Chapter 31: Capacitors and Dielectrics
📋 Table of Contents
- 1. Introduction to Capacitors
- 2. Capacitance Definition
- 3. Parallel Plate Capacitor
- 4. Cylindrical Capacitor
- 5. Spherical Capacitor
- 6. Isolated Sphere Capacitance
- 7. Energy Stored in Capacitors
- 8. Energy Density in Electric Fields
- 9. Capacitance with Dielectrics
- 10. Dielectric Materials
- 11. Gauss's Law in Dielectrics
- 12. Solved Problems
- Frequently Asked Questions
Introduction to Capacitors
⚡ What is a Capacitor?
A capacitor is a device that stores energy in an electrostatic field. It consists of two conductors separated by a small distance, with either vacuum or a dielectric medium between them.
🔬 Applications of Capacitors
- Energy Storage: Flash bulbs require short bursts of energy that exceed what batteries can provide. Capacitors draw energy slowly from batteries and release it rapidly.
- Laser Pulses: Large capacitors provide intense laser pulses for thermonuclear fusion experiments, reaching power levels of about \(10^{14}\) W for durations of \(10^{-9}\) s.
- Electric Field Production: Parallel plate capacitors create electric fields to deflect beams of charged particles.
- Electronic Circuits: Capacitors are fundamental components in radio and TV signal transmission and reception, and in electromagnetic oscillators.
Capacitance Definition
📏 Definition of Capacitance
When plates of a capacitor are connected to a battery of emf V, charge q is stored. This stored charge is directly proportional to the potential difference applied between the plates:
Here C is the constant of proportionality called capacitance. The SI unit of capacitance is the farad (F).
🎯 Definition of Farad
"If one coulomb of charge given to the plates produces a potential difference of one volt, then the capacitance of the capacitor is one farad."
Question: An electrometer is a device used to measure static charge. Unknown charge is placed on the plates of a capacitor and the potential difference is measured. What minimum charge can be measured by an electrometer with a capacitance of 50 pF and a voltage sensitivity of 0.15 V?
Solution:
Question: A storage capacitor on a random access memory (VRAM) chip has a capacitance of 55 fF. If it is charged to 5.3 V, how many excess electrons are there on its negative plate?
Solution:
Parallel Plate Capacitor
🔋 Structure of Parallel Plate Capacitor
A parallel plate capacitor consists of two parallel conducting plates separated by a distance d, with area A for each plate. The plates carry equal and opposite charges.
31.1.1 Electric Field between Plates
Gaussian Surface Selection
Consider a box-shaped Gaussian surface that encloses part of one plate. The electric field inside a conductor is zero, and between plates it's uniform.
Apply Gauss's Law
31.1.2 Potential Difference between Plates
Potential Difference Formula
For Parallel Plates
Final Expression
If initial plate is positively charged and final plate is negatively charged:
31.1.3 Capacitance of Parallel Plate Capacitor
Start with Electric Field
Relationship with Potential
Equate and Solve
With Dielectric Medium
where \( \kappa_e \) is the dielectric constant.
Question: The plates of parallel plate capacitor are separated by a distance d=1mm. What must be the plate area if the capacitance is to be 1 F?
Solution:
Question: A parallel plate capacitor has circular plates of 8.22 cm radius and 1.31 mm separation. (a) Calculate the capacitance. (b) What charge will appear on the plates if a potential difference of 116 V is applied?
Solution:
Question: An air filled parallel plate capacitor has capacitance of 1.32 pF. The separation of the plate is doubled and wax inserted between them. The new capacitance is 2.57 pF. What is dielectric constant of wax?
Solution:
Cylindrical Capacitor
🔋 Structure of Cylindrical Capacitor
A cylindrical capacitor consists of two coaxial cylinders of radii 'a' and 'b', with length L. The inner cylinder is positively charged and the outer cylinder is negatively charged.
Electric Field Between Cylinders
Consider a cylindrical Gaussian surface of radius r (a < r < b):
Potential Difference
Capacitance
Question: The plate and cathode of a vacuum tube diode are in the form of two concentric cylinders with the cathode as central cylinder. The cathode diameter is 1.62 mm and the plate diameter 18.3 mm with both elements having a length of 2.38 cm. Calculate the capacitance of the diode.
Solution:
Spherical Capacitor
🔋 Structure of Spherical Capacitor
A spherical capacitor consists of two concentric spherical shells of radii 'a' and 'b' (b > a). The inner sphere is positively charged and the outer sphere is negatively charged.
Electric Field Between Shells
Consider a spherical Gaussian surface of radius r (a < r < b):
Potential Difference
Capacitance
Question: What is the capacitance of Earth? Take it as an isolated sphere of radius 6370 km.
Solution:
Isolated Sphere Capacitance
🌐 Capacitance of Isolated Sphere
An isolated sphere can be considered as a spherical capacitor where the outer sphere has infinite radius (b → ∞).
Start with Spherical Capacitor Formula
Take Limit as b → ∞
📐 Final Expression
The capacitance of an isolated sphere of radius R is:
Energy Stored in Capacitors
⚡ Energy Storage in Capacitors
When a capacitor is charged, work is done to transfer charge from one plate to the other. This work is stored as potential energy in the electric field between the plates.
Work Done in Charging Process
At any instant during charging, the potential difference is v = q/C. The work done to add a small charge dq is:
Total Work
Alternative Forms
Since Q = CV, we can write:
Question: A parallel plate capacitor has plates of area A and separation d. A potential difference V is applied between the plates. Calculate the stored energy.
Solution:
Question: A certain thundercloud has a potential difference of \( 1 \times 10^8 \, V \) relative to a tree. If during a lightning flash 50 C of charge is transferred, what is the change in energy of that charge?
