hnc question 4 (last one i hope!!!)

Discussion in 'Homework Help' started by redshaw, Aug 4, 2008.

  1. redshaw

    Thread Starter Member

    Jul 15, 2008
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    a coil of resistance 8 ohms and inductance 150 mH is connected in series with a 100uF capacitor across a 240V 50 Hz ac supply. calculate a.) the circuit current, b.) the circuit phase angle, c.) the p.d across the coil, d.) the p.d across the capacitor and e.) the power dissipated.

    i have calculated a.)

    Xc=1/2piefc
    Xc=1/2pie*50*100*10^-6
    Xc=31.83 ohms

    Xl=2piefl
    Xl=2pie 50*150*10^-3
    Xl=47.12 ohms

    Z=square root r^2 + (Xl-Xc)^2
    Z=17.25 ohms

    I=V/Z
    I=240/18.37
    I=13.9 A

    b.) as

    tan theta=47.12-31.83/8
    tan thata=1.911
    theta=arctan 1.911
    theta= 62.38 degree

    for c and d

    is pd phase difference, potential drop, power dissipated - any ideas

    i have read section in principles of elec technology (cotton) about phase difference but am struggling to understand it and put it into place with the figures/data i have.

    e1=OPsinwt
    e2=OQsin(wt-theta)

    did consider Ur=IR for the couil but was unsure aboutcapacitor

    any help/formulas would be welcome

    d.)

    P=I^2/R
    P=13.9^2/8
    P=24.15 W

    hopefully the rest of my working is correct
     
    Last edited: Aug 5, 2008
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You might want to recheck your Xc calculation.
     
  3. redshaw

    Thread Starter Member

    Jul 15, 2008
    12
    0
    thank you for pointing that out -

    i have now edited my thread with correct figures

    cheers
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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  5. redshaw

    Thread Starter Member

    Jul 15, 2008
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    is it just simply

    p=I^2/R

    so for coil it would be 4.1W

    and for capacitor 6.07W

    i know the equation for pd voltage for a coil

    but what is the equation for voltage pd for a capacitor??

    any help much appreciated
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    what is pd? power disipation? Potential Drop?

    I originally thought you were talking about the potential acrossed the components when you wanted the phase angle as well. That led me to recomment that page in the ebook.

    Being reactive, I would tend to think the power would be VA (voltamperes) and not watts unless you factor in the power factor.

    Your stated power formula is incorrect.

    Is this from an instructor led course or are you doing self-study?
     
  7. redshaw

    Thread Starter Member

    Jul 15, 2008
    12
    0
    i am doing an hnc self study course i recieve assignments through post with a list of books to read - however i am serving in the royal air force and access to books when i am away is limited.

    reading through previous questions when they talk about pd it refers to voltage

    i have done these calculations

    Vc=IXc=13.9*31.83
    Vc=442.437V

    Zcoil=square root of R^2+Xl^2
    =47.79

    Vcoil=IZcoil=13.9*47.79
    =664.34V

    I am terrible at electrics as i am a mechancial engineer
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You have the potential differences (voltage drops) slightly incorrect, you need to watch your rounding more carefully.

    Attempt to keep your decimal places consistent throughout. The coil/resistor impedance is 47.798 ... which you could round up to 47.80 if your going for two decimal places. In the scheme of reality, when you consider your measuring instrument tolerances, 47.79 is within those tolerances. In academia, your answer could be incorrect.
     
    Last edited: Aug 6, 2008
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