couple simple electric force questions


by JimiJams
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#1
Feb15-11, 02:11 PM
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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
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Feb15-11, 02:15 PM
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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.)
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Feb15-11, 04:47 PM
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Quote Quote by JimiJams View Post
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.
Magnitudes are always positive, if that's what's confusing you.

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Feb15-11, 05:14 PM
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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.
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Feb15-11, 05:26 PM
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Quote Quote by JimiJams View Post
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.
When finding the force you must apply the rule that like charges repel and unlike charges attract. That tells you the direction of the force. (The sign depends on your coordinate system.)

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.
Not sure what you mean here, but when adding up components one usually finds components with respect to the horizontal. And that depends on your coordinate system.

Post the exact problem and we can get into more detail.
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Feb15-11, 05:34 PM
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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????
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Feb15-11, 05:50 PM
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Quote Quote by JimiJams View Post
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.
Again, magnitudes are always positive (by definition). The force between q3 and q1 is repulsive, so the force on q1 from q3 points in the direction of q3-to-q1. Taking horizontal and vertical as the x and y axes, the x and y components of that force will both be negative.

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????
It depends on the direction of the force and how you define the angle. See my comments for this example above.
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Feb15-11, 08:13 PM
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thanks a lot for the help. I actually just got back from the school tutor and it's crystal clear now. I wasn't drawing out vector lines, I was just doing the calculations. After seeing the vectors it all makes sense. Thanks again though!


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