Solution:
Energy Density in Electric Fields
📊 Energy Density Concept
The energy stored in a capacitor is not localized on the plates but is distributed throughout the electric field between the plates. The energy density u is the energy per unit volume.
Start with Parallel Plate Capacitor Energy
Substitute C and V
Energy Density
Since Ad is the volume between plates:
🔍 General Result
The energy density in any electric field (not just in capacitors) is:
This result is valid for all electric fields, regardless of their source.
Question: The average breakdown electric field in air is \( 3 \times 10^6 \, V/m \). What is the energy density in this field?
Solution:
Capacitance with Dielectrics
🔬 Dielectric Materials
A dielectric is an insulating material that, when placed between the plates of a capacitor, increases its capacitance. The dielectric constant \( \kappa_e \) is defined as:
where \( C_0 \) is the capacitance with vacuum between plates and C is the capacitance with dielectric.
Effect on Electric Field
The electric field inside a dielectric is reduced by a factor of \( \kappa_e \):
Effect on Potential
For the same charge, the potential difference decreases:
Effect on Capacitance
The capacitance increases by a factor of \( \kappa_e \):
Question: A parallel plate capacitor has plates of area \( 2 \, m^2 \) separated by 1 cm. What is its capacitance if the space between plates is filled with paper of dielectric constant 3.7?
Solution:
Dielectric Materials
🧲 Types of Dielectrics
Dielectrics can be classified based on their molecular structure and behavior in electric fields.
Nonpolar Dielectrics
Molecules have no permanent dipole moment. Examples: \( O_2 \), \( N_2 \), \( CH_4 \). In an external field, induced dipoles are created.
Polar Dielectrics
Molecules have permanent dipole moments. Examples: \( H_2O \), \( NH_3 \). In an external field, dipoles tend to align with the field.
Polarization of Dielectrics
When a dielectric is placed in an electric field, it becomes polarized. This means that electric dipoles are induced or aligned within the material.
Induced Surface Charges
Polarization creates induced surface charges on the dielectric. These charges create an electric field that opposes the external field.
Net Electric Field
📊 Common Dielectric Constants
- Vacuum: \( \kappa_e = 1 \)
- Air (1 atm): \( \kappa_e = 1.00054 \)
- Paper: \( \kappa_e = 3.7 \)
- Glass: \( \kappa_e = 5-10 \)
- Water: \( \kappa_e = 80 \)
- Titanium dioxide: \( \kappa_e = 100 \)
Gauss's Law in Dielectrics
📐 Modified Gauss's Law
When dielectrics are present, Gauss's law must be modified to account for both free charges and bound (induced) charges.
Original Gauss's Law
Introduce Electric Displacement
Define the electric displacement vector \( \vec{D} = \epsilon_0 \vec{E} + \vec{P} \), where \( \vec{P} \) is the polarization vector.
Gauss's Law in Dielectrics
For Linear Dielectrics
For linear dielectrics, \( \vec{P} = \epsilon_0 \chi_e \vec{E} \), where \( \chi_e \) is the electric susceptibility.
🔍 Key Points
- Gauss's law in dielectrics involves only free charges, not bound charges.
- The electric displacement \( \vec{D} \) depends only on free charges.
- The relationship between \( \vec{D} \) and \( \vec{E} \) depends on the dielectric properties.
Solved Problems
Question: A cylindrical capacitor has length L = 6.28 cm, inner radius a = 1 mm, outer radius b = 4 mm. The space between cylinders is filled with paper of dielectric constant 3.7. Calculate the capacitance.
Solution:
Question: A spherical capacitor has inner radius a = 2 cm and outer radius b = 2.2 cm. The space between spheres is filled with oil of dielectric constant 4.5. Calculate the capacitance.
Solution:
Question: A parallel plate capacitor has plates of area 0.12 \( m^2 \) and separation 1.2 cm. A potential difference of 120 V is applied. Calculate (a) the capacitance, (b) the charge on each plate, (c) the energy stored.
Solution:
Frequently Asked Questions
The capacitance increases by a factor equal to the dielectric constant of the material. For example, if a dielectric with \( \kappa_e = 3 \) is inserted, the capacitance becomes three times its original value.
When the capacitor remains connected to a battery, the potential difference remains constant. Inserting a dielectric increases the capacitance, which means more charge flows from the battery to the capacitor. However, the energy stored is given by \( U = \frac{1}{2} CV^2 \), so with constant V and increased C, the energy stored actually increases, not decreases.
Polar dielectrics have molecules with permanent electric dipole moments (e.g., water), while nonpolar dielectrics have molecules with no permanent dipole moments (e.g., oxygen). In an external electric field, polar dielectrics experience alignment of existing dipoles, while nonpolar dielectrics develop induced dipoles.
No, capacitance is always a positive quantity. It represents the ability of a system to store charge per unit voltage, and both charge and voltage are positive quantities in this context.
The energy density \( u = \frac{1}{2} \epsilon_0 E^2 \) comes from the work done in establishing the electric field. Since the electric force is proportional to E, and work is force times distance, the energy stored ends up being proportional to \( E^2 \).
📚 Continue Your Physics Journey
Mastering Capacitors and Dielectrics is fundamental to understanding electromagnetism and circuit theory. These comprehensive notes based on Halliday, Resnick and Krane provide a solid foundation for further studies in physics including current electricity, alternating currents, and electromagnetic waves.
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These comprehensive notes are designed to help B.Sc. Physics students understand fundamental concepts of Capacitors and Dielectrics based on Halliday, Resnick and Krane
Author: Muhammad Ali Malik | Contact: +923016775811 | Email: aliphy2008@gmail.com
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