Register to reply 
Couple simple electric force questions 
Share this thread: 
#1
Feb1511, 02:11 PM

P: 53

hey everyone, We're doing electric force and fields right now in our physics class. The trouble I'm having is with signs. I'm doing a prblem right now where I need to solve the magnitude and angle of electric force on one charge from a couple other charges and I don't know what sign to apply to the answer. Can anyone help? If you need more details about the problem just let me know.
Thanks a lot 


#2
Feb1511, 02:15 PM

Mentor
P: 41,563

Post the details of the problem, what you've done so far, and where you are stuck.
(I'll move this to Intro Physics, where it probably belongs.) 


#3
Feb1511, 04:47 PM

Mentor
P: 41,563




#4
Feb1511, 05:14 PM

P: 53

Couple simple electric force questions
it's hard to explain but suppose a problem involves three charges, one at 3 of the 4 corners of a rectangle. I need to find the force on one of the charges. well one charge is diaganol from the other. so you use pythagorean's theorem to find the distance. Then I find the force using coulomb's law. How do I know whether that number is positive or negative, because when I see the work done by others in finding the components they sometimes have cos or sin as positive or negative. These signs are killing me.
Also when finding the components (multiplying the magnitude by cos and sin of theta) I don't know which angles considered theta. The angle at the charge we're assessing or the other charge diaganol from the charge we're assessing. 


#5
Feb1511, 05:26 PM

Mentor
P: 41,563

Post the exact problem and we can get into more detail. 


#6
Feb1511, 05:34 PM

P: 53

ok lower left corner of rectangle is positive charge (5nC) q1, 3cm to the right is negative charge (5nC) q2, 4cm above q2 is a positive charge (10nC) q3. I need to find the force on q1. when finding the magnitude of the force between q3 and q1 how do I know whether it's negative or positive.
And when finding the components, in other words multiplying the magnitude by cos theta and sin theta, I've seen people applying a negative sign to cos or sin or both???? 


#7
Feb1511, 05:50 PM

Mentor
P: 41,563




Register to reply 
Related Discussions  
Couple Multiple Choice questions on force and centripetal acceleration.  Introductory Physics Homework  6  
Couple of simple (I hope) questions  Calculus & Beyond Homework  4  
A Couple More Simple Think critically and Explain Questions  Introductory Physics Homework  3  
Some electric charge and force questions need help on how to set them up  Introductory Physics Homework  2  
Electric Fields: A couple of general questions  Introductory Physics Homework  0 