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Homework Help: Need help on these problems

  1. Oct 19, 2005 #1
    Need help on these problems!!

    Hello all,

    I just need a little direction on these problems. Any help that you could give me would be appreciated.

    1) A typical american family used 900 kWh of electricity a month. What is the average current in the 120 V power line to the house? On average, what is the resistance of a household?

    2) A standard 100 W (120 V) lightbulb contains a 7.90 cm-long tungsten filament. The high-temperature resistivity of tungsten is 9.0*10^-7 m. What is the diameter of the filament?

    3) An isolated 4.60 uF parallel-plate capacitor has 4.40 mC of charge. An external force changes the distance between the electrodes until the capacitance is 2.20 uF. How much work is done by the external force?

    Thanks for any help you can give me on these, I have solved all the 7 others and cant seem to arrive at the correct answer for these.

  2. jcsd
  3. Oct 20, 2005 #2
    I imagine you know the formula that relates power to voltage and current.

    For the first one, you need to find the power (Watts) and it gives you the amount of kiloWatt-hours in one month.

    For the second, you're given power, voltage, and resistivity and you're asked to find diameter. I'll give you a hint: Look for a formula that has both resistivity and diameter (or radius). Then find out what other unknown is in that equation. Then see if you can find an equation with that unknown that you can solve knowing the power and voltage.

    For the third: It requires a certain amount of energy to charge a capacitor to a certain voltage. You should know the formula for this. It takes a different amount of energy to charge a capacitor with a different capacitance to a different voltage. The difference of in these energies is the work required to change one to the other.

    Hope that helps.
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