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P=vi, v=ir

  1. Jul 18, 2010 #1
    If an appliance is rated 60W 120V, R=1 ohm. Then I = V/R= 120/1 = 120A. But the household current is 13 A (max). Will the fuse blow?
     
  2. jcsd
  3. Jul 18, 2010 #2

    ehild

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    The power rating is P=VI=V2/R = I2R.

    If P=60 W, V = 120 V, R=V2/P=240 Ω. It can not be 1 Ω.
    If you connect an 1 Ω resistor to 120 V, the 13 A fuse will blow.

    ehild
     
  4. Jul 18, 2010 #3
    If a 1 ohm resistor is connected, the 13A fuse blows. Why? The max. current supplied to the house is 13A. Where does the extra current comes from to blow the fuse?
     
  5. Jul 18, 2010 #4
    Something is wrong in those figures... From P=IV then I = P/V = 60/120 = 0.5 A. Using V = IR then I = V/R = 120/1 = 120 A. So something isnt right there...

    J
     
  6. Jul 18, 2010 #5

    Redbelly98

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    In reality, more than 13A would flow for a brief time, until the fuse wire is heated up to it's melting point.
     
  7. Jul 18, 2010 #6

    ehild

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    The voltage is supplied, and you can use more current if you have a higher-amps fuse.

    ehild
     
  8. Jul 19, 2010 #7
    Where does the extra Amps come from?
     
  9. Jul 19, 2010 #8

    Redbelly98

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    It is supplied by the electric company, just like all of the electricity that is used.
     
  10. Jul 21, 2010 #9
    How can the home pull in more than 13A from outside. The 13A fuse will melt.
     
  11. Jul 21, 2010 #10

    Redbelly98

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    You must have missed Post #5. It takes time, a very short time, for the fuse to heat up to its melting point. This does not happen instantaneously. Current can be more than 13A during that brief time.
     
  12. Jul 30, 2010 #11
    Why are transmission cables have high voltage but low current. Should not V=IR, high voltage, high current?
     
  13. Aug 1, 2010 #12

    Redbelly98

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    The fixed quantity in designing a transmission system would be power, not resistance. So use the equation that relates power to voltage and current ... that provides the relation between V and I that is relevant here.
     
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