Calculations involving different Dielectrics and Capacitors

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To determine the dielectric constant for a capacitor that can store 7.4 μJ at 652 V, the correct formula to use is U = ½CV², not the charge-based approach initially applied. The initial calculations mistakenly used charge instead of energy, leading to an incorrect value for the dielectric constant. The required capacitance with the dielectric can be found using the energy stored, and then compared to the original capacitance of 7.4 pF to find the dielectric constant. The dielectric constant corresponding to the correct calculations is 4.7, which matches with Pyrex in Table 25-1. This clarification resolves the confusion regarding the calculations and the appropriate formulas to use.
as2528
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TL;DR Summary: Need dielectric constant for given capacitor

Given a 7.4 pF air-filled capacitor, you are asked to convert it to a capacitor that can store up to 7.4 mJ with a maximum potential difference of 652 V. Which dielectric in Table 25-1 should you use to fill the gap in the capacitor if you do not allow for a margin of error?

I did the following:

C=(k*e*A)/d and Q=C*V=>C=Q/V

So:

Q/V=(k*e*A)/d=>k=Q/V*d/(A*e)=>k=7.4*10^-6/652*7.4*10^-12=>k=8.7616*10^-20

The answer is 4.7, and uses the potential between capacitors formula. Why are the formulas I used wrong? It seems to me like it makes sense.
 
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as2528 said:
the potential between capacitors formula
What does that mean? What is Table 25-1?
 
In addition to what @hutchphd said:

Is the energy really 7.4mJ (millijoules) or did you mean 7.4μJ (microjoules)?

What formula relates energy (not charge) stored to C and V?
 
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hutchphd said:
What does that mean? What is Table 25-1?
Table 25-1 was a table showing a few dielectric constants along with the materials they corresponded to. I was supposed to get 4.7 from the question, which was the part I failed on. On the table it said Pyrex was corresponding to that kappa.
 
Steve4Physics said:
In addition to what @hutchphd said:

Is the energy really 7.4mJ (millijoules) or did you mean 7.4μJ (microjoules)?

What formula relates energy (not charge) stored to C and V?
That was u=.5c*v^2. So I calculated with charge which causes the error?
 
as2528 said:
That was u=.5c*v^2. So I calculated with charge which causes the error?
And it was the microjoules.
 
as2528 said:
That was u=.5c*v^2.
That's the correct formula. But as far as I can see, you didn't use it.

as2528 said:
So I calculated with charge which causes the error?
The charge (Q) is not needed. Try this:

Step 1: With the dielectric present, U(energy stored) =7.4μJ when V =652V. Use the formula U=½CV² to find C (the required capaicitance with the dielectric in place).

Step 2: Note that without the dielectric, the capacitance is 7.4pF. Use this and your result from Step 1 to find the dielectric constant.
 
Steve4Physics said:
That's the correct formula. But as far as I can see, you didn't use it.The charge (Q) is not needed. Try this:

Step 1: With the dielectric present, U(energy stored) =7.4μJ when V =652V. Use the formula U=½CV² to find C (the required capaicitance with the dielectric in place).

Step 2: Note that without the dielectric, the capacitance is 7.4pF. Use this and your result from Step 1 to find the dielectric constant.
Thank you! This cleared it up for me.
 
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