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Homework Help: More conceptual MCQs

  1. Aug 7, 2006 #1
    More conceptual MCQs :)

    Hi guys,

    After the concept test that I found quite difficult for some.. apologise for asking so many questions at a go. I try my best to solve for all just hope that any of you would be kind enough to correct any wrongs. I always have the urge to know the correct answers for those that I don't know! :smile:

    1) For a cylinder to continue rolling on a frictionless surface, when viewed from its cross sectional area, which is true:

    a) Magnitude of CF at top, bottom, left and right the same.
    b) Magnitude of CF at top is greater than bottom.
    c) magnitude of CF at bottom is greatest.
    d)none of the above.
    My answer is (c) but I don't really know the explanation. I only recall that for example in vertical rotations, the bottom of the loop always requires a larger CF than the bottom as it needs to overcome the weight.
    Centripetal Force = CF

    2) Bubbles in a boiling pot get larger as they rise to the surface. Which accounts for this?
    (i) Temp at surface higher than bottom of pot.
    (ii) External Pressure on bubbles less at the surface
    (iii) Additional water vapourise into the bubbles.
    I would say the first 2 is true, however for the (iii)?

    3) 5J of heat flowed from a cooler region to a warmer region. Which of the statements are possibly or is/are true?
    (i) Work can be extracted during heat transfer and used to generate 5J of electrical energy.
    (ii) Work has been expended to transfer heat
    (iii) No work is extracted during heat transfer.
    My question is that how is it possible for heat to be extracted?

    4) Which of the following does not involve diffraction?
    a) Hearing a person speak around a corner of a building even if he cannot be seen
    b) splitting of visible light into a colourful spectrum by a glass prism
    c) sea waves passing through a gap in the breakwaters.
    d) transmission of electrons through a thin film of diamond
    e) None of the above
    All seem to involve diffraction to me.. but seldom the answer is none of the above.. but in this case I think it is.

    5) Which of the following is not an example of free oscillations?
    a) Swinging pendulum
    b) Ice cube bobbling up and down in water
    c) vibrations on a drum skin after it has been hit
    d) light rays in space travelling from sun to earth.
    e) tidal variations in sea level.
    I am stuck between b and e. In b there seem to be a driving freq - the water but it does not transfer energy to the cube right? for d I can't see how it is oscillating in the first place?

    6) An ideal gas is confined in a container with flexible walls that always keep the pressure inside constant. Suppose the gas shrinks slowly to half its original volume. Which is definately false?
    a) Walls of container conduct heat and it is cooler outside
    b) walls of the container are semi-permeable and gas is diffusing out of container through walls.
    c) walls of container impermeable and av KE of gas molecules has fallen
    d) walls of container impermeable and made of perfect insulator to heat
    e) walls of container semi-permeable and conduct heat.
    I found this a tough one. As gas volume dcreases its temperature decreases at constant pressure. But I don't know how this links to the permeability of the walls?

    Thanks a lot guys... I assure you this is not homework it's a past test and I seek understanding rather than free answers from this wonderful forum. Thanks!
  2. jcsd
  3. Aug 7, 2006 #2


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    2> It is that the pressure at a point inside a liquid is given by hdg, where h is the height of the water column above it, d is the density of liquid and g is acceleration due to gravity.Now as the bubble was much below the surface of water, the pressure on it was much more(h is more), so it was small. Now as the bubble rises, the external pressure on it decreases(h decreases).So it becomes larger.
    But how can additional vapour vaporise at the surface.In the heating of any fluid, convection currents are set up. first the water near the bottom gets heated, so it rises and the cold water from above takes its place.this in turn gets heated and this cycle continues until the whole water is heated.
    Last edited: Aug 7, 2006
  4. Aug 8, 2006 #3
    Thanks app. Anyone else have other inputs for the other questions?
  5. Aug 8, 2006 #4


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    Homework Helper

    6. The meaning of the flexible walls is that they can expand and contract to keep the pressure of the gas constant. since there is no mentioning of temperature change in the answers this probably implies an application of Boyle's law.

    5. (b) Is there a driving force that will cause the cube to bob up and down when disturbed from its natural floating position?

    4. How do you define the phenomena of diffraction?

    3. Heat do not flow by itself from a cooler to a warmer region.
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