le1 sample exam

8/18/2019 LE1 Sample Exam

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University of the Philippines

College of Science

Physics 72

Set A

First Long Exam

First Semester, AY 2014–2015

Name: Instructor:

Section/Class Schedule: Student Number:

Course: College:


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First Long Exam First Semester, AY 2014–2015 Physics 72

INSTRUCTIONS: Choose the best answer and shade the corresponding circle on your answer

sheet. To change your answer, cross-out and sign your original answer and then shade your

new answer. No computational devices allowed (e.g. calculators, mobile phones). Any form

of cheating in examinations or any act of dishonesty in relation to studies, such as plagiarism,

shall be subject to disciplinary action. Following instructions is part of the exam.

Useful formulas:

Area Volume

Sphere (radius=r) 4πr2 43

πr3

Cylinder (radius=r, height=h) 2πr2 + 2πrh πr2h

sin(π/3) = cos(π/6) =√

3/2cos(π/3) = sin(π/6) = 1/2

sin(π/4) = cos(π/4) = √ 2/2

Useful constants:

e −1.60× 10−19 Cme 9.1× 10−31 kg0 8.854× 10−12 C2 /Nm2k 8.988× 109 Nm2 /C2

1. Electric Field. Given the charges +Q and −2Q, which of the following diagrams shows acorrect representation of the electric field lines?

A. B.

C.

D.

E.

2. Coulomb Force. What is the net Coulomb force on

charge +Q3 due to the other two identical charges +Qas shown in the figure?

A. kQQ3s2

(1 +√ 2

2 )̂ı − kQQ3

s2

√ 2

2 ˆ

B. kQQ3s2

(1 +√ 2

2 )̂ı + kQQ3

s2

√ 2

2 ˆ

C. 2kQQ3s2

ı̂ − kQQ3s2

ˆ

D. kQQ3s2

ı̂ + kQQ3s2

ˆ

E. kQQ3s2 (1 + √ 2

4 )̂ı − kQQ3s2 √ 24 ˆ

A – 2


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3. Die Paul, die! Which of the following is a correct de-

scription of the motion of the dipole immersed in a uni-

form electric field as shown in the figure?

A. It will rotate clockwise.

B. It will rotate counterclockwise.

C. It is static but unstable

D. It is static and stable.

E. It will move to the right.

4. Electric Failed Lines. Consider the electricfield lines shown in the figure. Which of the

following is true about the charges P , Q, andR?

A. P , Q, and R are all negative

B. P = Q = R

C. Q


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6. Dipole on a uniform electric field. A dipole is placed

along the x-axis as shown. Which of the following elec-tric field(s) will give the minimum potential energy?

I. E = E 0ı̂

II. E = −E 0ı̂III. E = E 0ˆ

A. I only

B. II only

C. III only

D. I and II only

E. I and III only

7. Coaxial Cable. Two very long coaxial cylindrical shells have radii a < b as shown. Theinner shell has a uniform linear charge density +λ while the outer shell has no net charge.Let E bet be the electric field at the region between the cylinders (a < r < b) and E out bethe electric field at the outermost region (r > b). Which of the following is true about themagnitudes of the electric fields?

A. E bet < E out

B. E bet = E out > 0

C. E bet > E out

D. E bet = E out = 0

E. E bet = 0, E out > 0

8. Conducting Sphere & Shell. Consider a solid sphere

inside a spherical conducting shell as shown. The solidsphere has charge +4Q and the spherical shell has charge+5Q. What is the electric field in the region between thesphere and shell?

A. 0

B. kQ/r2

C. +4kQ/r2

D. +5kQ/r2

E. +9kQ/r2

A – 4


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9. Rotating E-field. Consider a sheet of area A with normal vector

n̂ as shown. It is placed in a uniform electric field of magnitude

E . Which of the following is the correct relationship of the electricflux through the sheet if the electric field is rotated in three different

directions I, II and III?

A. I > II > III

B. I > III > II

C. III = I > II

D. III > I = II

E. I = II = III

For the next two items, consider five point charges enclosed by five different surfaces I, II, III, IV and V where Q > 0.

