urgent help

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.

 

<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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"Quando Omni Flunkus Moritati" (Red Green)

 

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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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
numbers.

(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

 

----------------------
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:

ftp://ftp.armory.com/pub/user/rstevew/TUTS/voltdvdr.txt

-Steve