Homework Help: New to this yet needing some help

1. Aug 23, 2007

immeteen

It's the first week of school and my professor has already assigned homework and I don't have my book yet. I am completely lost on two problems:

1)Calculate the magnitude of the force between two 3.85 µC point charges 7.8 cm apart.

Of course the answer is going to be in newtons due to force.

2) How many electrons make up a charge of -90.0 µC?

There is one thing with this assignment, sig figs are extremely sensitive and I only have a 2 percent error. I have worked the problems two ways but I only have two more trys before I can receive credit. Please help. Thnx

2. Aug 23, 2007

learningphysics

Can you show how you approached these problems and what answers you got?

3. Aug 24, 2007

olgranpappy

You better buy that book right quick--or check it out from the library...

Your book will have an expression for the force between two point charges (which depends on the value of the two charges and the distance separating the charges). You have all the needed info, so plug and chug.

Your book will tell you the charge of a single electron. Then you will have to figure out what number to multiply that single-electron charge by to get -90.0 micro-Coulombs.

There is one thing with this assignment, sig figs are extremely sensitive and I only have a 2 percent error. I have worked the problems two ways but I only have two more trys before I can receive credit. Please help. Thnx[/QUOTE]

4. Aug 24, 2007

immeteen

I've finally got the right answer for the first one. Here's my work for anyone having a similiar problem:
f=k[(3.85x10^-6)(3.85x10^-6)/(7.8 x10^-2) ]

k[(1.48 x10^-11)/(.006084)]=

21.89

I used coulomb's law which states f=k [(q1)(q2)/(r2)]. k is a constant.

5. Aug 24, 2007

Leong

1 electron : charge of 1 electron

? many electrons : -90.0 micro C.

6. Aug 24, 2007

immeteen

electron question:

(6.241509*10^18 electrons/coulomb) * 90*10^-6=5.617*10^14 electrons

7. Aug 24, 2007

Leong

wrong? then, i don't know...

8. Aug 24, 2007

Leong

maybe you should try putting your answer as 5.62*10^14 electrons if you take the sig figures into account...

9. Aug 24, 2007

immeteen

ahhh merci beaucoup! tu es tres intelligent et je suis bete.

incase you speak 0 french: thank you so much. you are very intelligent and i am stupid.

10. Aug 24, 2007

Leong

you should not say like that... it's not good for you... but i like the part when you say i am INTELLIGENT.....