Mike Hermele, Naresh Sen

Starting time of your Tues recitation (write time in box: 9am, noon, 1pm, etc)

Please do not open the exam until you are told to.Your exam should have 12 pages, numbered 1 thru 12.This exam consists of 18 multiple-choice questions worth 75 points total, and two multipart long-answer questions worth 25 points total. Fill in the bubble sheet with a #2 pencil.

PLEASE follow all directions carefully – if you don’t, we may not be able to identify your exam (which would be a disaster for you!) Place a check in each box on the left as you complete the instructions.

Circle your TA's name above.

Print and bubble in your name on the bubble sheet.

Print and bubble in your student Identification Number.

Print and bubble in your Exam version, 0001 or 0002, in the upper left of your bubble sheet in the area marked 1234.

Erase mistakes as thoroughly as possible. Ask for a fresh bubble sheet if you fear you cannot thoroughly erase mistakes.

After you begin, be sure to neatly print your name and ID on the long answer question pages.

At the end of the exam, check that you have filled in the first 18 questions on the bubble sheet, with only one bubble filled in for each question.

I have read and followed the instructions above.

Signature______________________________________________

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1. A small positive test charge +q flies by a large positive unmovable charge +Q along the trajectory shown. How do the magnitudes of the acceleration at points A, B, and C (which we denote as aA, aB, aC) compare with each other?

A) aC < aA < aB

B) aA = aB = aC

C) aA < aB < aC

D) aC < aA = aB

E) aC < aB < aA

2. The electric field throughout a region of space is given by the formula

€

r E = Axyˆ i + Bxˆ j where (x,y) are the coordinates of a point in space, and A, B are constants. What is

€

r E • ˆ j ?

A) Axy

B) Bx2 C) Bx D) B E) none of these

3. Three charges are placed as shown. If released, the charge at the bottom of the picture will initially start accelerating in what direction?

A) leftB) rightC) upD) downE) the acceleration is zero

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r

+Q !Q

r

+Q

A

C

B+q

+Q

up

right

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4. A balloon of mass M has been positively charged (so that its charge is Q) and placed next to a vertical blackboard, as shown. The blackboard polarizes, so the balloon sticks to it. The friction coefficient between the balloon and the blackboard is µ, and we will assume that the electric field created by the blackboard is uniform and directed towards the blackboard perpendicular to it. What is the magnitude E of this electric field?

A)

B)

C)

D)

E)

5. Three positive charges are placed as shown. If released, which way will the charge +q initially accelerate?

A) rightB) leftC) the acceleration is zeroD) downE) up

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E =Q

µMg

E =µQMg

E =MgµQ

E =µMgQ

E =µMg

Q

+2Q +q

2r r

+Q rightup

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6. A point charge with mass m and positive charge q is suspended on a massless string in a uniform horizontal electric field of magnitude E. The electric field is adjusted until the string makes a 45 degree angle with the vertical. Assuming we know E and q, find the mass of the object. (HINT: draw a free-body diagram.)

E

m, q

+

A) qE/g

B) qE

€

2 / g

C) g/(q E

€

2 )

D) qE/(g

€

2 )

E) None of these is correct.

7. A uniformly-charged insulating rod is bent in the shape of a semi-circle of radius R. The total charge on the rod is +Q. There are no other charges nearby. What can you say about the magnitude E of the electric field at the center of the semi-circle?

E=?

R

+Q

A) E= kQ/R2 B) E< Q/R2

C) E> kQ/R2 D) Not enough information to decide

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45!

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8. A uniformly charged rod with linear charge density λ is positioned vertically as shown, so that the distance between its lower end and the origin is L. What is the magnitude of the electric field |dE| at the origin (O), created just by a small piece of the rod of length dl located at a height H above the lower end of the rod?

9. Two boxes of unequal masses M1 and M2 have been placed on a flat frictionless surface some distance away from each other, and given unequal positive charges Q1 and Q2. The first box is heavier than the second, M1 > M2, however the charge on the first box is smaller, Q1 < Q2. Which of the following statements are true?

A) Both boxes start accelerating, and the first one (M1) has bigger acceleration.B) Both boxes start accelerating, and the second one (M2) has bigger acceleration.C) The accelerations are equal and nonzero. D) Neither of the boxes accelerate.E) None of the above

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d!E

O

H

L

x

y dl

D) |dE| = kL!dl

(H2 +L2)32

E) |dE| = k!dl

H2 +L2

M1, Q1 M2, Q2

A) |dE| = kdl

H2 +L2

B) |dE| = kd!

