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Homework Help: More Homework due tomorrow.

  1. Dec 6, 2004 #1
    22)What is the total charge of 75kg of electrons.
    25) You are given a number of 10Omega resistors, each capable of dissipating only 1W without being destroyed. What is the minimum number of such resistors that you need to combine in series or parallel to make a 10omega resistance that is capable of dissipating at least 5.0 W?

    For 22 do I just divide 75 by the still mass then multiply by the electon's charge?

    And for 25 I am completely stumped.

    A lot of detailed help before tomorrow would be swell.
  2. jcsd
  3. Dec 6, 2004 #2
    Alright I may be wrong here but I am going on a hunch
    take the fact the mass of an electron is

    9.10938188 × 10-31 kilograms

    find how many electrons you have and and a negative sign in front of it for 22.

    and for 25. is it 100 or 10
  4. Dec 6, 2004 #3


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    If by "still mass", you mean rest mass = 9.11 * 10^-31 kg, then YES, that's right.

    For this, let's make sure we have the question correct. You have a bunch of [itex]10 \Omega [/itex]resistors...yes ? And you want an effective resistance of exactly [itex]10 \Omega [/itex] ? I suggest you look at the question and make sure you have copied it down exactly. It's not a particularly hard problem, but a slight change in the language can result in a big change in the solution.
  5. Dec 6, 2004 #4


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    25)First : If each resistor can handle only 1W, is there any way that you can dissipate 5W using only 2 resistors...or 3 ? So that should tell you what number to start with. Keep this in mind for later.

    Second : What is the effective resistance of a pair of 10 ohm resistors in series ?...and in parallel ? What if we had 3 resistors in series ? Or 3 in parallel ? Notice the pattern. Finally, what is the effective resistance if you put two resistors in series and put another two in series, and then put these pairs in parallel ?

    That should get you going.
  6. Dec 6, 2004 #5
    I had the question wrong
  7. Dec 7, 2004 #6
    Yeah thanks for the tip on 22, but 25 is still confusing.

    I'm guessing I'd need a minimum of 5 10(Omega) resistors. I'm super stumped here and I only have a few hours before my HW is due. Could you help me out with a starting formula?
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