Coulomb's Law and Electric Forces

  • Coulomb's Law and Electric Forces Notes

    • Understand positive and negative charges and measurement in coulombs.

    • Calculate net electric force on a point charge from a system of point charges.

    • Solve problems involving electric charges, dipoles, forces, fields, and flux.

    Electric Force Overview

    • Electric force is a non-contact force that acts over a distance, influenced by:

  • Charge type (positive or negative)

  • Distance between charges

  • Examples: Interaction between different materials (plastic vs paper, balloons vs styrofoam vs aluminum).

Characteristics of Electric Force

  • Expressed in Newtons (N) and is a vector quantity (has both magnitude and direction).

  • Direction depends on the charge type:

  • Like charges repel (e.g., two positively charged balloons)

  • Unlike charges attract (e.g., a positively charged sphere and a negatively charged sphere).

Fundamental Rules of Charge Interaction

  • Opposites attract, Likes repel.

Factors Affecting Electrical Force

  • Magnitude of Charge:

  • Increasing the charge on either charged object increases the force.

  • Method: Rubbing balloons increases their charge.

  • Distance:

  • The force is strongest when the objects are close together.

  • It decreases as distance increases (inversely related to distance).

Coulomb's Law

  • Definition: Describes the relationship between electrical force and charge.

  • Equation:
    [ F = k \frac{Q1 Q2}{d^2} ]
    Where:

  • F = Electric force

  • Q1 and Q2 = charges in coulombs

  • d = distance between charges in meters

  • k = Coulomb's law constant (9.0 × 10^9 N • m²/C²)

Practice Problem: Electric Force

  • Example: Calculate the force between q1 = +5 C and q2 = -3 C at a distance of 30,000 m.

  • Given: q1 = +5 C, q2 = -3 C, d = 30,000 m

  • Solution:
    [ F = 9.0 \times 10^9 \frac{(5)(-3)}{(30,000)^2} ]

  • Result: 150 N (Attractive)

Superposition of Forces

  • When multiple charges are present, the resultant force on any charge is the vector sum of all forces exerted by other charges.

  • Example: Three charges lie on x-axis with interactions calculated from each charge's influence.

Free-Body Diagrams

  • Used to illustrate all forces acting on an object in a scenario, depicting relative magnitude and direction of forces.

  • Given three charges and distances, use free-body diagrams to calculate the position of a charge where the forces balance to zero.

  • Charge calculations involving multiple electric forces and distances to demonstrate Coulomb's law and interactions using real values.

  • Example: Three point charges along a line and determining resultant forces based on distance and charge magnitude.