Exam 3

PHY 2140

Date, 2003

Please type your name here / Please type your student number here

Instructions:

The following procedure must be followed in order to correct any (unlikely) mistakes in the grading.

  1. Do all the questions. Show your work for partial credit. I must be able to understand what you have done while I am reading the exam – not when you explain it to me after the exam is graded and returned.
  1. For each problem: (a) write down any formula(s) used

(b) justify why you used that particular formula(s)

(c) copy the answer you have circled from the exam pages to the General Purpose Answer Sheet and fill in the ovals.

(d) show your work! No work – no point.

3. The formula sheet is on the last page of the exam. You may detach it from the rest of the exam.

I read and understood the statement above

Your signature

NAME

  1. An inductor has a 54 Ohm reactance at 60 Hz. What will be the peak current if this inductor is connected to a 50 Hz source that produces 100 V rms voltage?
  1. 0.69 A
  2. 1.41 A
  3. 2.71 A
  4. 3.14 A
  5. none of the above
  1. An inductor and a resistor are connected in series. When connected to a 60 Hz, 80 V source, the voltage drop across the resistor is found to be 35 V rms and the power dissipated in the circuit is 11 W. What is the value of the resistance?
  1. 97 
  2. 111 
  3. 123 
  4. 134 
  5. 151 
  6. 167 
  1. If a 2.0 H inductor is used in a tuning circuit in a radio, what is the capacitance needed to tune in Phat Matt on 89X at a frequency of 88.7 MHz?
  1. 0.54 pF
  2. 0.71 pF
  3. 0.91 pF
  4. 1.61 pF
  5. That station sucks!

NAME

  1. A new elementary particle is detected. Moving with a speed of 0.98c, the lifetime of the particle is measured to be 1.3 x 10-7 s by a stationary observer. What is its proper lifetime?
  1. 1.12 x 10-9 s
  2. 2.45 x 10-9 s
  3. 4.52 x 10-9 s
  4. 6.02 x 10-9 s
  5. 2.59 x 10-8 s
  1. Marilyn Monroe and Albert Einstein go for a ride in a rocket ship at a speed of 0.7c. Albert finds that Marilyn’s heart beats 90 times per minute (I think she likes him). What is her heart rate according to a stationary observer on the Earth? Hint: what is the time interval between heart beats?
  1. 36 beats/min
  2. 64 beats/min
  3. 122 beats/min
  4. 160 beats/min
  5. zero beats per minute
  1. Stephen Hawking and Albert Einstein each blast off in rockets, labeled “e” and “h” in the figure. In a cosmic game of “chicken,” they fly towards one another on a collision course. Each rocket has a speed of 0.8c as measured by a stationary observer on the Earth. What is the speed of Einstein’s rocket as measured by Hawking?
  1. zero
  2. 0.26c
  3. 0.74c
  4. 0.98c
  5. 1.60c

NAME

  1. A radio wave has a frequency of 5.00 MHz. Find its wavelength.
  1. 60 mm
  2. 15 mm
  3. 60 m
  4. 15 m
  5. zero
  1. If the total energy of a rocket ship is three times its rest energy, what is its speed?
  1. 0.63c
  2. 0.72c
  3. 0.94c
  4. 0.99c
  5. none of the above
  1. The phasor diagrams below represent threeoscillating emfs having different amplitudesand frequencies at a certain instant of time t =0.As t increases, each phasor rotates counterclockwiseand completely determines a sinusoidaloscillation.At the instant of time shown,the magnitude of E associated with each phasorgiven in ascending order by diagrams?
  1. a, b, c
  2. b, c, a
  3. a, c, b
  4. c, b, a
  5. need more info
  1. Radio The light bulb has a resistance R, and theemf drives the circuit with a frequency .The light bulb glows most brightly at
  1. Very low frequencies
  2. Very high frequencies
  3. Resonant frequency
  1. A supertrain of proper length 100 m travels at a speed of 0.95c as it passes through a tunnel having proper length 50 m. As seen by a trackside observer, is the train ever completely within the tunnel?
  1. yes
  2. no
  3. halfway
  4. not enough information
  5. what are you talking about?
  1. (bonus) Imagine that the entire Sun collapses to a sphere of radius Rgsuch that the work required to remove a small mass m from the surface would be equal to its rest energy, mc2. This radius is called the gravitational radius for the Sun. Find Rg. (It is believed that the ultimate fate of very massive stars is to collapse beyond their gravitational radii into black holes.)
  1. 3347 km
  2. 127 km
  3. 1.47 km
  4. 1.14 m
  5. impossible to find

/ , / /
/ / c = 3.00 x 108 m/s / h = 6.63 x 10-34 J s
/ / / 1 eV = 1.6 x 10-19 J