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Discussion in 'Electronic Basics' started by irfan aziz, Dec 26, 2003.

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  1. irfan aziz

    irfan aziz Guest

    I am having problems working out these three questions, please can someone
    help me?

    1) three point charges are situated on the x-axis of a co-ordinate system.
    Q1= +2.0 nC is +0.05m from the origin
    Q2= -3.0 nC is +0.1m from the origin
    Q3= -10.0 nC is -0.1m from the origin
    what is the total force exerted by the three charges on a fourth charge Q4=
    + 5.0 nC situated at the origin? (1 nC = 10 to the (-9)c).

    2) the element of an electric fire consists of 5 m of wire of 0.5mm
    diameter. the resistivity of the wire at 15 oC (degrees celecus) is 1.12 x
    10 (-6) ohms meter. when connected to a 240v supply the fire dissipates 2kw
    and the tempreture of the element is 1015oC (degrees celecus). determine the
    value for the mean tempreture co-effivient of resistance of the wire.

    3) a simple potential dividing circut is to be used to supply 10 V to a
    device which draws zero current. if the supply is 100V and no more than 1 uA
    is to be drawn from this supply, calculate the value of the resistors in the
    potential dividing chain.
  2. <snip>

    I don't know how it is in your part of the world, but it's
    customary in the U.S. for students to do their own homework.

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  3. Well, the last one is easy, 100V / 1 uA is 100 megs, so you could
    divide it down with 90 meg and 10 meg resistors. But you should
    really be doing these homework problems yourself, and not asking
    others for help.

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  4. John Fields

    John Fields Guest

  5. All the concepts and equations you need should be given
    to you in your textbook before the questions are posed. Find
    them in the index.

    (A big help in doing homework is to try to work your
    solutions backwards; see if you can start from your answers
    and use the relevant equations to find the questions.)
    Review the equation for finding the force between pairs
    of charges. What kind of force is it (scalar, vector, etc
    [hint; it's along the x-axis])? Review how forces add.

    (This is hard to do backwards since there's an infinite
    number of charge distributions that can produce a given
    force; I think the best you can do is to assume two charges
    to find where you _can't_ put the third. This just decreases
    the infinity so to speak.)
    Review the definition of "temperature coefficient" and
    the definition of "mean". Review the definition of
    resistivity and find it for the wire at 1015C. Assume T.C.
    linear (since you weren't told it isn't) and plug in your

    (Much easier to work backwards; fewer unknowns.)
    Review how a resistive voltage divider works.

    Use Ohm's Law to find the total resistance of the chain
    (you're given the supply's E and allowed I draw, find the R
    that will draw that I). It's dropping 100V, so since you're
    given a very convenient requirement (1/10th of the total)
    just divide the total resistance by 10 to determine how many
    ohms the bottom resistor must be to drop 10V. Subtract that
    from the total resistance to get the top resistor value.

    (Easiest to do backwards.)

    Mark L. Fergerson
  6. ----------------------
    Just do the vector sum of the KQq/r^2's for each pair, or do the
    combined vectorial field KQ/r*2 and then F=Eq. Whassa problem,
    no read the chapter?

    Draw a temp dependant graph of resistivity, which is R*A/l for the
    resistive wire.

    Oh come on, didn't you read anything? See here:

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