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Homework Help: Current & Power

  1. Aug 10, 2009 #1
    Problem 1

    Just starting on the Current & Power chapter and I'm a little confused. I know they're just basic definitions and concepts, but I'm having trouble wrapping my head around it. Easy questions follow, but I'm lost.

    1. The problem statement, all variables and given/known data

    How many electrons flow past a point in a wire each second if the wire has a current of 1.00 Amperes?

    2. Relevant equations

    1 Ampere = 1 Coulomb/1 second
    Avogadro's number = 6.02x10^23

    3. The attempt at a solution

    I don't know where to start.

    Problem 2

    1. The problem statement, all variables and given/known data
    A lamp draws 0.50 A from a 120 V generator. (a) How much power does the generator deliver to the lamp? (b) How much electric energy does the lamp convert to light and heat in a period of 5.0 minutes?

    2. Relevant equations

    P=Energy/time or dU/dt or current*Voltage.

    3. The attempt at a solution

    a.) P=current*Voltage
    P= (.5 Amps)(120 Volts)= 60 Watts

    b.) I don't know where to start with this.

    Maybe I'm just missing some key concepts, or something just isn't sticking in my stupid head due to the way it's been explained. I don't know.

    Any help?
  2. jcsd
  3. Aug 10, 2009 #2
    1) How much charge is flowing through the wire per second if the current is 1 ampere? What is the charge of an electron?

    2b) The bulb converts electrical energy to light and heat, right? How much much energy does the bulb use in 5 mins?
  4. Aug 10, 2009 #3
    1.) I get the right answer if I divide 1/1.6e-19 but I'm not entirely sure why.

    2b.) P=dU/dt
    dU=60*300=18kJ which is right.

    I didn't realize it said five MINUTES.
  5. Aug 10, 2009 #4
    Each electron has a fixed charge of -1.6e-19 C.
    So with 1C/s it follows that there are

    Units in brackets
    electrons/second = 1 (C/s)/1.6e-19 (C) = 6.25E18 electrons per second.

    It bit like saying a brick cost 25p, you have £1000 how many bricks can you get £1000/£0.25= 4000 bricks.
  6. Aug 10, 2009 #5
    Gotcha. As soon as I started reading your second sentence, it hit me. :)

    Thanks guys. or girls. or dogs. or whatever you might be.
  7. Aug 10, 2009 #6
    None of the above I am actually a fish.
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