Odd electronics test question

Discussion in 'Homework Help' started by xaos, Apr 9, 2012.

  1. xaos

    Thread Starter New Member

    Apr 9, 2012
    10
    0
    I could really use some help. I am taking an electronics test, and this is one of the questions:

    Direct current would NOT ordinarily be used in which of the following?
    A. Ships
    B. Airplanes
    C. Battery Charging
    D. Arc Welding
    E. Electric clocks



    Now I may be totally crazy, but don't all on the list use or are capeable of using both AC & DC? I've pretty much narrowed this down to either battery charging or arc welding, but both of these have AC & DC versions... Battery chargers are typically run on AC, but output DC, though there are the 'quick charge' packs that use entirely dc...

    Any help appreciated.
     
  2. justtrying

    Active Member

    Mar 9, 2011
    329
    335
    I agree with you.

    I think it is one of those multiple choice questions that we all fear. Sitting at home, the key seems to be in the wording "would NOT ordinarily be used" - in which case I would say battery charging.
     
  3. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    i wanna get in on this, isnt it an aeroplane? sounds a bit stupid, but i remember a physics lesson where they said a voltage was generated on wires as it cut through the magnetic flux of the earth... i guess that could screw up chips if it raised the voltage enough? an ac voltage would be raised or decreased yes but if your anticipating an ac signal youd be prepared for offset..? interesting question!
     
  4. justtrying

    Active Member

    Mar 9, 2011
    329
    335
    then again it's the silliness of MC question, as I believe an airplane uses both AC and DC but for different purposes. So main voltage would be AC (I think), with DC powering systems such as lights, etc...
     
  5. paulktreg

    Distinguished Member

    Jun 2, 2008
    608
    120
    E. Electric Clocks.

    If it's Electric I take that to mean they need the AC mains frequency to keep time?
     
  6. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    unless its one of themon the wall with the battery? lol
    you could be right though, with the phrasing of the original question

    Also, is there any way of knowing the answer? or is this one of those scratchy head forever questions?
     
  7. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    There are probably more battery-powered clocks out there than line-powered ones :p

    That's a tough question. Arc welding is often very low voltage, high amperage DC, ships run on batteries, and battery chargers usually output DC. I would have to lean towards airplanes, since you really don't want heavy batteries on an airplane. They often use 400hz (?) AC. Don't quote me on this, but I would definitely lean towards B.
     
  8. jtrent

    New Member

    Mar 11, 2012
    26
    4
    Electric Clocks is my choice. An electric clock would be one that is plugged into AC and runs an AC motor in the clock turning the gears that move the hands. No DC is required.

    A portable battery operated clock would be different. It would require DC. Also a digital electric clock would require DC.

    As "JustTrying" said, the key is in the wording.
     
  9. justtrying

    Active Member

    Mar 9, 2011
    329
    335
    could be, but they don't work very well :) also the wording... What about battery operated electric clocks? Got to love these questions.
     
  10. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    What makes you think that? There is nothing to suggest that an "electric clock" would be one that's plugged in. There are probably ten times more battery-powered electric clocks out there than AC-powered ones. Especially the household ones with the motors--those are usually DC.
     
  11. panic mode

    Senior Member

    Oct 10, 2011
    1,318
    304
    there is no good answer to this question. who ever asked it should be punished...
     
    chrisw1990 and DerStrom8 like this.
  12. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    I definitely agree with that! :eek:
     
  13. paulktreg

    Distinguished Member

    Jun 2, 2008
    608
    120
    xaos likes this.
  14. justtrying

    Active Member

    Mar 9, 2011
    329
    335
    I guess it comes down to the confusion between electric clock, quartz and digital. I haven't seen an electric clock in a while since even the ones that are plug in all have a back up battery and are quartz movement which means that they will have an internal oscillator. Whoever asked this question had better covered the history of clocks!
     
  15. xaos

    Thread Starter New Member

    Apr 9, 2012
    10
    0
    The gereral consensus seems to be electric clock.

    As a bit of background info, this test appears to be pretty old. In part of the test, they give you a schematic and ask questions about it. One of the schematics is for a photoelectric switch using a 2d21 thyratron to rectify 50V to 50VDC. I'd say this test is likely from the 60s... Why the company chose this test I have no idea.

    So it actually makes sense given the age of the test, that the answer may very well be electric clock (as linked by Paulktreg).

    Thanks guys for taking the time to help me with such a stupid question. I appreciate it greatly.
     
  16. mlog

    Member

    Feb 11, 2012
    276
    36
    I agree the question is terrible. However, aircraft power systems can be direct current. See MIL-STD-704, which devotes part of the specification to 28 volt DC power systems, as does the commercial aircraft specification, RTCA DO-160 section 16. Now that's not to say that the DC won't come from rectification of AC generators, because it often does, but there are also batteries and permanent magnet generators.
     
  17. justtrying

    Active Member

    Mar 9, 2011
    329
    335
    however wrote this question clearly knew what the answer was, and that's the problem. I have an instructor like that. The skies are dark and gloomy...
     
  18. #12

    Expert

    Nov 30, 2010
    16,247
    6,742
    Old fart on board. In the 1950's and 1960's wall clocks in schools, hospitals, and office buildings were synchronized and kept that way by using a synchronis electric motor in each one. The AC power grid is frequency accurate enough to keep time with. That is the missing clue: accuracy of the 60 Hz line frequency.
     
    xaos likes this.
  19. mlog

    Member

    Feb 11, 2012
    276
    36
    Didn't they also send a "sync" signal on the power line to move the clock to the hour in case there had been a small power outage?
     
  20. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Those old clocks in the schools could also be set forward or backward by an hour for daylight savings time. I'm not sure quite how they did it.

    I just came across this thread and I agree that the answer is not at all obvious, though maybe it was a lot less clear a few decades ago. Although I don't know that this was the case, I can easily see AC electric clocks that used the mains frequency as a time base becoming common before battery powered clocks with sufficient accuracy to be widespread came along. It would be an interesting topic to look at.

    Most light aircraft only have DC, just like most cars. I don't know of any aircraft (but I'm sure there's a few and, for all I know, a lot) that have AC systems but no DC systems.

    I, too, narrowed it to arc welding and electric clocks, but wrote off electric clocks because battery powered clocks are so common. I just couldn't remember whether arc welding had a strong preference for one over the other, but thought that DC was preferred so that you could sustain a continuous, stable arc instead of having it extinguish and relight 120 times a second (in the U.S.).
     
    xaos likes this.
Loading...