Lecture 1 Electric force and Coulomb’s Law

Lecture 1: Electric Force and Coulomb's Law

Course: Engineering Physics (24BSCS201)

  • Instructors:

    • Dr. Hassanein Shaban (Computer Engineering Group)

      • Email: Hassanein.Shaban@bue.edu.eg

      • Room: A207

    • Prof. Elsayed Salama (Others)

3. Electric Charge

  • Definition: Fundamental property of matter based on the presence or absence of electrons.

  • Symbol: Q, measured in Coulombs (C).

    • 1 C = 6.25 × 10¹⁸ electrons.

    • Charge of an electron is approximately -1.6 × 10⁻¹⁹ C.

  • Types of Charge:

    • Positive (deficit of electrons)

    • Negative (excess of electrons)

  • Charge Conservation: In any interaction, the total charge is conserved.

4. Measurement and Experiments

  • Millikan Oil Drop Experiment (1910):

    • Measured the charge of the electron.

  • Charges Q can be expressed as an integer multiple of the charge of an electron (Q = ±Ne).

5. Structure of Matter

  • Atomic Structure:

    • Protons (positive), neutrons (neutral), electrons (negative).

    • The attraction between protons and electrons holds the atom together.

  • Atoms and Ions:

    • Neutral atoms have equal protons and electrons.

    • Ions are charged atoms with either lost (positive ion) or gained (negative ion) electrons.

6. Charging Methods

  1. Charging by Rubbing:

    • Hard rubber rubbed with wool = negative charge (excess electrons).

    • Glass rubbed with wool = positive charge (few electrons).

  2. Charging by Induction:

    • Neutral objects can become charged when exposed to charged objects, resulting in polarization.

7. Conductors vs Insulators

  • Conductors:

    • Materials that allow electrons to move freely (e.g., metals, water with salts).

  • Insulators:

    • Materials where electrons do not move freely (e.g., glass, pure water).

8. Static Electricity Applications

  • Electrostatic Painting:

    • Charged paint droplets attract to oppositely charged objects.

  • Static Electric Discharge:

    • Everyday occurrences like shocks after walking across carpets.

    • Natural displays like lightning.

  • Industrial Uses:

    • Static charges used in air filters to trap pollutants.

9. Coulomb's Law

  • Definition: Describes the force between two charged particles.

  • Formula:

    • F = k * (|q1 * q2|) / r²

    • Where k = Coulomb's constant (≈ 9 × 10⁹ N·m²/C²).

    • Forces are attractive between opposite charges and repulsive between like charges.

10. Applications of Coulomb's Law

  • Superposition Principle:

    • The net force on a charge is the vector sum of all forces exerted on it.

  • Examples:

    • Calculating forces between multiple charges in various configurations.

11. Case Studies and Examples

  • Comparing Forces:

    • Example: Electric force versus gravitational force in a hydrogen atom.

  • Practical Problems:

    • Understanding net forces acting on point charges in different configurations (right triangles, linear arrangements, etc.).