Lecture Section: A-Duffy B- Goldberg C-Narain

Lecture Section: A-Duffy B- Goldberg C-Narain

Page 1of 9 PY106 TEST 1

Feb 23, 2000

Name______ID______

Lecture Section: [ ] A-Duffy [ ] B- Goldberg [ ] C-Narain

Room: [ ] CAS 213 [ ] CAS 222 [ ] CAS 224

[ ] CAS 226 [ ] CAS 313 [ ] SCI 115

  • Put your name and ID number on each page and lecture section and room of exam on the top of this page.
  • The test is 90 minutes long and has a total of 100 points
  • Answer ALL problems below. Indicate your answers in the space provided. If you need more space, write on the back of the pages.
  • Express all your numerical answers to the appropriate number of significant digits and with appropriate units.
  • It is very important to show ALL your work. Even if you have the right answer, you may not receive full credit if you don't show your work.
  • This is a closed-book test with an equation sheet provided.

Grades

Problem 1 ______(/10)

Problem 2 ______(/20)

Problem 3 ______(/15)

Problem 4 ______(/20)

Problem 5 ______(/15)

Problem 6 ______(/20)

Total ______(/100)

PROBLEM 1. QUICKIES [10 points– 2 points each]

a)We consume about 1 x 10 J of energy a day from food. If we could get this energy by plugging ourselves into an electrical outlet instead of by eating food, about how much would it cost us per day if the electric company charges $0.10/ kW-hr?

[ ] $0.30[ ] $3[ ] $30

b)A series circuit consists of a 6.0 V battery, a 12 ohm resistorand a 24 ohm resistor. Which of the following statements is true?

[ ] The voltage across each resistor is the same

[ ] The current through each resistor is the same

[ ] The power dissipated in each resistor is the same

c)A series RC circuit consists of a resistor, a capacitor charged to 6.0 volts,

and a switch that is initially open. When the switch is closed, it takes 2.5 seconds

for the capacitor voltage to reach 4.0 volts. After an additional 2.5 seconds, what

is the capacitor voltage?

[ ] less than 2.0 V[ ] 2.0 V[ ] more than 2.0 V

d) A proton is launched into a region where there is a uniform magnetic field directed vertically down. If the force on the proton is initially to the North, what can you say about the proton’s initial velocity?

[ ] The proton was definitely launched horizontally to the East

[ ] The horizontal component of the proton’s velocity is definitely East, but the proton’s velocity could also have a vertical component

e)Along straight wire carries current into the page. The magnetic

field lines are circular loops centered on the wire. Does the

magnetic field go clockwise or counterclockwise?

[ ] Clockwise[ ] Counterclockwise

PROBLEM 2A. [10 points]– Point charges

Four charges are arranged at the corners of a square, as shown.

The square is 25 cm on each side.

(a)[4 points] Calculatethe magnitude of the net electric field at the center of the square due to the four charges.

(b)[2 points] What is the direction of the net electric field at the center of the square?

(c)[4 points] Calculate the electric potential at the center of the square due to the four charges.

PROBLEM 2B. [10 points]– Point charges

With the same charges arranged at the corners of

the same square, answer the following questions.

(d)[5 points] Which of the following changes would result in a net electric field of zero at the center of the square? Select all that apply.

[ ] Replace the +3.0 C charge by a –3.0 C charge

[ ] Replace the +3.0 C charge by a –1.0 C charge

[ ] Make all four charges +2.0 C

[ ] Replace the +3.0 C charge by a –3.0 C charge and remove one of the +2.0 C charges

[ ] Replace the +3.0 C charge by a –2.0 C charge, and replace the -1.0 C charge by a –2.0 C charge

(e)[5 points]Which of the following changes would result inan electric potential of zero at the center of the square? Select all that apply.

[ ] Replace the +3.0 C charge by a –3.0 C charge

[ ] Replace the +3.0 C charge by a –1.0 C charge

[ ] Make all four charges +2.0 C

[ ] Replace the +3.0 C charge by a –3.0 C charge and remove one of the +2.0 C charges

[ ] Replace the +3.0 C charge by a –2.0 C charge, and replace the -1.0 C charge by a –2.0 C charge

PROBLEM 3. [15 points]

Electric fields and conductors

The figure to the right shows a cut-away viewthrough two concentric metal spherical shells. There is a charge Q1 placed on the inner shell and a charge Q2 placed on the outer shell.

Q1 = 4.0 C; Q2 = -2.0 C

(a)[6 points] Show the electric field lines in all regions, from the very center of the inner shell to beyond the exterior of the outer shell. Be careful that the number of lines you draw clearly represents any changes in the strength of the field between regions.

(b)[6 points] Draw three equipotential lines in the region outside the outer shell such that the potential difference between the lines is approximately the same.

(c)[3 points] Now we add a dielectric that completely fills the space between the two spherical shells. On the diagram to the right, showthe modification (if any) to the electric field between the shells. The dielectric constant is  = 2.0.

PROBLEM 4. [20 points]– Light bulbs


The bulbs in the circuit are identical.

(1)[6 points] In the circuit on the left, rank bulbs 1-6 in order of decreasingbrightness. Briefly explain your answers.

(2)[6 points] If each bulb has a resistance of 12 ohms, what is the equivalent

resistance of the circuit on the left?

(3)[8 points] When bulb 1 is removed, leaving a break in the circuit….

a)The brightness of bulb 3 ______

[ ] increases[ ] decreases[ ] stays the same

Briefly explain your answer:

b)The brightness of bulb 6 ______

[ ] increases[ ] decreases[ ] stays the same

Briefly explain your answer:

PROBLEM 5. [15 points]–Kirchoff’s rules

The resistors are:

R1= 10 ohms

R2=20 ohms

R3=5 ohms

The battery emf’s are:

 = 3 volts

 = 6 volts

 = 1.5 volts

(a)[9 points] Using the currents defined as in the figure, write down the junction and loop equations you will need to solve for the currents in the circuit:

(b)[ 6 points]Solve for the three currents.

I1= ______

I2 = ______

I3 = ______

PROBLEM 6. [20 points]– A bubble chamber


In bubble chamber experiments, charged particles leave small hydrogen gas bubbles along their paths. The paths of three such charged particles are shown below; the paths are in the plane of the page. There is auniform magnetic field directed into the page, and the three particles have charges of equal magnitude.

a)[6 points] Are the charged particles negative or positive?

Particle 1 is[ ] positive[ ] negative

Particle 2 is[ ] positive[ ] negative

Particle 3 is[ ] positive[ ] negative

b)[3 points] If the masses of the particlesare equal, which one has the highest speed?

[ ] Particle 1[ ] Particle 2[ ] Particle 3

c)[5 points] If the radius of the path followed by particle 3 is 4 times larger than the radius of particle 1’s path, determine the ratio of their kinetic energies. (Assume they have the same mass.)

d)[6 points] Particle 3 enters the chamber at point A with a speed of 2.4 x 10m/s. If the magnetic field in the chamber is 0.13 T, what is the magnitude of the electric field that can be introduced so particle 3 moves as if it does not experience any force?Sketch the electric field on the diagram above, clearly showing its direction.

Top View