10. Flux of my heart. What is the electric flux through sur-

face II?

A. 2Q/0

B. Q/0

C. zero

D. −Q/0

E.

−2Q/0

11. Largest flux. Which surface has the largest electric flux?

A. I B. II C. III D. IV E. V

12. Bear flux. Consider a charged bear of irregular shape enclosed by a Gaussian sphere of

radius R. Which of the following statement(s) is/are true?

I. Gauss’ law cannot be used to calculate the electric flux through the surface of the

sphere.

II. The electric flux through the sphere is smaller when the bear is moved a distance R/4from the center but still inside the sphere.

III. The electric flux through the sphere is proportional to the charge of the bear.

A. I only

B. II only

C. III only

D. I and II only

E. II and III only

A – 5


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13. Dipole inside. A dipole with charges of magnitude 2 nC is placed inside the cavity of asolid conductor. The solid conductor has a net charge of +5nC. What is the total chargeat the outer surface of the conductor?

A. +1 nC B. +2nC C. +3nC D. +4nC E. +5nC

For the next two items, consider the equipotential curves for

the point charges q 1 and q 2 as shown.

14. Charge Nature. What is the nature of the charges?

A. |q 1| < |q 2|, same signsB. |q 1| = |q 2|, same signsC. |q 1| > |q 2|, same signsD. |q 1| < |q 2|, opposite signsE. |q 1| = |q 2|, opposite signs

15. EField. Which of the following is the correct electric field vector at point P ?

A. ↑ B. ↓ C. ← D. → E.

16. Potential Gradient. Consider the potential V (x,y,z ) = A + Bx − Cxy2z 3 at a certainregion in space where A, B, and C are constants. Which of the following is the electricfield E(x,y,z )?

A. E = (Bx + Cxy2z 3)̂ı + (Cxy2z 3)ˆ + (Cxy2z 3)k̂

B. E = −(Bx − Cxy2z 3)̂ı − (−Cxy2z 3)ˆ − (−Cxy2z 3)k̂ C. E = (Bx − Cxy2z 3)̂ı + (−Cxy2z 3)ˆ + (−Cxy2z 3)k̂ D. E =

−(B

−Cy2z 3)̂ı

−(

−2Cxyz 3)ˆ

−(

−3Cxy2z 2)k̂

E. E = (B − Cy2z 3)̂ı + (−2Cxyz 3)ˆ + (−3Cxy2z 2)k̂

A – 6


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17. Equipotential. Consider the spherical equipotential sur-

faces of a positive point charge as shown in the fig-

ure. The potential difference between the surfaces is

measured to be 2.00 V. Another positive test chargeq = 1.00nC is brought from A to C, and then to B. Whatis the total work done by the electric field in moving the

test charge?

A. 2.00nJ

B. 1.00nJ

C. zero

D. −1.00nJE. −2.00nJ

18. Paths. Consider an electron moving along one of the

three paths shown. The correct increasing order of the

work done by the electric field on the electron is

A. 2, 3, 1

B. 3, 1, 2C. 2, 1, 3

D. 1, 2, 3

E. 1, 3, 2

19. 4charge. Four identical point charges with charge +q are placed at the corners of a square as shown. What is

the electric potential at the center of the square P?A. 4kq

d

B. 2√ 2kq

d

C. zero

D. −2√ 2kq

d

E. −4kq d

A – 7


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20. Solid. Consider a solid spherical conductor with radius R andcharge −Q at electrostatic equilibrium. What is the correct rank-ing of the Points A, B, C and D according to the values of theirelectric potential?

A. A = B < C = D

B. A = B > C > D

C. A = B < C < D

D. A > B > C = D

E. A < B < C < D

21. Dielectric. Consider an isolated capacitor with initial charge Q. Which of the followingis/are true about the effect of inserting an uncharged dielectric between conducting plates?

I. The magnitude of charge on each plate remains the same.

II. The capacitance decreases.

III. The electric field between the plates decreases.

A. I only

B. II only

C. III only

D. I and III only

E. II and III only

22. Capacitor Network. What is the equivalent capacitance of

the capacitor network shown in the figure?

