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Having lots of trouble with a few problems that are due tonight please help

  1. Jun 24, 2004 #1
    having lots of trouble with a few problems that are due tonight...please help!!!

    1. A 0.28F capacitor is desired. What area must the plates have if they are to be separated by 1.2mm air gap?

    I used C=E0 (A/d) and I keep getting the same and wrong answer. What am I doing wrong?

    2. What potential difference acting over a distance of 3.5cm would be needed to balance the downward force of gravity so that an electron would remain stationary? Assume the electric field is uniform.

    I used V=mass of electron*gravity*0.035m/1.6*E-19, and that gave me the wrong answer. Help anyone.

    3. A 9.4V battery is connected to a bulb whose resistance is 2.1ohms. How many electrons leave the battery per minute.

    I used ohm's law I=9.4/2.1 to get the current 4.476C/s. Then I multiplied 4.476c/s*60s/1.6E-19 and got 1.68E-17 electrons/minute...and that answer was wrong...any help?
     
  2. jcsd
  3. Jun 24, 2004 #2

    Doc Al

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    Staff: Mentor

    Your methods all look correct. Show your work in the first two problems--I suspect an arithmetic error. That's what happened in problem 3: check those exponents.
     
  4. Jun 24, 2004 #3
    I still keep getting the wrong answers can you show me what to do...I just plugged the numbers into the formulas
     
  5. Jun 25, 2004 #4

    Doc Al

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    Staff: Mentor

    check your arithmetic

    You know what to do, you are just making arithmetic errors. Show the actual numbers you are plugging into the equations.

    For example, you showed your work for problem 3: The numbers are right, but the answer is wrong. Redo your calculation carefully. You messed up the exponent in the answer.
     
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