Making a square with point charges (alternative solutions)

In summary, the first charge costs no work to bring in, the second charge costs kq2/A work to bring in, the third charge costs qV work to bring in, and the fourth charge costs (kq^2/d) * (4+sqrt(2))A work to bring in.
  • #1
narfarnst
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Homework Statement


Calculate how much work it takes to make a square of side length A with point charges (all of charge q) at each vertex.


Homework Equations





The Attempt at a Solution


I know one way to do it. (find the difference in potential for each point and then go from there to find the work).
But I have two other ways that I'm not sure if they'll work or not.

First way
Since you'll have to do more work each time to overcome the newly added charges, can you just find the work it takes to place one charge and then multiply by six? (Because for each charge you have to do that much more work. i.e, 1 charge is 1W, 2 charges is 2W, 3 charges is 3W, so total charge would be (3*2*1)W=6W... or would it be (3+2+1)W=6W?).

Second Way
Can I just find the potential at the center of the finished square and use that final potential to find work?

Also, am I correct in assuming that because there are initially no charges (and no surroundings to speak of) the first charges needs 0 work?


Thanks!
 
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  • #2
The best way to do this is to assemble the charges one at a time. The work to bring in the first charge is zero. For the second charge it is kq2/A. For the third charge it is qV, where V is the electric potential at the location of the third charge due to the presence of the other two charges, and so on. Add all the works to get the total. I wouldn't trust any other method.

You are correct in assuming that initially there are no charges. That's why it costs no work to bring in the first charge.
 
  • #3
This is more of an accounting problem, no really a whole lot of math.

say you have charges oriented in this formation at the end
1 2

3 4

and they all have an equal distance of d

what is the energy to bring the first charge in?

1 -- all by itself

well its zero right. so U1 = 0

Now bring in charge 2

1 2

now we have work being done. -- the potential will be U2 = kq^2/d

now bring in charge 3

1 2

3 -- we are being affect by charge 1 and charge 2

U3 = Kq^2/d + kq^2/(d*sqrt(2))

Kq^2/d - being the potential due to charge 1 and 3
kq^2/(d*sqrt(2)) - being the potential due to charge 3 and 2, d*sqrt(2) is the distance between them

and lastly add the final charge

1 2

3 4do you see the logic here?

once your done you sum up all of your energy
 
  • #4
Yes, I get that. That's the method I already know how to do (thanks, by the way. I hadn't actually worked it out yet, but I knew how :) ).
I'm asking if there is an easier way to do the problem, namely, one of the two ways I suggested.
Thanks.
 
  • #5
>>Can I just find the potential at the center of the finished square and use that final potential to find work?

Oh alright :) well this is potential would be different in that scenario

that would be what the potential is at a point in the center of the square, not what work it took to build the square by pulling charges from infinity

the potential at the center of the square:

4*kq^2/(d*sqrt(2)/2) -- assuming all charges are the same

to build the square it would be :

(kq^2/d) * (4 + sqrt(2))
 
  • #6
A-ha!
Thanks.
 

FAQ: Making a square with point charges (alternative solutions)

How can I make a square with point charges?

Making a square with point charges can be achieved by arranging four equal charges at the corners of a square and then placing a fifth charge in the center. This configuration creates an electric field that forms a square shape.

What are some alternative solutions for making a square with point charges?

Aside from the traditional method of using four charges at the corners and one in the center, there are other ways to create a square with point charges. Some alternative solutions include using four charges on the sides of a square and one in the center, or using four charges on the sides and one at each corner.

Do the charges used to make a square have to be equal?

Yes, for a perfect square shape to be formed, the charges at the corners should be equal to each other and the charge in the center should be opposite in sign to the corner charges. However, if the goal is to create a square-like shape rather than a perfect square, the charges can be varied.

How does the distance between the charges affect the shape of the square?

The distance between the charges can affect the shape of the square by altering the strength of the electric field. As the distance between the charges increases, the electric field weakens, and the square shape becomes less defined. On the other hand, as the distance decreases, the electric field strengthens, and the square shape becomes more defined.

Can a square be made with more than five point charges?

Yes, a square can be formed with more than five point charges. The key is to ensure that the charges are arranged in a way that creates an electric field with a square shape. This can be achieved by using a combination of charges at the corners, sides, and center of the square.

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