# Find the electric field

Istiak
Homework Statement:
Electric field
Relevant Equations:
F=kq/r^2
Solution attempt

#### Attachments

• e4e2c5e8-5062-4bbf-94a7-55bbd56e393c.jpeg
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Mentor
Find the electric field where? Can you post the full problem statement, and maybe the figure that is in the problem sheet that you are working from? Thanks.

Homework Helper
Gold Member
Seems to me you don't like to write a lot. Maybe english is not your mother language ok.

The attachment explains the problem setup but then it offers some choices but it doesn't mention what is the question. It looks like it is asking what is the total potential energy of the system.

And besides that, on your own work you seem to work towards finding the equilibrium position for charge q. Is that what you trying to find?

Istiak
Seems to me you don't like to write a lot. Maybe english is not your mother language ok.

The attachment explains the problem setup but then it offers some choices but it doesn't mention what is the question. It looks like it is asking what is the total potential energy of the system.

And besides that, on your own work you seem to work towards finding the equilibrium position for charge q. Is that what you trying to find?
Yes! It is. That's what I wrote. I was trying to find that value. Potential energy depends on the X²

AF Fardin
AF Fardin
Yep

Homework Helper
Gold Member
Well I still don't understand your work.

At start you write ##F=qE## or ##k''x=qE##
But after some point you write ##F'+F=qE##
And later ##F''+F'+F=qE##
only one case can be true, or I just don't understand something here.

AF Fardin
Mentor
Yep

I was about to delete your reply as unnecessary, but when I checked your previous posting history, I see that you posted about a similar/same problem earlier today and the post was deleted because you showed no effort. Your attachment is added below; are you and @Istiakshovon in the same class?

#### Attachments

• IMG_20210512_130821_065.jpg
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Delta2
Homework Helper
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Hi @Istiakshovon. I will also chip in.

Title: Find the electric field”
The question is apparently not about finding the electric field!

Homework Statement: Electric field”
That’s not a homework statement. You need to give the complete question.

Relevant Equations: F = kq/r^2
The equation is wrong. Maybe you mean E = kq/r^2 or F = kq₁q₂/r^2. However, neither of these equations is relevant to your question!

Like @Delta2, I too I can’t understand your solution attempt.

You have ignored @berkeman's post (#2). Is there a reason? I don’t think you will get help unless you at least post the complete question, as requested. (I can possibly guess what the question is but I might be wrong.)

Istiak
I was about to delete your reply as unnecessary, but when I checked your previous posting history, I see that you posted about a similar/same problem earlier today and the post was deleted because you showed no effort. Your attachment is added below; are you and @Istiakshovon in the same class?

View attachment 284973
Actually, he was new to the community. He didn't understand how the community works. He wasn't familiar with these tools. So, he told me to create new thread here cause, I had talked about the community to him. Then, I had posted it. I hope you understood. I know that "Yep" comment is unnecessary. So, you can delete that. He doesn't know code of conduct of the community either. I haven't read COC also.

Istiak
Hi @Istiakshovon. I will also chip in.

Title: Find the electric field”
The question is apparently not about finding the electric field!

Homework Statement: Electric field”
That’s not a homework statement. You need to give the complete question.

Relevant Equations: F = kq/r^2
The equation is wrong. Maybe you mean E = kq/r^2 or F = kq₁q₂/r^2. However, neither of these equations is relevant to your question!

Like @Delta2, I too I can’t understand your solution attempt.

You have ignored @berkeman's post (#2). Is there a reason? I don’t think you will get help unless you at least post the complete question, as requested. (I can possibly guess what the question is but I might be wrong.)
As I mentioned in my previous reply that this question is not mine. It's another person's. I didn't have much more time to look at his works. So, that what I simply understood without reading answer and question, I had just added that. As I said I didn't have much more time, that's why I added unnecessary **Homework Statement**

Istiak
I was trying to edit my question. Unfortunately, there's no edit button. Is it bug? The thread was migrated from Calculus and Physics. I think that's why I don't have anymore access to edit the question.

