Physics BAS-101: Foundations of Electricity

Course Information

• Course: Physics BAS-101
• Level: First Level
• Semester: Fall 2024-2025
• Instructors: Ass. Prof. Mohamed Abdelghany, Dr. Nermin Ali Abdelhakim, Dr. Enas Lotfy

Foundations of Electricity

• Electricity is a fundamental physics concept.
• Lecture 4 covers the foundations of electricity.

Capacitance

• Capacitors consist of two isolated conductors of any shape.
• The plates of a charged capacitor carry charges of equal magnitude but opposite signs (+q and -q).
• The plates are equipotential surfaces with all points at the same electric potential.

Capacitance Formula

• The charge (q) and the potential difference (V) across a capacitor are proportional: q = CV
• The proportionality constant (C) is capacitance, measured in farads (F).
• Capacitance depends only on the geometry of the plates, not their charge or potential difference.

Definition of Capacitance

• Capacitance is a measure of how much charge must be put onto the plates to produce a certain potential difference between them.
• The formula for capacitance is C = Q/V, where Q is the charge and V is the voltage across the capacitor.

SI Unit of Capacitance

• The SI unit of capacitance is the farad (F).
• 1 farad (F) = 1 coulomb/volt (1 C/V)
• Sub-units like microfarad (µF) and picofarad (pF) are more commonly used.

Purpose of Capacitors

• Capacitors store electric charge (Q = CV).
• They are used in DC and AC circuits.
• They store energy (U = Q²/2C = ½C V²).
• They are used in timing circuits in DC and resonance circuits in AC.

Capacitors in Parallel

• The voltage across each capacitor in parallel is the same (V₁ = V₂ = V₃).
• The total charge is the sum of the charges on each capacitor (qtot = q₁ + q₂ + q₃).
• The equivalent capacitance (C_eq) is the sum of individual capacitances (C_eq = C₁ + C₂ + C₃).

Capacitors in Series

• The charge on each capacitor in series is the same (q₁ = q₂ = q₃).
• The total potential difference is the sum of the potential differences across each capacitor (V = V₁ + V₂ + V₃).
• The reciprocal of the equivalent capacitance is the sum of the reciprocals of individual capacitances (1/C_eq = 1/C₁ + 1/C₂ + 1/C₃).

Concept Test

• The configuration with capacitors in series (C/2) has the lowest equivalent capacitance.

Capacitors in Circuits

• Determining the total capacitance in circuits often requires a multi-step process combining parallel and series rules.

Another Example

• Capacitors in parallel are added directly, while those in series use reciprocal addition for the total capacitance.

Capacitor with a Dielectric

• A dielectric is a non-conducting material, like rubber, glass, or wax paper, placed between capacitor plates.

Dielectric Constant

• The dielectric constant (k) is a dimensionless factor that represents the increase in capacitance due to a dielectric material.

Effect of Dielectric

• Inserting a dielectric increases the capacitance by a factor of k. This is because it decreases the voltage across the capacitor, while preserving the same charge. (C = kC_o)
• A dielectric also provides mechanical support between plates, allowing them to be closer without touching.

Advantages of Dielectric

• Increased capacitance
• Increased maximum operating voltage
• Mechanical support allowing the plates to get closer and increasing capacitance

This quiz covers the foundations of electricity, focusing on capacitance. It includes concepts such as the structure of capacitors, the capacitance formula, and the definition of capacitance. Test your understanding of these fundamental principles in physics.