A. 73

C

B. 3

7C C. 6

5C

D. 56

C

E. 5C

A – 8


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23. Potential Difference. What is the potential difference

between points A and B in the circuit shown?

A. 0

B. 13

V

C. 12

V

D. 23V

E. V

24. Connected Capacitors. Two initially uncharged capacitors with

capacitance C and 3C are connected to a constant voltage source

V as shown in the figure. What is the total stored energy in bothcapacitors at equilibrium?

A. 12

CV 2

B. CV 2

C. 32

CV 2

D. 2CV 2

E. 4CV 2

25. Compressed Plates. A parallel plate capacitor consists of conducting plates with equal

areas, separated by some distance D. The capacitor is connected to a constant voltagesource. Which of the following statements is/are true if we decrease the separation distance

between the capacitor plates, while it is connected to the voltage source?

I. The capacitance will decrease.

II. The stored charge will increase.

III. The stored energy will stay the same.

A. I only

B. II only

C. III only

D. I and II only

E. II and III only

For the next two items, consider a parallel plate capacitor with a capacitance of 10mF and charge density σ. The separation distance between the plates is 10 mm. A potential differenceof 500 V is applied across the capacitor.

26. Area . What is the area of each plate (in terms of 0)?

A – 9


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A. 10

(1× 10−4) m2B. 1

0(1× 10−5) m2

C. 0(1× 10−4) m2

D. 0(1× 10−5) m2E. 1

0(1× 10−6) m2

27. What the E.F. What is the electric field magnitude between the plates?

A. 500 kV/m

B. 50 kV/m

C. 0.02 kV/m

D. 0.2 kV/m

E. 100 kV/m

28. Hatit-Taas. A conductor has resistivity ρ, length L, radius r and resistance R. Which of the following changes on the conductor will reduce its resistance by 1/4?

I. Double both the resistivity and radius.

II. Halve the length and double the radius.

III. Double the radius.

A. I only

B. II only

C. III only

D. I and II only

E. II and III only

29. Curly Tops. Which of the following statements is/are TRUE about the I-V curve of an

Ohmic material?

I. The plot is exponentially increasing.

II. The plot is linearly increasing.

III. The plot is linearly decreasing.

A. I only

B. II onlyC. III only

D. I and II only

E. II and III only

For the next two items, consider a 10.0 Ω resistor connected to a power supply of unknownvoltage and without internal resistance.

30. Ipaglaban Mo. If a 1.50 A current flows through the resistor, what is the voltage acrossthe resistor?

A – 10


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A. 7.50 V

B. 15.0 V

C. 22.5 V

D. 30.0 V

E. 45.0 V

31. Vary-Vary. If the resistance is doubled, what will happen to the power dissipated?

A. Quartered

B. Quadrupled

C. Halved

D. Doubled

E. Remains constant

32. Omega Rho. The resistivity of Metal X was found to increase linearly by 20% (ρ1/ρ0 =1.20) as its temperature increases from T 0 to T 1. Given a piece of wire made of pure MetalX, if its dimensions and the voltage across it are kept constant, what fraction of the current

I 0 at temperature

T 0 remains after the temperature is increased to

T 1?

A. I 1 = 1

6I 0

B. I 1 = 5

6I 0

C. I 1 = 1

5I 0

D. I 1 = 4

5I 0

E. I 1 = 6

5I 0

33. Current density. Given a system of n positive charges moving in a wire with diameter D,drift velocity vd, and current density J. Which of the following is the drift velocity of a

system moving in a wire with diameter 12

D having the same magnitude of J?

A. 1

4vd

B. −14vd

C. 4vd

D. −4vdE. vd

For the next four items, consider the circuit

shown. Each resistor has the same resistance

1.0 Ω.

34. Requivalent. What is the equivalent resistance of the circuit?

A – 11


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39. I 2. What is the current passing through R1?

A. −2.0 AB. +2.0 A

C. +1.0 A

D. +0.5 A

E. −0.5 A

40. I 1. What is the current passing through R2?

A. 2.0 A

B. −2.0 AC. +4.0 A

D. −4.0 AE. 6.0 A

A – 13

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