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Homework Help: Multiple choice circuit help

  1. Jul 23, 2006 #1
    Ok guys, I've got a few multiple choice questions I'm having trouble with....

    1. If the current increases in a solenoid, the induced emf acts to...
    A. increase the flux
    B. decrease the flux
    C. either increase or decrease the flux
    D. have no affect of the flux
    ----I'm thinking B, because with I=V/R....and what little I know about self inductance, I think if current increases, the emf will attempt to reduce the flux.

    2. The current in an LRC circuit is....
    A. sinusoidal
    B. cosinusoidal
    C. either sin... or cos...
    D. neither sin... or cos...
    ----Here, I've looked in my book, and it doesn't say. I'm thinking A, but the only reason I know is because of this equation...I=I(knot)sin(wt)

    3. In a series LRC circuit, Kirchoff's loop rule...
    A. is obeyed
    B. not obeyed
    C. would be obeyed but the voltages are not in phase
    D. would be obeyed but the currents are not in phase
    ----I am thinking C here, because voltages in phase don't reach peaks at the same time, thus the peak voltages of the source don't add up.....and neither do the Vrms.

    4. The power factor for an RC circuit is
    A. 1
    B. 0
    C. between 0 and 1
    D. negative
    ----I'm not sure on the answer here. I think I was told it was C, from what I can recall from lectures. Only reasoning I know is that cos = R/Z....

    Thanks a ton guys.....any help is appreciated....I can figure on all of these....if you will just get me started. I wasn't explained some of this stuff, so its hard for me to understand some of it...by just what the book says. Thanks again....Brad
  2. jcsd
  3. Jul 23, 2006 #2


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    Staff: Mentor

    -1- I think you're right.

    -2- Poorly worded question, but I would think C, since cos and sin are the same thing, with just a phase (or time) shift.

    -3- AFAIK, Kirchoff's loop rule is always obeyed. That's how you solve for the currents and voltages in circuits, after all.

    -4- I'm not real familiar with the definition of power factor (but there was a recent PF thread about it, I think in the Homework Forums somewhere). But I'd guess C also, since you get a phase shift through the RC. The problem should state more clearly whether it's an RC or CR circuit as well. I'd recommend doing a PF search for that other thread for a better interpretation of power factor.
  4. Jul 23, 2006 #3
    ok, thanks a lot......if anyone else has some input, please feel free to leave any info you have too.......

    Thanks again,
  5. Jul 23, 2006 #4
    Can someone else look these over too.....just to make sure we're correct. I'd like to know this info before I start studying it again.

    I've also got one more question...

    5. Sources are coherent if
    A. they have a zero phase difference
    B. they have a constant phase difference
    C. the distance to them is the same
    D. they only come from lasers
    ----First let me say, does answer A mean they are in phase then? I've looked into this problem, and it said they need constant phase relation to eachother. I think they need to be in the same phase. Also, I know they come from lasers...but I thought like 2 light bulbs could be coherent too. So....as of now, I'm between answer A and D.
    Last edited: Jul 23, 2006
  6. Jul 24, 2006 #5
    anyone know?
  7. Jul 24, 2006 #6


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    Staff Emeritus
    Science Advisor
    Gold Member

    The only stipulation for two coherent sources are they must have a constant phase difference. So lets examine each of the options A to D.
    A: Zero Phase Difference If the two sources have a phase difference of zero, it is implied that there is a constant phase difference (namely zero), so this can be considered true.
    B: Constant Phase Difference This is the definition of a coherent source and therefore this must be true.
    C: Equal Path Difference Does the path difference have any effect on the sources?:wink:
    D: They Only Come from Lasers It is true that lasers emit a coherent beam of photons. However, it is an assumption of Young's double slit that two coherent sources must be used and do we commonly use lasers to perform this experiment (or do we use something else)?:wink:
    Last edited: Jul 24, 2006
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