ap-phys2_electric-force-and-field-njctl-SR-Students

Electric Force and Field

Table of Contents

• Electric Charge

• Atomic Structure and Source of Charge

• Conduction and Induction

• Electroscope

• Electric Force (Coulomb's Law)

• Electric Force in Two Dimensions

• Electric Field

• Electric and Gravitational Fields

• Electric Field of Multiple Charges

• Electric Field in Two Dimensions

• Superimposition of Electric Forces

• Uniform Electric Field

Electric Charge

Charging by Rubbing

• Rubbing non-metallic objects (e.g., plastic ruler and fur) leads to charge transfer.

• Uncharged ruler attracts paper bits after rubbing.

• Charging involves attraction (as seen with clothes from a dryer) and historical observations of amber.

Historical Context

• Benjamin Franklin named two types of charge based on experimental findings: positive and negative.

• Glass rods (positive) and rubber rods (negative) show attraction and repulsion.

Conservation of Electric Charge

• Electric charge is conserved; it cannot be created or destroyed.

• Total charge in a closed system remains constant.

• Charging by friction retains important concepts of electronegativity and electron transfer (e.g., balloon and hair).

Atomic Structure and Source of Charge

Basic Atom Composition

• Atoms consist of protons (positive), neutrons (neutral), and electrons (negative).

• Protons and neutrons form the nucleus; electrons orbit around it.

Measurement of Charge

• J.J. Thomson discovered the electron in 1897.

• Robert Millikan measured the charge of an electron as approximately 1.602 x 10^-19 coulombs.

• A coulomb represents a huge number of electrons (about 6.24 x 10^18).

Properties of Electrons

• Electrons are fundamental particles with a mass of 9.1 x 10^-31 kg.

• Charge of an electron is -e, which is equal in magnitude to the charge of a proton (+e).

• Atoms are usually neutral due to equal numbers of protons and electrons.

Nature of Charge

• Like charges repel while opposite charges attract.

• Electrical neutrality results from equal proton and electron counts in an atom.

Visualizing Atoms

• Atoms are mostly empty space; a nucleus can be compared to a baseball within a large sphere.

• Illustrations such as the Bohr Model represent basic atomic structure but do not accurately depict electron locations.

Charge Transfer Mechanisms

• Must understand conduction (contact-based charge transfer) and induction (charge transfer without direct contact).

• Grounding allows for control of charge neutrality.

Electroscope

• Device measures electric charge.

• Through conduction or induction, it can show electrical charge presence and its effect on gold leaves, which repel each other when charged.

Coulomb's Law

Electric Force Overview

• Electric force decreases with distance (inverse-square law) and depends on charge magnitude.

• Studied and formulated by Charles Coulomb in the late 18th century.

Electric Force Magnitudes

• The formula defines the electric force:

  F = k * (|q1||q2|)/r^2

  Where k = 9.0 x 10^9 N-m²/C².

• Electric fields measure interactions with a unit of N/C.

Comparing Electric and Gravitational Forces

• Similar mathematical descriptions, with electric forces being significantly stronger than gravitational forces (on the order of 10^36).

Electric Fields

Introduction to Electric Fields

• Electric fields represent the influence of a charge within space where it affects other charges.

• Depicted visually through field lines originating from positive charges and terminating at negative charges.

Uniform Electric Fields

• Occur when two charged plates create a consistent field strength within their space, differing from point charges that vary with distance.

Electric Field Behavior

• Charge will experience different forces based on its magnitude and distance to other charges, and visualizing field lines helps to understand force directions.

Superimposition of Electric Forces

Force Composition

• Analyze systems with multiple charges similar to vector additions, accounting for attraction and repulsion.

• Register individual charge interactions (forces) and sum them for the net force.

Two-Dimensional Electric Fields

• Extend previous findings to systems involving more complex geometries; resolving vector forces is essential to calculating net electric forces upon charges.

Key Takeaways

• Conservation of charge, principles of attraction and repulsion, and concepts of conduction and induction form the base for understanding electric forces and fields.

• Charge transfer abilities through various methods highlight the importance of grounding and measurement devices like electroscopes.

• The foundational theories and laws established by early scientists continue to serve as the background for modern electrical principles.