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Energy in Electric Circuits

  1. May 10, 2010 #1
    1. The problem statement, all variables and given/known data
    A 100-W heater is designed to operate with an applied
    voltage of 120V.

    a) What is the heater's resistance, and what current does the heater carry?

    b) Show that if the potential difference V across the heater changes by a small
    amount ΔV, the power P changes by a small amount ΔP, where ΔP/P = 2ΔV/V. (Hint:
    Approximate the changes by modeling them as differentials, and assume the
    resistance is constant.

    c) Using the part b result, find the approximate power delivered to the heater
    if the potential difference is decreased to 115V. Compare your result to the
    exact answer.


    2. Relevant equations
    P = IV = V2/R

    3. The attempt at a solution
    I'm stuck on part b). So far I have
    ΔP=ΔV2/R

    ΔP/P=(ΔV/V)2

    I'm not really sure what to do.
     
  2. jcsd
  3. May 10, 2010 #2

    ehild

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    Gold Member

    The change of a function f(x) can be approximated by Δf=f'(x)Δx, where f' is the derivative of f(x) taken at the original value of x, and Δx is a small change of x.


    ehild
     
  4. May 10, 2010 #3
    Thanks.

    Another question, for part c) when you change the voltage on a circuit does the resistance stay the same? Or the current? Although I don't really see how the current could but maybe i suppose.
     
  5. May 10, 2010 #4

    ehild

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    Homework Helper
    Gold Member

    The heater stays the same, isn't it? And it consist of a resistor, and some other parts which do not change either, if you plug it into an other socket or the household voltage decreases because of some overload happening in the network.
    Question c. wants you to apply the formula in question b, if V and ΔV are given.

    ehild
     
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