Parallel Disk Capacitor E field

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Homework Help Overview

The discussion revolves around calculating the electric field strength at the midpoint between two charged disks, each with a diameter of 2.4 cm and separated by 1.9 mm. The disks are charged to ± 12 nC, and participants are exploring the implications of their calculations and assumptions regarding the electric field.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss their calculations for the electric field strength, questioning where errors may have occurred. There is mention of checking significant figures and units, as well as the potential need to use a more precise formula for the electric field of circular disks rather than approximating them as infinite sheets of charge.

Discussion Status

The conversation is ongoing, with participants sharing their calculations and results. Some suggest that using a precise formula may yield different results, while others express uncertainty about the implications of their findings. There is no explicit consensus on the correct approach or answer yet.

Contextual Notes

Participants are considering the effects of the disks' size relative to their separation distance and are questioning the accuracy of their approximations. There is also a reference to a calculus-based physics example related to the electric field of a circular disk.

PhysKid45
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Homework Statement


Two 2.4-cm-diameter disks face each other, 1.9 mm apart. They are charged to ± 12 nC .
What is the electric field strength at the midpoint between the centers of the disks?

Homework Equations


ppcap5.gif


The Attempt at a Solution


Q=12*10^-9
A= Pi * r^2 = pi*(.012)^2
Q/A= (12*10^-9)/(pi*(.012)^2)
E=(Q/A)/epsilon naught (8.85*10^-12)
E= 3*10^6

I am not sure where I go wrong, but I have one attempt left and keep getting the same answer

[Edit] answer: 2.7*10^6 but I would appreciate if someone could explain why
 
Last edited:
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PhysKid45 said:

Homework Statement


Two 2.4-cm-diameter disks face each other, 1.9 mm apart. They are charged to ± 12 nC .
What is the electric field strength at the midpoint between the centers of the disks?

Homework Equations


ppcap5.gif


The Attempt at a Solution


Q=12*10^-9
A= Pi * r^2 = pi*(.012)^2
Q/A= (12*10^-9)/(pi*(.012)^2)
E=(Q/A)/epsilon naught (8.85*10^-12)
E= 3*10^6

I am not sure where I go wrong, but I have one attempt left and keep getting the same answer

Are you checking sig figs and units if entering online?
 
Hey student100, I am checking sig figs. It tells me to use 2
 
PhysKid45 said:
Hey student100, I am checking sig figs. It tells me to use 2

So you're entering 3.0?
 
Yup. I ended up guessing correctly, in an attempt to figure out the second part of the problem and the correct answer was 2.7*10^6 which I do not understand. Any ideas on how it would work out to be that?
 
PhysKid45 said:
Yup. I ended up guessing correctly, in an attempt to figure out the second part of the problem and the correct answer was 2.7*10^6 which I do not understand. Any ideas on how it would work out to be that?

Only thing that comes to mind is that they wanted you to use the percise formula for disks and not approximate them as infinite sheets of charge. Have you tried that?

There size compared to distance of separation leads me to be skeptical of that however. Are all the numbers presented correct?
 
Following Student100's advice, you should find that it does make a difference if you don't approximate the disks as infinite. However, I get 2.8 × 106 N/C rather than 2.7 × 106 N/C.
 
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All numbers presented are correct.
I am not sure what you both mean by using the precise formula vs. approximating as an infinite sheet.
 
PhysKid45 said:
All numbers presented are correct.
I am not sure what you both mean by using the precise formula vs. approximating as an infinite sheet.
If you are taking a "calculus-based" physics course, then a standard example is the electric field of a circular disk.
http://www.phys.uri.edu/gerhard/PHY204/tsl36.pdf
 

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