H2 +L2

C) |dE| = kdl

(H2 +L2)32

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10. Two rigid rods with charges attached at the end are arranged as shown. Do the rods repel each other, attract each other, or neither attract nor repel? (in answering this question, neglect gravity and friction)

A) attractB) repelC) Neither attract nor repel. If released, the rods

will remain stillD) Neither attract nor repel. If released, the

rods will nevertheless moveE) Impossible to tell from the information given

11. An electroscope (like the one shown on the picture) is negatively charged by touching it with a negative glass rod. The electroscope leaves spread apart and the glass rod is removed. Then a positively charged plastic rode is brought closer to the top of the electroscope, but it does not touch. What happens to the leaves as the positive rod starts to approach?

A) The leaves spread father apartB) The leaves get closer togetherC) One leaf moves higher, the other lowerD) The leaves don’t move

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!Q +Q

+Q !Q

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12. Two positive +Q charges and one negative -2Q charge are placed as shown on the picture. An electron of negative charge -e is placed as shown. Where is the force acting on the electron directed?

A) to the right (positive x direction)B) upwards (positive y direction)C) to the left (negative x direction)D) downwards (negative y

direction)E) the force is zero

13. Two large parallel planes are shown edge-on. The left plane has positive charge density η, while the right plane has negative charge density -η. How do the magnitudes of the electric field at points A, B, and C compare with each other?

A) EA < EB = EC

B) EA = EB > EC

C) EA > EB >EC

D) EA < EB < EC

E) EA < EC < EB

14. An electron, a particle with a negative charge -e, is injected horizontally into a box bounded by two charged plates as shown on the picture. The top plate is charged negatively, and the bottom plate is charged positively. Which trajectory will the electron follow?

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!2Q

r30!

+Q

+Q

30!

x

y

electron

A B C

! !!

E

A

!e

- - - - - - - - - - - - - - - - B

CD

+ + + + + + + + + + +

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15. An electric field is given by the expression

(given in units of N/C). What is the flux of the electric field though the surface shown on the picture (a horizontal square, sitting in the x-y plane, with a side of one meter long)?

A) -4 N m2/CB) 5 N m2/CC) 0 N m2/CD) -20 N m2/CE) Impossible to determine from the information given

16. An insulating sphere has been charged throughout its volume, with the charge density ρ. Subsequently the sphere is broken into two pieces, with the first piece smaller and the second piece larger. Is the charge density of the first piece ρ1 bigger or smaller than the charge density of the second piece ρ2?

A) ρ1 = ρ2

B) ρ1 > ρ2

C) ρ1 < ρ2

D) Impossible to tell with the information given

17. The picture shows the electric field lines in some region in space. Compare the magnitude of the electric field at points A, B, and C.

A) EA < EB < EC

B) EA < EB = EC

C) EA = EB > EC

D) EA > EB > EC

E) EA < EC < EB

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1m

1m x

y

z

!E = 5i!4 j.

A B C

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18. Four charges are arranged at the vertices of the square as shown. Which way does the electric field at the center point?

A) RightB) UpC) LeftD) DownE) The field is zero

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up

right

!2Q r

r

+Q r

+Q

!2Q

r

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Your Name ______________________________________Student Number__________________________________

Question 1. A dipole was placed in a box with the electric field lines shown in the figure below. In which direction will the dipole initially move? (Neglect gravity).(a) Please carefully CIRCLE the correction direction:

left, right, up, down, won’t move at all

In the space below, please explain your reasoning, briefly but clearly.

(b) Now the dipole has been rotated slightly, as shown in this figure. In the space below, first describe the initial (instantaneous) behavior of the dipole after it is released, and then also tell us what will happen to this dipole as time goes by. Explain your reasoning briefly but clearly.

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!E

!E

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Your Name ______________________________________Student Number__________________________________

Question 2. Two identical conducting balls, A and B, are attached to insulating stands. The net charge on ball A is +Q0, and there is no net charge on ball B.(a). In the diagram at right, sketch the charge distribution on ball B. (b). Balls A and B are observed to attract. Use your sketch (and a brief explanation in the space below) to account for this attraction.

A third identical conducting ball, ball C, with zero net charge is brought into contact with ball B. The distance between balls A and B does not change. (c). Does the attraction between balls A and B (circle one!)increase, decrease, or remain the same? Explain.

(d).We have redrawn ball C (only) (for your convenience) below: please indicate the direction of the electric force on ball C by ball A. If this force is zero, state that explicitly. Explain your reasoning.

(e). The three balls are arranged so that they all touch, as shown in the top view diagram. What is the sign and the magnitude of the charge on ball C? Explain.

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AB

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