Zero Hertz or Infinity

Discussion in 'Homework Help' started by TheSpArK505, Jan 21, 2015.

  1. TheSpArK505

    Thread Starter Member

    Sep 25, 2013
    92
    0
    Hi everyone
    I wonder what is the frequency of a DC signal? Basically i know its zero Hertz DA!! But what about infinity frequency for DC o_O >>>
    And how do Passive elements behave with the variation of frequency?
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    How many cycles does a DC signal make per second?

    The only way you could claim a non-zero frequency would be to claim a zero amplitude, in which case the distinction between AC and DC and what frequency it is at is intrinsically meaningless since the signal is identically zero.

    As for how passive elements behave at different frequencies, first you start with the constitutive equations for the elements (i.e., the defining relationships between voltage and current for that component). For a resistor that's Ohm's Law, V=IR. What are the constitutive equations for an inductor and capacitor?
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,055
    3,245
    Infinity frequency is the polar opposite of DC (0 Hz).
     
  4. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,692
    2,762
    And, to be completely pedantic, flicker (1/f) noise pretty much guarantees there is really no such thing as DC.
     
    #12 likes this.
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Good answer.

    Like how can a cop give you a ticket for not stopping at a STOP sign? What is the definition of "coming to a complete stop"?
     
  6. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,692
    2,762
    0 Kelvin?
     
    #12 likes this.
  7. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    DC is 0Hz. You gotta measure the period!
     
  8. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,692
    2,762
    Is this a reply to my 0K post? If so, you asked "what is the definition of coming to a complete stop."

    Well, a complete stop implies zero kinetic energy. This occurs only at 0K (vacuum energy not considered here). Thus, my answer in non-time based units.
     
    jjw likes this.
  9. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Sorry. No, I was replying to the DC problem.

    As for "Complete Stop" we have a mountain here and I swear it moved since yesterday.
     
  10. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,692
    2,762
    Give it time. It'll move.
     
  11. MrAl

    Well-Known Member

    Jun 17, 2014
    2,440
    492
    Hi,

    Yes zero Hertz is considered DC, but infinite frequency is often considered to be zero time, or saying it differently, zero time is often considered infinite frequency.

    So we have zero frequency is like DC, and zero time is like infinite frequency.
    We also end up with infinite time is like zero frequency.
     
  12. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    When you say zero time I'm assuming you are talking about the period.

    Well, what's the period of something that never changes? Infinite.

    Actually, in both cases it is undefined except in the limit.
     
  13. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    velocity = Δd/Δt
    Hence zero velocity is in the limits Δd→0/Δt→∞

    Similarly,

    frequency = n/Δt
    Hence DC is in the limits n→0/Δt→∞
     
  14. TheSpArK505

    Thread Starter Member

    Sep 25, 2013
    92
    0
    Thanks all for your answers . It seems that it needs a PROOF to find outo_O
     
  15. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    To find out what? How passive components behave at different frequencies?

    Try doing what I suggested and start from the constitutive equations. The "proof" is only a few lines long.

    It might help if you told us what the homework problem is you are trying to solve.
     
  16. MrAl

    Well-Known Member

    Jun 17, 2014
    2,440
    492
    Hi,

    Maybe it would help if we looked at at filter circuit and it's response for AC and DC and at various times.
     
  17. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,692
    2,762
    A filter will have a different response tomorrow than today?
     
  18. MrAl

    Well-Known Member

    Jun 17, 2014
    2,440
    492
    Hi,

    Ha ha, over milliseconds or microseconds, not really days :)
     
  19. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    If it's nonstationary (which of course means it's nonlinear).
     
Loading...