Electricity and Charge Summary

Electricity and Charge

Course Overview
  • Instructor: Prof Fakhrul Alam, PhD
Standard SI Prefixes
  • pico (p): $10^{-12}$
  • nano (n): $10^{-9}$
  • micro (µ): $10^{-6}$
  • milli (m): $10^{-3}$
  • kilo (k): $10^{3}$
  • mega (M): $10^{6}$
  • giga (G): $10^{9}$
  • tera (T): $10^{12}$
Fundamental Concepts
  • Electric Current (I): Movement of charge, measured in Amperes (A).
  • Charge (Q): Measured in Coulombs (C).
  • Voltage (V): Drives charge flow.
  • Conductors: Allow easy charge flow (e.g., Copper, Aluminium).
  • Insulators: Resist charge flow (e.g., Plastics, Ceramic).
Charge-Current Relationship
  • Formula: I=QΔtI = \frac{Q}{\Delta t}
    • I: Current (A)
    • Q: Charge (C)
    • Δt: Time (s)
Examples
  • Battery Charger: 12 A for 1 hour → Q=I×Δt=12×3600=43.2kCQ = I \times \Delta t = 12 \times 3600 = 43.2\text{kC}
  • Lightning Bolt: 20,000 A for 70 μs → Q=20,000×70×10−6=1.4CQ = 20,000 \times 70 \times 10^{-6} = 1.4C
Properties of Charge
  • Charge types: Positive (+) and Negative (-).
  • Like charges repel; opposite charges attract.
Coulomb's Law
  • Describes electrostatic force between point charges.
  • Formula: F=kQ<em>1Q</em>2r2F = k \frac{Q<em>1 Q</em>2}{r^2}
    • F: Force (N)
    • Q1, Q2: Charges (C)
    • r: Distance (m)
    • k: Coulomb's constant (9 \times 10^9 \text{ N m}^2/ ext{C}^2)
Examples of Coulomb's Law
  1. Example 1: Two charges $q1 = 3 \times 10^{-6} C$ and $q2 = -2 \times 10^{-6} C$ separated by 0.5 m: F=0.216NF = 0.216 N (Attractive force)
  2. Example 2: Given forces and charge magnitudes, find distance using:
    r=k∣Q<em>1Q</em>2∣Fr = \sqrt{\frac{k|Q<em>1 Q</em>2|}{F}}
Electric Field
  • Definition: Force per unit charge in the field of a charge.
  • Formula: E=kQr2E = k \frac{Q}{r^2} (N/C)
  • Field direction:
    • Positive charges: Field points outward.
    • Negative charges: Field points inward.
Example of Electric Field
  • For a charge of $5 \times 10^{-6} C$ at point P, the electric field vector points in the +x direction.
  • For two charges $q1$ and $q2$, field at midpoint → net field computed as:
    E<em>net=E</em>1+E2E<em>{\text{net}} = E</em>1 + E_2
Important Note
  • Discussions are geared towards point charges. For distributed charges, consider charge density and integration techniques to find the electric field.