Lecture 4

Lecture Overview

• Insulators vs. Conductors

  • Insulators: Electrons remain close to their respective atoms.

  • Conductors: Charges can flow freely like a liquid.

  • Electric field (E) is zero inside a conductor in equilibrium.

  • Discussion of ionic solutions (e.g., NaCl in water) and metals.

  • Topics include how to charge and discharge conductors.

Van der Waals Forces

• Induced Dipoles:

  • Induced dipoles lead to weak attraction between atoms and molecules.

  • Electron clouds fluctuate, creating temporary dipoles.

  • Molecules do not need to possess permanent dipoles to exhibit attraction.

  • When fluctuations synchronize, they generate attractive forces.

• Examples in Nature:

  • Geckos can hold up to a human due to van der Waals forces at their feet.

  • Demonstrates process without reliance on sticky secretions or suction cups.

  • Maximized contact through tiny hairs on gecko feet.

Conductors and Insulators

• Basic Characteristics:

  • Conductors:

    • Allow for the movement of charges (e.g., ionic solutions and metals).

  • Insulators:

    • Electrons are bound and cannot move freely; examples include plastic, wood, and glass.

• Polarization in Materials:

  • Insulators:

    • Electrons can shift slightly when in an external electric field but remain bound.

    • Rapid polarization occurs; significant net effects can arise from many molecules.

    • No free mobile charged particles; excess charges remain in place.

• Inside Conductors:

  • Charges flow similarly to a liquid within the material.

  • Proved that net electric field (Enet) is zero as excess charges are always on the surface in equilibrium.

Ionic Solutions

• Ionic Solutions Behavior:

  • Examples: Sodium (Na+) and Chloride (Cl-) ions in aqueous solutions.

  • Under applied electric fields, ions move causing polarization of the liquid.

  • Net electric field inside becomes a combination of applied and polarization fields.

  • Drift speed is determined by ion mobility and average speed due to microscopic collisions.

Electric Fields in Conductors

• Net Electric Field:

  • In a conductor: Electric field is zero when in equilibrium.

  • Proving Equilibrium:

    • If there are excess charges or disruptions, they will redistribute until equilibrium is achieved.

    • Under static conditions, charges will reside on the surface only.

• Excess Charge Behavior:

  • In cavities of conductors, if no external charge is present, excess charge on the inner surface is zero.

  • When external charges are introduced, charge redistributes to maintain overall zero electric field inside the conductor.

Applications in Technology

• Practical Applications:

  • Understanding polarization has implications for creating devices like climbing robots inspired by nature (geckos).

  • Techniques for charging and discharging include grounding and induction.

  • Ionic liquids (e.g., in biological systems) show greater polarization effects than standard atom-based systems.

Summary of Topics

• The fundamental principles of insulators and conductors impact various technological applications and natural phenomena.