PHYSICS 111(02): QUIZ 04

Answers are at bottom of page.
  1. The difference in height between two sections of a pipeline carrying water is h. Assuming laminar, steady, non-viscous flow through the pipeline, under which of the following conditions will the pressure in both sections of the pipeline be the same?

  2. A wave is described by the equation

    where y and x are in metres and t is in seconds. The frequency of this wave is:

  3. A sound source is emitting sound waves uniformly in all directions. At a distance of 1 m from the source the sound intensity level is 80 dB. At a distance of 2 m from the source, the sound intensity level is:

  4. A red car approaches a stationary black car. Both cars have horns that emit sounds of the same frequency f. Let fR be the frequency heard by the driver of the red car when the black car's horn sounds and let fB be the frequency heard by the driver of the black car when the red car's horn sounds.

  5. Two speakers are vibrating in phase and emitting identical sound waves of amplitude A, frequency f, and wavelength. The amplitude of the sound wave heard by a listener who is equal distance from each speaker is

  6. A guitar string under a tension F has a fundamental frequency f. If the tension is increased to 4 F, the new fundamental frequency is

  7. A positively-charged rod is brought near a conducting sphere that is initially grounded. The rod does not touch the sphere. The wire from the sphere to ground is then disconnected (refer to diagram below). Which of the following statements is correct?
    • (A) The sphere will not acquire a net charge
    • (B) The sphere will acquire a net negative charge and the net positive charge on the rod will be unchanged.
    • (C) The sphere will acquire a net positive charge and the net positive charge on the rod will be diminished.
    • (D) The sphere will acquire a net negative charge and the net positive charge on the rod will be increased.
    • (E) Both the sphere and the rod will acquire net negative charges.

  8. Two charged particles are located such that they attract each other with an electrostatic force F. If the charge on each particle is doubled and the distance between them is also doubled, then the electrostatic force will be:

  9. Consider a charge +Q1 and a charge –Q2 arranged as shown:

    The electric field at P due to Q1 has a magnitude |E1| and the electric field at P due to Q2 has a magnitude |E2|. |E2| > |E1|. The total (net) electric field at P is:

  10. The absolute electrostatic potential at the centre of a square is 3V when a charge of +Q is located at one of the square's corners. What is the absolute potential at the square's centre when a second charge of –Q is placed at one of the remaining corners?

ANSWERS:

  1. (B) - From Bernoulli's equation, for the pressures in both sections to be the same, the flow speed must be smaller in the upper section. According to the continuity equation, this will be the case if the upper section has a larger cross-sectional area (and thus larger diameter).
  2. (A) - From the general equation for a harmonic wave, the factor multiplying the time variable is 2 pi times the frequency. Therefore, pi equals 2 pi times f, so f = ½ s-1 = 0.500 Hz.
  3. (E) - Since the sound is emitted uniformly in all directions, the wavefronts are spherical shells, and the intensity varies as the inverse square of the distance from the source. Therefore, at twice the distance, the intensity is decreased to ¼ its original value. Now, recall that a 10 dB change in level corresponds to a factor of 10 change in intensity. Therefore, a factor of 4 change in intensity corresponds to a change in level of < 10 dB, i.e. the intensity level at 2 m will be > 70 dB.
  4. (D) - fR > f since observer approaches source and fB > f since source approaches observer.
  5. (D) - Equal distance from each source means 0 path length difference and therefore constructive interference.
  6. (D) - Frequency is proportional to wave speed since the wavelength is fixed by the length of the string, and wave speed varies directly as the square root of the string tension.
  7. (B) - This is an example of charging by induction. Electrons flow from ground onto the sphere.
  8. (C) - Coulomb's Law
  9. (D) - Electric field points away from a positive charge and toward a negative charge. Therefore, the electric field at P due to charge 1 is directed to the right, and the electric field at P due to charge 2 is directed to the left. Since the magnitude of the field due to charge 2 is greater than the magnitude of the field due to charge 1, the net field is directed to the left and has magnitude |E2| - |E1|.
  10. (A) - Since the distance from the centre to a corner is the same for all corners, the potential due to the added charge of -Q is -3 V (everything same except for polarity of charge), and therefore the net potential is 0.

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