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Find the Y component of Electric field at a point

  • #1

Homework Statement


The figure below shows a thin, vertical rod of length L with total charge Q. The indicated point P is a horizontal distance x from the one end of the rod. What is the electric field at point P. Express your answer in component notation in the two blanks below.

L = 5.0 cm, Q = 3.0 nC, and x = 3.0 cm.
physs2.jpg

Homework Equations



kq/r^2

The Attempt at a Solution


I found the x component to be 15000N/C

theta = arctan(5/3) = 59 deg
total charge = [(8.99x10^9)(3x10^-9)]/((0.03)^2) = 29966
x component = 29966cos59=15417 with sig figs is 15000

so now the y component should just be 29966sin59 = 25685 rounded to 26000 N/C but that doesn't make sense because the y component should be facing down
 

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Answers and Replies

  • #2
even when I did theta = -59 degrees the answer was incorect
 
  • #3
haruspex
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total charge = [(8.99x10^9)(3x10^-9)]/((0.03)^2) = 29966
Not all the charge is at distance 3cm from P.
 
  • #4
Not all the charge is at distance 3cm from P.
So did I just get lucky with the first answer??
 
  • #5
im confused
 
  • #6
Ok I think I see what youre saying. I got the distance from the top of the rod as
[(8.99x10^9)(3x10^-9)]/((0.058^2))=7932
7932sin59 = 6800

incorrect

@haruspex
 
  • #7
I tried generalizing from the middle

[(8.99x10^9)(3x10^-9)]/((0.039^2))=17685
(new angle arctan(2.5/3) = 39 deg
17685sin(39) = 11321

incorrect
 
  • #8
haruspex
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Ok I think I see what youre saying. I got the distance from the top of the rod as
[(8.99x10^9)(3x10^-9)]/((0.058^2))=7932
7932sin59 = 6800

incorrect

@haruspex
Consider an element length dy at distance y up the rod. What is the charge on it? What field does it generate at P? What is the x component of that?
 
  • #9
Consider an element length dy at distance y up the rod. What is the charge on it? What field does it generate at P? What is the x component of that?
so the charge on dy is kq/r^2 at point P the x component is (kq/r^2)cos(theta)
 
  • #10
my x component is correct tho, I thought the y should be simple just sin of the angle @haruspex
 
  • #11
haruspex
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so the charge on dy is kq/r^2
That s not a charge.
What is the charge on a section of length dy?
If it is distance y up the rod, how far is it from point P?
 
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  • #12
That s not a charge.
What is the charge on a section of length dy?
Of it is distance y up the rod, how far is it from point P?
would it be dy/3? im really not understanding. dy is sqrt(y^2+9) away
 
  • #13
haruspex
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would it be dy/3?
No.
What is the charge density along the rod? If the whole rod length L carries charge Q, how much charge is there on section length dy?
dy is sqrt(y^2+9) away
Units! But better still, don't plug in numbers yet, just call it x2, not 9.
 
  • #14
No.
What is the charge density along the rod? If the whole rod length L carries charge Q, how much charge is there on section length dy?
Is it Q*dy????
 
  • #15
Help?
 
  • #16
GOT IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! >=[
 
  • #17
haruspex
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Is it Q*dy????
The charge Q is spread out along a rod length L, what is the charge density (charge per unit length)?
 
  • #18
The charge Q is spread out along a rod length L, what is the charge density (charge per unit length)?
Then I think it is dQ*dL

I got the answer partial correct 1.8/2 points with the answer -7900N/C
 
  • #19
Im confused why im getting partial credit

[(8.99x10^9)(3x10^-9)]/((sqrt(.05^2+.03^2)^2) = 7932 taking the negative because +/- sqrt = -7932

1.8/2 points
 
  • #20
SammyS
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Is it Q*dy????
You seem to be guessing, rather than answering the specific question haruspex asked you.

To ask basically the same thing:
If the length of the rod is 5.0 cm, and the charge on the rod is 3.0 nC, how much charge is on each 1 cm of the rod?
How would you write that using symbols: L for length and Q for the charge?
 
  • #21
SammyS
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Im confused why im getting partial credit

[(8.99x10^9)(3x10^-9)]/((sqrt(.05^2+.03^2)^2) = 7932 taking the negative because +/- sqrt = -7932

1.8/2 points
Slow down. You're nowhere near ready to solve this. The solution will require that you use Calculus to do integration.
 
  • #22
Slow down. You're nowhere near ready to solve this. The solution will require that you use Calculus to do integration.
Oh god.. I have to go to physics class right now. im getting frustrated awith this anyway I have to step away
 
  • #23
SammyS
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So did I just get lucky with the first answer??
You mean to ask if you got lucky with getting the correct numerical answer for the x component of the leectric field (which is in your "other" thread.)

The answer is absolutely yes, you got lucky.

You used cosine, when what was required there was to use sine.

Both components require the use of integral calculus.
 
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  • #24
hey @SammyS do I have to use trig substitution?
 
  • #25
SammyS
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hey @SammyS do I have to use trig substitution?
Most likely you will.

You will first have to come up with a linear charge density and will need to set up an integral.
 

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