Coloumb's Law problem (calculating Electrical force on a charge)

XuFyaN

1st Problem

Two positive charges, each 4.18C, and a negative charge, -6.36C are fixed at the vertices of an equilateral triangle of side 0.13m.
Find the electrical force on the negative charge.

I don't know how to calculate electrical force on a specific charge :-S
and please explain why we take negative charge always as positive ?

please explain in detail why we take negative charge always as positive when applying coulomb's law.

2nd problem:

two charges q1 and q2, held a fixed distance d apart. (a) Find the strength of electric force that acts on qi. Assume that q1=q2=21.3C and distance=1.52m
(b)
A third charge q3=21.3C is brought in and placed as shown in figure, find the electric force on q1 now.




i've solved part (a) of the 2nd problem but i don't know how to solve the second part of it :-S
please explain
( I don't want the answers of these problems , i just need some help and explanation )

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tiny-tim

Hi XuFyaN! Welcome to PF!
If E is the field and q is the charge, the the force is F = qE.
I don't understand what you mean.
Find the force on q₁ as if q₃ wasn't there, and the force on q₁ as if q₂ wasn't there.

That will give you two vectors, so add those vectors (either by adding the components, or by using a vector triangle ).

XuFyaN

for Problem 1:

no E is given there :-S , how can i solve it ?
first i applied coloumb's law on two charges q1 and q2 , what should be the next step ?

for problem 2

i have solved (a) part by using F=kq₁q₂/r²
and got the answer 1.76N , but for part (b),
i don't know how to find the strength of the electric force on q1 charge ?

Edit:

part (b) of 2nd problem has been solved but i don't know its correct or incorrect , please have a look
F1= F₁₂+F₁₃sin45
F1=1.76+1.76(0.707)
F1=3.00N

is it correct or incorrect ?

XuFyaN

please reply

tiny-tim

(is it 45º? the original question was about an equilateral triangle (3 x 60º), is this the same?)

No, you've assumed the resultant force is along one of the sides, it isn't.

XuFyaN

so should i have to multiply sin60 by 3 ??
bec0z according to b0ok the answer is 3.02N and i am not getting this correct :(

tiny-tim

Stop guessing!

Which direction do you think the total force is in?