Capacitor Charge Density Calculation

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SUMMARY

The discussion focuses on calculating the surface charge density of two metal disks and the glass dielectric between them, given a potential difference of 1100 V. The relevant equations include capacitance (C = (ε₀A)/d) and charge density (σ = Q/A). The dielectric constant for Pyrex glass is noted as 4.7, which is essential for accurate calculations. The user initially struggled with unit conversions, specifically between microfarads and farads, which led to confusion in their results.

PREREQUISITES
  • Understanding of capacitance and its formula (C = (ε₀A)/d)
  • Knowledge of charge density calculation (σ = Q/A)
  • Familiarity with dielectric materials and their constants
  • Basic unit conversion skills, particularly between farads and microfarads
NEXT STEPS
  • Study the impact of dielectric constants on capacitance in various materials
  • Learn about the relationship between electric field strength and surface charge density
  • Explore practical applications of capacitors in electronic circuits
  • Review unit conversion techniques for electrical measurements
USEFUL FOR

Students in physics or electrical engineering, educators teaching electromagnetism, and anyone involved in capacitor design and analysis.

PhysKid45
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Homework Statement


Two 10-cm-diameter metal disks separated by a 0.64-mm-thick piece of Pyrex glass are charged to a potential difference of 1100 V .
What is the surface charge density on the disks?
What is the surface charge density on the glass?

Homework Equations


C=(e_0A)/d
C=Q/V
charge density=Q/A

The Attempt at a Solution



Q/V=(e_0A)/d)
Q=((e_0A)/d)*V
charge density= ((e_0A)/d)*V)/A
charge density=(((8.85*10^-12)(pi*.05^2)*1100/.00064)/(pi*.05^2)

I have also tried multiplying the dielectric constant for pyrex (google gave me 4.7) by e_0 and nothing has worked.

I am not sure what the difference between the surface charge density on the glass vs. the disks would be, or how to find it.
 
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What do you mean by "nothing has worked"? What were the results you got?
Where is the formula taking the dielectric constant into account?
PhysKid45 said:
I am not sure what the difference between the surface charge density on the glass vs. the disks would be, or how to find it.
Imagine adding a very small air gap between glass and conductor. What would be the field strength there? How is the difference between that and the field strength in the glass related to the surface charge density of the glass?
 
Hey, so apparently the answer was in microfarads but I was entering it in farads, but thanks for the help!
 

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