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# Coulomb's law and electric field (calculations)

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For the folloing 6 questions, I got completely different (way off) answers from the ones given in the textbook. This is really driving my crazy. I checked my work many times but can't find out what I did wrong. If anyone is interested in physics problems and have the time, I hope you can help me check out whether the answers in the textbook are correct or not, so that I can know if I am correct. I really appreciate for your help! :shrug:

I am wondering if I am calculating something seriously wrong so that I am getting way off answers for all these questions. I am afraid and frustrated now that I am losing confidence in this topic. :hihi:

1) A point charge of +3.8x10^-6 C is placed 0.20 m to the right of a charge of -2.0x10^-6 C. What is the force on a third charge of +2.3x10^-6 C if it is placed

1a) 0.10 m to the right of the second charge?

[assuming the second charge is the charge on the right side(+3.8x10^-6C), I got an answer of 7.4N

, but the answer is 7.9N

]

1b) where would the third charge experience a net force of zero?

[i got an answer of 0.53 m to the left of the -2.0x10^-6C charge, but the answer given is 1.2x10^-2 m [right of smaller charge] ]

2) 4 objects, each with a positive charge of 1.0x10^-6 C, are placed at the coreners of a 45-degree rhombus with sides of length 1.0m. Calculate the magnitude of the net force on each charge.

[i got 2.2x10^-2 N and 1.9x10^-2 N respectively, but the answers provided are 8.4x10^-3 N and 1.9x10^-2 N respectively.]

3) 3 charges of +1.0x10^-4 C form an equilateral triangle with side length 40cm. What is the magnitude and direction of the electric force on each charge?

[i got an answer of 9.7x10^2N[90 degrees from the line joining the other charges], but the answer provided is 8.9x10^2 N[90 degrees away from the line connecting other charges] ]

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4) 2 charges of +4.0x10^-6 C and +8.0x10^-6 C are placed 2.0m apart. What is the field strength halfway between them?

[i got an answer of 3.6x10^4N/C toward the 4.0x10^-6C charge, but the answer given is 3.6x10^-4N/C toward the larger charge]

5) A point charge of 2.0x10^-6C experiences an electric force of 7.5N to the left. What force would be exerted on a -4.9x10^-5 C charge placed at the same spot?

, but the answer given is 1.5x10^5N/C

]

6)

[i got 3.2x10^5N/C [right], but the answer in the book is 1.8x10^5N/C

]

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I know you physictists can calculate these very quickly. :shrug:

Could somebody help me with 1 or 2 questions? I still can't get the answers in the book! Thanks!

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1) this is a three stage process.

Stage 1. Calculate the force on the charge from one of the charges.

F1= q1q2/r^2 4*pi*E0 q1 = +3.8x10^-6 C, q2 = + 2.3x10^-6 C r = .2 + .1 = .3

F1 = 0.873N

Stage 2. Calculate the force on the charge from the other charge.

F2 = q3q2/r^2 4*pi* E0 q3 = -2.0x10^-6 C, q2 = + 2.3x10^-6 C, r = .1

F2 = -4.13N (ie to the left).

Stage 3. Calcultate the resulting force

F = F1 + F2 = -3.26N (ie to the left).

That's how to work it out.

However, I notice I have got the wrong answer. That is because I did not draw the charge picture correctly. This is partly because I don't have a clue what the question means by '0.1m to the right of the second charge' and partly because I didn't read it properly.

Nevertheless, the simple easy method works.

If the question were in 3 dimensions, it would still work. You just add the Force vectors together.

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for q1a, I think we should assume the second charge is the charge on the right side(+3.8x10^-6C, but I still don't get the answer. This set of questions is making me crazy...

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I don't know if it helps or not but the system presented is attractive in nature.

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