Lecture 1-1

Electricity and Charge

• Course Instructor: Prof. Fakhrul Alam, PhD.

Acknowledgement

• Some lecture slides based on Prof. Peter Chong's notes (AUT ENGINEERING).

Review: Standard SI Prefixes

• Common SI Prefixes for measurement:

  • 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

Basic Concepts of Electricity

• Electric Current: Movement of charge creates electric current.

• Current (I): Measured in Amperes (A).

• Charge (Q): Measured in Coulombs (C).

• Conductors: Materials that allow charge to flow easily.

• Insulators: Materials that resist charge flow.

• Voltage (V): Drives charge flow.

Charge-Current Relationship

• Electric current measures the amount of charge passing a specific point over time.

  • Formula: I = Q / Δt, where

    • I = Current (A)

    • Q = Charge (C)

    • Δt = Time (s)

Charge-Current Examples

• Example 1:

  • Given a 12-A battery charger operating for one hour:

    • Q = I × Δt = 12 A × 3600 s = 43.2 kC.

• Example 2:

  • Lightning bolt of 20,000 A lasting for 70 μs:

    • Q = I × Δt = 20,000 A × 70 μs = 1.4 C.

Atomic Structure and Charges

• Atoms consist of protons (p), neutrons (n), and electrons (e).

  • Negatively charged materials: e > p.

  • Positively charged materials: e < p.

  • Neutral materials: e = p.

  • Adding/removing electrons alters the charge (positively or negatively charged).

Conductors and Insulators

• Conductors:

  • Allow free movement of electrons (e.g., Copper, Steel, Aluminium).

• Insulators:

  • Do not allow free electron movement (e.g., Plastics, Ceramics, Air, Oil).

Properties of Charge

• Charge is a fundamental property of matter, measured in Coulombs (C).

• Two types: Positive (+) and Negative (-).

  • Like charges repel; opposite charges attract.

Electrostatic Interactions

• Static charges can result in sparks or attraction (e.g., when opening car doors, hair sticking to a comb).

Coulomb's Law

• Coulomb's Law describes the electrostatic force between two point charges:

  • Attractive for opposite charges, repulsive for like charges.

  • Formula:

    • F = k * (Q1 * Q2) / r^2

  • Where k = Coulomb's constant.

• The magnitude of electric force is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

Electric Field

• Defined by the force exerted on a charge within its vicinity.

  • Direction indicated by a positive test charge.

• Electric field strengths change with distance—stronger near charges and weaker at greater distances.

Electric Field Examples

1. Example 1: 5 × 10^-6 C charge creates an electric field pointing radially outward.

2. Example 2: For two charges (+3 × 10^-6 C and -3 × 10^-6 C) placed 4 m apart, net electric field at midpoint calculated as:

   • E_total = E1 + E2, with direction towards the negative charge.

Practical Considerations

• Discussing Coulomb’s law and electric field regards point charges.

• For charged objects with distributed charges (e.g., rods, spheres), needs different approaches (like integrating contributions from charge elements).

• Electric fields decay faster than inverse square law for point charges.