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Ohm's law and AC circuits

  1. Jun 5, 2005 #1
    Ohm's law and AC circuits/ Biots / Oscilliscope

    hey wasssup...

    i'm doing a practice final and need some help ...
    It's askin which is true
    (1) Ohm's Law is true for all materials;
    (2) Resistance may depend on temperature
    (3) REsistance may depend on current
    (4) Ohm's law is empircal

    (a) 1 and 2
    (b) only 1
    (c) only 2 and 3 and 4
    (d) none of the above

    (1)Clearly Ohm's Law states that V = IR or in some other order
    Now it doesn't have any material constant in there so would one be ruled out ?
    (2) Resistance may depend on temperature : This is true but it's not stated in ohms law
    (3) Resistance doesn't depend on current, but current and resistance is needed for ohm's law
    (4) Ohm's law is empircal ... it's not an observation but a LAW....

    so is my choice D?

    (2) The voltage that is delivered by the electric power companies to domestic consumers has a frequency of 60z and PtP amplitude is closest to :
    (a) 330 Volts
    (b) 100 Volts
    (c) 50 Volts
    (d) 1200 Volts

    I think in the us the AC voltage is 120 volts... (JPN is 100 V.. i always get these two mized up) anyhow assuming it's 120 Volts and Hz = 60 then wouldn't t = 1/2 * f because it's PtP

    So V(t) = Vptp * Sin (w * t) .... yea i did this and i didn't get a matchin number.. hahahaha... thanks ahead a times
    Last edited: Jun 5, 2005
  2. jcsd
  3. Jun 5, 2005 #2
    Here's another one ...

    Question 4)
    (A): P = IV hold for any cuductor
    (B) P = I^2* R holds only for conductors which happen to obey Ohm's Law

    (a) only (A) is true
    (b) only (B) is true
    (c) both (A) and (B) is true
    (d) both are false
  4. Jun 5, 2005 #3
    For the last question, all the numbers sounds pretty bad.

    The power companies deliver [itex]v(t) = 170 \cos (377 t)[/itex], the rms is 120 V. That is [itex]\frac {170}{\sqrt 2} = 120[/itex].
    None of the possible answers are even "close".
  5. Jun 5, 2005 #4


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    Science Advisor

    1 is clearly false
    2 is clearly true
    3 is unclear
    4 is clearly true

    Thus I'd go for 2,3,4, option c.
  6. Jun 5, 2005 #5
    another one

    Question 24)

    When DC coupling is chosen on the Oscilliscope:
    (a) input signal is grounded
    (b) the input signal is directly connected directly to the amplifier and not altered in any way
    (c) the input signal is connected to the amplifier through a low pass filter
    (d) the input signal is connected to the amplifier through a high pass filter

    thank everyone for the reponses.... these questions are tough... what happened to F = ma .. damn...
  7. Jun 5, 2005 #6
    Question 25) High biot number (>>1) imply:
    (a) Temperature gradients within sphere are great
    (b) Temperature gradients inside sphere are small
    (c) Reynolds number greater than 2300
    (d) high friction factors
  8. Jun 5, 2005 #7
    i hope this will be the last one

    Question 32)
    For a 60Hz sinusoidal signal, the sampling rate (in samples per second) that would only record 4 ponts per cycle is :
    (a) 15
    (b) 30
    (c) 120
    (d) 240

    i'm guessing 15 .... 60/4... yea.. i dont' get this question .. i think it goes with aliasing and nyquist
  9. Jun 6, 2005 #8
    no one else?
  10. Jun 6, 2005 #9
    TA sent the answer in ...

    1.d, 2.b, 3.b, 3.b, 4.c, 5.e, 6.a, 7.b, 8.d, 9.c, 10.c, 11.b, 12.a, 13.d, 14.b, 15.a, 16.b, 17.d, 18.d, 19.d, 20.a, 21.a, 22.b, 23.a, 24.b, 25.a, 26.b, 27.d, 28.a, 29.b, 30.b, 31.d, 32.d, 33.c, 34.b
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