Electric field = 0 at what point on x axis?

In summary, the conversation discusses finding the point on the positive x-axis where the electric field is 0, as well as the point where the potential is 0. The summary also includes a correction to the original solution and a clarification on using distance versus coordinates in calculations. It is determined that the point where the electric field is 0 is x=2.366, and the potential is 0 at both x=1.5 and x=3/4. The use of absolute value in the denominator of the potential equation is also explained.
  • #1
connor02
27
0

Homework Statement



you have a +3C and a -1C charge on the x axis. the +3C charge is at the origin and the -1C is at x=1. At what point on the positive x-axis does E=0?

Homework Equations



E=kQ/r^2


The Attempt at a Solution



So i have

E=(-1k/r^2)+(3k/(1+r)^2)

Setting E=0

1k/r^2=3k/(1+r)^2

r= sqrt(0.5) = 0.707

is this correct?
 
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  • #2
Will your answer be a point in between the two charges, or to the right of each of them?
 
  • #3
connor02 said:

The Attempt at a Solution



So i have

E=(-1k/r^2)+(3k/(1+r)^2)

What does "r" mean?

connor02 said:
Setting E=0

1k/r^2=3k/(1+r)^2

r= sqrt(0.5) = 0.707

is this correct?

No, r= sqrt(0.5) is not correct. How did you get it?

ehild
 
  • #4
ah, r=1.707?
 
  • #5
What is r?
 
  • #6
r=the point on the x-axis where the E firld is 0

right?
 
  • #7
You used the expression of the electric field at distance "r" from a point charge. "r" is distance, not a point. What do you substitute for "r" for the 3C and -1C charges? ehild
 
  • #8
my bad. r=distance to the right of the -1C charge. x=1.707 is the point on the x-axis where E=0.

right?
 
  • #9
connor02 said:
r=the point on the x-axis where the E firld is 0

right?
No, it isn't. Your r is clearly a number, not a point. I suspect you mean it is the x-coordinate of such a point. In that case, the distance to 0 is r but the distance to 1 is |1- r| not 1+ r.
 
  • #10
connor02 said:
my bad. r=distance to the right of the -1C charge. x=1.707 is the point on the x-axis where E=0.

right?

If so, your equation 1k/r^2=3k/(1+r)^2 is correct, but the solution is wrong. Check it.

ehild
 
  • #11
WOW i feel STUPID.

r=-0.366 or 1.366

Since x is to the right of the -1C charge, x = 2.366 when E=0.

I hope i am right now.
 
  • #12
It is correct now. Well done!

ehild
 
  • #13
Thanks man!

Now, I'm just curious, but if I wanted to find the spot where V=0, I would have

0=3Q/(1+r) + (-Q/r)

so r=1/2

Does that mean V=0 at x=1.5 or V=0 at both x=1.5 and x=0.5?
I think it is the second one as all potential lines have to form a closed surface.
Am I right?

thank you.
 
  • #14
It is less confusing if you calculate with the position of the point on the x axis, that is with its x coordinate.
If a point charge Q is at position x=a, its contribution to the potential at x is kQ/|x-a|. Note the absolute value in the denominator!

In this problem, U(x)=3k/|x|-k/|x-1|. U(x)=0 at x=1.5 and x=3/4.

ehild
 
  • #15
Yes you are right, it is a lot easier to make sense of the answer if i use the point on the x axis.

Why did you use absolute value?

I used

0 = 3k/x + [(-Q)/(x-1)]

and i get the solution x = 1.5 only

thanks.
 
  • #16
The term "r" in the denominator of the potential is distance. The distance is positive (or zero). The coordinate can be both positive and negative.
At distance r from a point charge, the potential is the same either at the right hand side of the charge or at the left hand side.
Find the potential of the -1 C charge at x=3/4. What do you get without absolute value and what is the right value of the potential?

ehild
 

1. What does it mean for the electric field to be 0 at a point on the x axis?

When the electric field is 0 at a point on the x axis, it means that there is no net flow of electric charge in that direction. This can occur if there are equal and opposite charges on either side of the point, canceling out the electric field.

2. How can we determine the point on the x axis where the electric field is 0?

To determine the point on the x axis where the electric field is 0, we can use the equation E = kq/r^2, where E is the electric field, k is the Coulomb's constant, q is the electric charge, and r is the distance from the charge. We can set E equal to 0 and solve for r to find the distance from the charge where the electric field is 0.

3. Can the electric field be 0 at more than one point on the x axis?

Yes, the electric field can be 0 at multiple points on the x axis. This occurs when there are multiple charges with equal and opposite values located at different distances from the point on the x axis.

4. What is the significance of the electric field being 0 at a point on the x axis?

The electric field being 0 at a point on the x axis indicates that there is no force acting on a charged particle placed at that point. This can be useful in understanding the behavior of charged particles in an electric field.

5. Can the electric field be 0 at a point on the x axis if there is a non-zero charge located at that point?

No, the electric field cannot be 0 at a point on the x axis if there is a non-zero charge located at that point. This is because the electric field is a measure of the force that a charge would experience at that point, and a non-zero charge would create a non-zero electric field at that location.

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