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    at what situations do i use these equations?
    1)P=I^2*R
    2)P=V^2/R
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    (Original post by djmans)
    at what situations do i use these equations?
    1)P=I^2*R
    2)P=V^2/R
    In direct current, working out the power dissipated in a resistor.
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    (Original post by morgan8002)
    In direct current, working out the power dissipated in a resistor.
    also works for AC if it's a resistive load - you will be given RMS values in an exam.

    P=I^2R means that if you know any 2 of P,I or R you can work out the third
    P=V^2/R means that if you know any 2 of P,V or R you can work out the third

    so you can tackle questions like
    what's the resistance of a 2000W kettle element that operates at 230V?

    or

    what's the current in a 800W heater with resistance of 5.5 ohms
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    (Original post by Joinedup)
    also works for AC if it's a resistive load - you will be given RMS values in an exam.

    P=I^2R means that if you know any 2 of P,I or R you can work out the third
    P=V^2/R means that if you know any 2 of P,V or R you can work out the third

    so you can tackle questions like
    what's the resistance of a 2000W kettle element that operates at 230V?

    or

    what's the current in a 800W heater with resistance of 5.5 ohms

    what's the resistance of a 2000W kettle element that operates at 230V?
    for this question cant i just use P=VI
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    (Original post by djmans)
    what's the resistance of a 2000W kettle element that operates at 230V?
    for this question cant i just use P=VI
    those equations are derived from combining Ohms law and P=VI... they don't tell you anything you couldn't get from using P=VI and Ohm's law
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    (Original post by Joinedup)
    also works for AC if it's a resistive load - you will be given RMS values in an exam.
    Yeah. In its above form it can only be used to find instaantaneous power though.
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    (Original post by djmans)
    what's the resistance of a 2000W kettle element that operates at 230V?
    for this question cant i just use P=VI
    this formula

     P=\dfrac{V^2}{R}

    as well as this one

    P=I^2R

    should be on the formula sheet
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    (Original post by djmans)
    what's the resistance of a 2000W kettle element that operates at 230V?
    for this question cant i just use P=VI
    Just to sum up,

    P=VI is the equation you are thinking of using right? along with V=IR...

    Now the equations you have given are a combination of these two.

    If you did:
    P=2000W; V=230V
    I=\dfrac{P}{V} = \dfrac{2000}{230} \approx 8.7

    V=IR, Therefore
    R=\dfrac{230}{8.7} \approx 26.45

    See how long winded this is? we can combine the equations into one to make this easier.

    P=VI, but we know that V=IR...
    So obviously substitute in and we get a nicer, one step equation to work with:
    P=I^2R.

    The same can be done for current:
    P=VI, I=\dfrac{V}{R}
    \therefore P=\dfrac{V^2}{R}.

    Using this equation makes the operation doable in one step:
    R=\dfrac{230^2}{2000} = 26.45.

    TL;DR combining is not a necessity and the end result can be achieved using the standard equations. But it makes it much nicer!
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    (Original post by The-Spartan)
    Just to sum up,

    P=VI is the equation you are thinking of using right? along with V=IR...

    Now the equations you have given are a combination of these two.

    If you did:
    P=2000W; V=230V
    I=\dfrac{P}{V} = \dfrac{2000}{230} \approx 8.7

    V=IR, Therefore
    R=\dfrac{230}{8.7} \approx 26.45

    See how long winded this is? we can combine the equations into one to make this easier.

    P=VI, but we know that V=IR...
    So obviously substitute in and we get a nicer, one step equation to work with:
    P=I^2R.

    The same can be done for current:
    P=VI, I=\dfrac{V}{R}
    \therefore P=\dfrac{V^2}{R}.

    Using this equation makes the operation doable in one step:
    R=\dfrac{230^2}{2000} = 26.45.

    TL;DR combining is not a necessity and the end result can be achieved using the standard equations. But it makes it much nicer!
    thx
 
 
 
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