Engineering Calculating the charge if the electric field density = 0

AI Thread Summary
The discussion focuses on calculating charge in a scenario where the electric field density is zero. Participants clarify the roles of variables and constants, specifically distinguishing between 'r' as a variable and 'R' as a constant. They confirm the correct volume element for integration and address mistakes in integral calculations, emphasizing the importance of consistent notation to avoid confusion. The final consensus is that the charge calculated should behave like a point charge of zero from outside the sphere, with a confirmed value of -2.09e-8 C. The conversation highlights the significance of careful mathematical treatment in physics problems.
falyusuf
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Homework Statement
Attached below.
Relevant Equations
Attached below.
Question:
846%2F8468799a-80b2-4052-85ae-161b13a4fffa%2Fimage.jpg

Relevant Equations:

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My attempt:
1637429910033.png

1637429929135.png

Could someone please confirm my solution?
 
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Doesn't look right.
1. Is r a variable or a constant?
2. Is dv = drdθdφ?
3. Is the integral of r3/100 = a3/100?
 
mjc123 said:
1. Is r a variable or a constant?
r is constant as it is the radius of the sphere whereas a is variable.
mjc123 said:
2. Is dv = drdθdφ?
yes, (for the sphere)
mjc123 said:
3. Is the integral of r3/100 = a3/100?
I was mistaken here, it should be a^4 / 400
 
falyusuf said:
r is constant as it is the radius of the sphere whereas a is variable.
You are integrating by dr from limits 0 to a, yet treat 4πr2 as a constant.
falyusuf said:
yes, (for the sphere)
Really? What are the dimensions on either side?
 
mjc123 said:
You are integrating by dr from limits 0 to a, yet treat 4πr2 as a constant.

Really? What are the dimensions on either side?
Is dv = r^2 sin theta dtheta dPhi dr ?
I tried to correct my mistakes and this is what I got:
1637438382719.png
 
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I still think you're using r in a confusing way. Use r for the integration variable and R (constant) for the distance at which you want to evaluate the field. Then you are not integrating r3, but r5/R2.
 
mjc123 said:
I still think you're using r in a confusing way. Use r for the integration variable and R (constant) for the distance at which you want to evaluate the field. Then you are not integrating r3, but r5/R2.
Am I right now?
1637445069498.png
 
The integration variable is r, not R. And the upper limit is a, not R.
 
mjc123 said:
The integration variable is r, not R. And the upper limit is a, not R.
Sorry I was confused.
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Right?
 

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  • #10
R is not 0.1 m. a is 0.1 m. You should have R2 in the denominator for both expressions, and it will cancel out.
You need to be very careful in distinguishing the various distances and using consistent notation for them. Too many rs is a recipe for confusion.
 
  • #11
mjc123 said:
R is not 0.1 m. a is 0.1 m. You should have R2 in the denominator for both expressions, and it will cancel out.
You need to be very careful in distinguishing the various distances and using consistent notation for them. Too many rs is a recipe for confusion.
Thanks for clarifying. I solved it using two methods and I got the same magnitude with opposite sign. Could you please figure out my mistake?
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1637589697386.png
 
  • #12
I'm not quite sure what you're doing in method 2, but the point charge at the centre should be equal to minus the charge obtained by integrating ρ over the volume of the sphere. So from outside it behaves like a point charge of zero.
I think your answer of -2.09e-8 C is correct. (I did it in my head and got -2.09e-4 C; I think I must have accidentally switched the factor of 100 from the bottom to the top:).)
 
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  • #13
mjc123 said:
I'm not quite sure what you're doing in method 2, but the point charge at the centre should be equal to minus the charge obtained by integrating ρ over the volume of the sphere. So from outside it behaves like a point charge of zero.
I think your answer of -2.09e-8 C is correct. (I did it in my head and got -2.09e-4 C; I think I must have accidentally switched the factor of 100 from the bottom to the top:).)
I got it now.. Thank you so much. Appreciate your help.
 
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