Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo

Section Summary

PhysicsSection Summary

18.1 Electrical Charges, Conservation of Charge, and Transfer of Charge

  • Electric charge is a conserved quantity, which means it can be neither created nor destroyed.
  • Electric charge comes in two varieties, which are called positive and negative.
  • Charges with the same sign repel each other. Charges with opposite signs attract each other.
  • Charges can move easily in conducting material. Charges cannot move easily in an insulating material.
  • Objects can be charged in three ways: by contact, by conduction, and by induction.
  • Although a polarized object may be neutral, its electrical charge is unbalanced, so one side of the object has excess negative charge and the other side has an equal magnitude of excess positive charge.

18.2 Coulomb's law

  • Coulomb’s law is an inverse square law and describes the electrostatic force between particles.
  • The electrostatic force between charged objects is proportional to the charge on each object and inversely proportional to the distance squared between the objects.
  • If Coulomb’s law gives a negative result, the force is attractive; if the result is positive, the force is repulsive.

18.3 Electric Field

  • The electric field defines the force per unit charge in the space around a charge distribution.
  • For a point charge or a sphere of uniform charge, the electric field is inversely proportional to the distance from the point charge or from the center of the sphere.
  • Electric-field lines never cross each other.
  • More force is applied to a charge in a region with many electric field lines than in a region with few electric field lines.
  • Electric field lines start at positive charges and point away from positive charges. They end at negative charges and point toward negative charges.

18.4 Electric Potential

  • Electric potential energy is a concept similar to gravitational potential energy: It is the potential that charges have to do work by virtue of their positions relative to each other.
  • Electric potential is the electric potential energy per unit charge.
  • The potential is always measured between two points, where one point may be at infinity.
  • Positive charges move from regions of high potential to regions of low potential.
  • Negative charges move from regions of low potential to regions of high potential.

18.5 Capacitors and Dielectrics

  • The capacitance of a capacitor depends only on the geometry of the capacitor and the materials from which it is made. It does not depend on the voltage across the capacitor.
  • Capacitors store electrical energy in the electric field between their plates.
  • A dielectric material is an insulator that is polarized in an electric field.
  • Putting a dielectric between the plates of a capacitor increases the capacitance of the capacitor.
Order a print copy

As an Amazon Associate we earn from qualifying purchases.


This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute Texas Education Agency (TEA). The original material is available at: . Changes were made to the original material, including updates to art, structure, and other content updates.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at
Citation information

© Jan 19, 2024 Texas Education Agency (TEA). The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.