Homework Helper
Gold Member
I was trying to edit my question. Unfortunately, there's no edit button. Is it bug?
No it isn't bug, the forum software doesn't let you edit the post a few hours after it was initially posted.

Istiak
No it isn't bug, the forum software doesn't let you edit the post a few hours after it was initially posted.
So, How can I edit Homework Statement?

Homework Helper
Gold Member
Just post a new message here with the full homework statement. Later when a moderator (for example @berkeman) might see your new message, he will copy paste the homework statement to the old message.

Istiak
Istiak
Homework Statement: The second mass (m) also carries a charge q while the mass M is uncharged and made of an insulator. The masses are allowed to move only along the horizontal direction. The distance between the plates A and B as shown in the figure is fixed. Moreover there is a horizontal uniform electric field E that runs between A and B plates and points from left to right.

https://www.physicsforums.com/attachments/e4e2c5e8-5062-4bbf-94a7-55bbd56e393c-jpeg.284961/

Homework Helper
Gold Member
Homework Statement: The second mass (m) also carries a charge q while the mass M is uncharged and made of an insulator. The masses are allowed to move only along the horizontal direction. The distance between the plates A and B as shown in the figure is fixed. Moreover there is a horizontal uniform electric field E that runs between A and B plates and points from left to right.

https://www.physicsforums.com/attachments/e4e2c5e8-5062-4bbf-94a7-55bbd56e393c-jpeg.284961/
Ok fine for the description of the problem setup, but you didn't tell us what is the question asked. Is it "Find the equilibrium position for mass m"?

Istiak
Ok fine for the description of the problem setup, but you didn't tell us what is the question asked Is it "Find the equilibrium position for mass m"?
I am forwarding it to @AFFardin. @AFFardin Reply to him using the reply button to his reply

AF Fardin
Well I still don't understand your work.

At start you write ##F=qE## or ##k''x=qE##
But after some point you write ##F'+F=qE##
And later ##F''+F'+F=qE##
only one case can be true, or I just don't understand something here.
F"+F'+F=qE
This is true I think !

AF Fardin
F"+F'+F=qE
This is true I think !
At First, I wasn't sure about it!
I was trying and thinking with some roughs!
And that's what made you confused. My final thought is this (F"+F'+F=qE).
Still don't know that my approach is correct or wrong!

Delta2
AF Fardin
I was about to delete your reply as unnecessary, but when I checked your previous posting history, I see that you posted about a similar/same problem earlier today and the post was deleted because you showed no effort. Your attachment is added below; are you and @Istiakshovon in the same class?

View attachment 284973
Yes

AF Fardin
Ok fine for the description of the problem setup, but you didn't tell us what is the question asked. Is it "Find the equilibrium position for mass m"?
[6/25, 09:49] AF_Fardin: Both X and Y
[6/25, 09:49] AF_Fardin: Means for both masses

Homework Helper
Gold Member
ok so you want the positions of the two masses for when the whole system is in equilibrium. Hold on while I work on my reply (might take a while cause I am a bit busy with other things at the moment.)

Homework Helper
Gold Member
Ok I think the correct system of equations is
$$Kx-K'y=0$$
$$K'y+Eq-K''z=0$$
$$x+y+z=L$$
The first equation is from the equilibrium condition on mass M, the second from the equilibrium condition on mass m, and the third is self explanatory

P.S We assume that the springs have zero natural length.

AF Fardin
Ok I think the correct system of equations is
$$Kx-K'y=0$$
$$K'y+Eq-K''z=0$$
$$x+y+z=L$$
The first equation is from the equilibrium condition on mass M, the second from the equilibrium condition on mass m, and the third is self explanatory

P.S We assume that the springs have zero natural length.
Wow thank you !
Could you please explain it explicitly for me?

