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24.41. The dielectric to be used in a parallel-plate capacitor has a dielectric constant of 3.60 and a dielectric strength of \(1.60 \times\) \(10^{7} \mathrm{V} / \mathrm{m}\) . The capacitor is to have a capacitance of \(1.25 \times 10^{-9} \mathrm{F}\) and must be able to withstand a maximum potential difference of 5500 \(\mathrm{V}\) . What is the minimum area the plates of the capacitor may have?

Short Answer

Expert verified
The minimum plate area is approximately 0.1364 m².

Step by step solution

01

Understand the Problem Requirements

We need to find the minimum area A of the plates of a parallel-plate capacitor which has a specified dielectric constant \( k = 3.60 \), dielectric strength \( E_{max} = 1.60 \times 10^7 \ \text{V/m} \), desired capacitance \( C = 1.25 \times 10^{-9} \ \text{F} \), and must withstand a potential difference \( V = 5500 \ \text{V} \).
02

Calculate Maximum Electric Field

The maximum electric field \( E_{max} \) is given by the dielectric strength. For the capacitor to withstand \( 5500 \ \text{V} \) without breaking down, the thickness \( d \) of the dielectric must not exceed \( d = \frac{V}{E_{max}} \). Calculate \( d \) using: \[d = \frac{5500}{1.60 \times 10^7} = 3.4375 \times 10^{-4} \ \text{m}\]
03

Use Capacitance Formula

The capacitance \( C \) of a parallel-plate capacitor with a dielectric is given by: \[ C = \frac{k \epsilon_0 A}{d} \] where \( \epsilon_0 = 8.85 \times 10^{-12} \ \text{F/m} \) is the permittivity of free space. We can rearrange this to solve for \( A \): \[ A = \frac{Cd}{k \epsilon_0} \]
04

Plug in the Values and Solve for A

Now substitute the known values into the formula for \( A \): \[A = \frac{1.25 \times 10^{-9} \times 3.4375 \times 10^{-4}}{3.60 \times 8.85 \times 10^{-12}}\] Calculate \( A \): \[A \approx 1.364 \times 10^{-1} \ \text{m}^2\]
05

Conclusion

Finally, verify the units and calculation to ensure the area makes sense. The area is expressed in square meters, which is appropriate for such a calculation.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Dielectric Constant
The dielectric constant, often symbolized as \( k \), plays a vital role in capacitors. It is a measure of a material's ability to increase the capacitance of a capacitor compared to a vacuum. The dielectric constant of a material quantifies how much the electric field within the capacitor is reduced when the material is inserted between the plates. A higher dielectric constant indicates a greater ability to store electric energy.
For our example, the dielectric constant is 3.60, meaning the capacitor can store 3.6 times more electric charge than it could without a dielectric. This value essentially modifies the capacitance by multiplying it, showing how significantly the dielectric material affects the capacitor's performance.
Dielectric Strength
Dielectric strength is a crucial parameter for dielectric materials used in capacitors. It represents the maximum electric field that a dielectric material can withstand without breaking down. Measured in volts per meter (V/m), it dictates the maximum voltage the capacitor can safely handle without the dielectric material failing.
In our context, the dielectric strength is given as \(1.60 \times 10^7 \ \text{V/m}\). This means the dielectric can withstand an electric field up to this magnitude before electrical breakdown occurs. This knowledge is vital to ensure that the capacitor operates safely under high voltage conditions without damage.
Capacitance
Capacitance is the ability of a capacitor to store charge per unit voltage, measured in farads (F). The formula connecting the capacitance \( C \), the dielectric constant \( k \), the permittivity of free space \( \epsilon_0 \), the plate area \( A \), and the distance \( d \) between the plates is \( C = \frac{k \epsilon_0 A}{d} \).
This relationship shows how the physical parameters of the capacitor influence its capacity to store electric charge. In our problem, a capacitance of \(1.25 \times 10^{-9} \ \text{F}\) is specified, which can be achieved by adjusting the size, separation, and dielectric properties between the plates.
Parallel-Plate Capacitor
A parallel-plate capacitor consists of two plates, spaced apart, with a dielectric material between them. Its simplicity makes it a fundamental model to understand how capacitors work. The parallel-plate capacitor's capacitance can be modified by changing the plate area, distance between them, and the type of dielectric material used.
The dielectric increases the capacitance by reducing the electric field between the plates, which allows the capacitor to store more charge. In this exercise, by setting the appropriate area \( A \) and thickness \( d \) of the dielectric, the desired capacitance \( C \) can be achieved while ensuring that the capacitor can withstand the required voltage.
Electric Field
The electric field in a capacitor is an important aspect that affects its operation. It is the field created between the plates of the capacitor, and it is directly influenced by the voltage applied across it. For parallel-plate capacitors, the electric field \( E \) is uniform and is calculated by dividing the voltage \( V \) by the separation \( d \) between the plates, \( E = \frac{V}{d} \).
Managing the electric field strength is essential to ensure the capacitor's longevity and reliability. If the electric field exceeds the dielectric strength, it can lead to a breakdown of the dielectric, causing the capacitor to fail.

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Most popular questions from this chapter

24.47. A \(12.5-\mu F\) capacitor is connected to a power supply that keeps a constant potential difference of 24.0 \(\mathrm{V}\) across the plates. A piece of material having a dielectric constant of 3.75 is placed between the plates, completely filling the space between them. (a) How much energy is stored in the capacitor before and after the diclectric is inserted? (b) By how much did the energy change dur- ing the insertion? Did it increase or decrease?

24.35. A cylindrical air capacitor of length 15.0 m stores \(3.20 \times 10^{-9} \mathrm{J}\) of energy when the potential difference between the two conductors is 4.00 \(\mathrm{V}\) . (a) Calculate the magnitude of the charge on each conductor. (b) Calculate the ratio of the radii of the inner and outer conductors.

24.65. A parallel-plate capacitor with only air between the plates is charged by connecting it to a battery. The capacitor is then disconnected from the battery, without any of the charge leaving the plates. (a) A voltmeter reads 45.0 \(\mathrm{V}\) when placed across the capacitor. When a dielectric is inserted between the plates, completely filling the space, the voltmeter reads 11.5 \(\mathrm{V}\) . What is the dielectric constant of this material? (b) What will the voltmeter read if the dielectric is now pulled partway out so it fills only one-third of the space between the plates?

24.4. Capacitance of an Oscilloscope. Oscilloscopes have parallel metal plates inside them to deflect the electron beam. These plates are called the deflecting plates. Typically, they are squares 3.0 \(\mathrm{cm}\) on a side and separated by \(5.0 \mathrm{mm},\) with vacuum in between. What is the capacitance of these deflecting plates and hence of the oscilloscope? (Note: This capacitance can sometimes have an effect on the circuit you are trying to study and must be taken into consideration in your calculations.)

24.48 . A parallel-plate capacitor has plates with area 0.0225 \(\mathrm{m}^{2}\) separated by 1.00 \(\mathrm{mm}\) of Tefion. (a) Calculate the charge on the plates when they are charged to a potential difference of 12.0 \(\mathrm{V}\) . (b) Use Gauss's law (Eq. 24.23 ) to calculate the electric field inside the Teflon. (c) Use Gauss's law to calculate the electric field if the voltage source is disconnected and the Teflon is removed.

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