irfan said:

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

help me?

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.)

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).

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.)

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.

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.)

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.

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