Istiak
Ok I think the correct system of equations is
$$Kx-K'y=0$$
$$K'y+Eq-K''z=0$$
$$x+y+z=L$$
The first equation is from the equilibrium condition on mass M, the second from the equilibrium condition on mass m, and the third is self explanatory

P.S We assume that the springs have zero natural length.
But, actually your approximation doesn't match with any option. What should I do next (with this equation)?

Homework Helper
Gold Member
But, actually your approximation doesn't match with any option. What should I do next (with this equation)?
Could be that my equations are wrong. Can you tell me what are the available options, maybe i can do some reverse engineering of some sort ...

Istiak
Could be that my equations are wrong. Can you tell me what are the available options, maybe i can do some reverse engineering of some sort ...
Look at this picture..
1.

$$\frac{1}{2} kx^2 + \frac{1}{2} ky^2 - qEY$$

2.

$$\frac{1}{2} kx^2 + \frac{1}{2} ky^2 - qE(Y+X) + \frac{1}{2} K''(L-x-y)^2$$

3.

$$\frac{1}{2}kx^2+\frac{1}{2}ky^2-qEY+\frac{1}{2} (L-x-y)^2$$

4.

$$\frac{1}{2}kx^2+\frac{1}{2}ky^2-qE(x+y)$$

5.

$$\frac{1}{2}kx^2+\frac{1}{2}ky^2+\frac{1}{2}k''(L-x-y)^2$$

Unfortunately, MathJax isn't working. I had tried with "####" also. I had tried as we use in SE or, CD

Last edited:
Homework Helper
Gold Member
Ehm I don't think these options are candidates to the equilibrium position , they are rather candidates for the total potential energy of the system. Try option 2 and tell me if it is correct.

Homework Helper
Gold Member
As I mentioned in my previous reply that this question is not mine. It's another person's. I didn't have much more time to look at his works. So, that what I simply understood without reading answer and question, I had just added that. As I said I didn't have much more time, that's why I added unnecessary **Homework Statement**
My apologies. You are just trying to help someone out. Well done for that!

But note, even after 28 posts, we do not know what the actual question is! If at all possible, post the complete question. We shouldn't have to make any assumptions such as "the springs have zero natural length" (@Delta , Post #23); all essential information should be included as part of the question.

Delta2
Istiak
Ehm I don't think these options are candidates to the equilibrium position , they are rather candidates for the total potential energy of the system. Try option 2 and tell me if it is correct.
Actually, that was like an exam (Olympiad). My friend had joined to that. Then, he thought his work isn't correct that's why he told me to post it here.

And, no matter what I choose the site won't show correct answer. That's why I/he wants to know the approximation/answer.

Istiak
My apologies. You are just trying to help someone out. Well done for that!

But note, even after 28 posts, we do not know what the actual question is! If at all possible, post the complete question. We shouldn't have to make any assumptions such as "the springs have zero natural length" (@Delta , Post #23); all essential information should be included as part of the question.
Actually, I had gave the whole question and, figure in following picture. You can see the whole question in the image also.

Homework Helper
Gold Member
Actually, I had gave the whole question and, figure in following picture. You can see the whole question in the image also.
Actually I can't see what the question is. I can read only the description of the system. What I can guess is asking (judging from the available options) is "What is the total potential energy of the system?"

Steve4Physics
Istiak
Actually I can't see what the question is. I can read only the description of the system. What I can guess is asking (judging from the available options) is "What is the total potential energy of the system?"
Yes! You got the question..

Homework Helper
Gold Member
Yes! You got the question..
The correct option according to my opinion is option 2.

Homework Helper
Gold Member
And, no matter what I choose the site won't show correct answer.
I'm not surprised!

@Delta has (I believe correctly) guessed that the question is 'What is the potential energy?'. But original question does not even mention potential energy!

The meaning of distances X and Y is very unclear. The question-wording does not mention X or Y. And the diagram in Post#1 does not show X and Y properly. (An accurate diagram showing the meanings of X and Y would help a lot.)

Assuming you have posted the full question, then it seems that whoever wrote the original question made a lot of bad mistakes/omissions. Without guesses/assumptions, the question is unanswerable.

Delta2