Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
College Physics 2e

Section Summary

College Physics 2eSection Summary

19.1 Electric Potential Energy: Potential Difference

  • Electric potential is potential energy per unit charge.
  • The potential difference between points A and B, VBVAVBVA, defined to be the change in potential energy of a charge qq moved from A to B, is equal to the change in potential energy divided by the charge, Potential difference is commonly called voltage, represented by the symbol ΔVΔV.
    Δ V = ΔPE q and ΔPE = qΔV . Δ V = ΔPE q and ΔPE = qΔV .
  • An electron volt is the energy given to a fundamental charge accelerated through a potential difference of 1 V. In equation form,
    1 eV = 1.60 × 10 –19 C 1 V = 1.60 × 10 –19 C 1 J/C = 1.60 × 10 –19 J. 1 eV = 1.60 × 10 –19 C 1 V = 1.60 × 10 –19 C 1 J/C = 1.60 × 10 –19 J.
  • Mechanical energy is the sum of the kinetic energy and potential energy of a system, that is, KE+PE.KE+PE. This sum is a constant.

19.2 Electric Potential in a Uniform Electric Field

  • The voltage between points A and B is
    V AB = Ed E = V AB d (uniform E - field only), V AB = Ed E = V AB d (uniform E - field only),
    where dd is the distance from A to B, or the distance between the plates.
  • In equation form, the general relationship between voltage and electric field is
    where ΔsΔs is the distance over which the change in potential, ΔVΔV, takes place. The minus sign tells us that EE points in the direction of decreasing potential.) The electric field is said to be the gradient (as in grade or slope) of the electric potential.

19.3 Electrical Potential Due to a Point Charge

  • Electric potential of a point charge is V=kQ/rV=kQ/r.
  • Electric potential is a scalar, and electric field is a vector. Addition of voltages as numbers gives the voltage due to a combination of point charges, whereas addition of individual fields as vectors gives the total electric field.

19.4 Equipotential Lines

  • An equipotential line is a line along which the electric potential is constant.
  • An equipotential surface is a three-dimensional version of equipotential lines.
  • Equipotential lines are always perpendicular to electric field lines.
  • The process by which a conductor can be fixed at zero volts by connecting it to the earth with a good conductor is called grounding.

19.5 Capacitors and Dielectrics

  • A capacitor is a device used to store charge.
  • The amount of charge QQ a capacitor can store depends on two major factors—the voltage applied and the capacitor’s physical characteristics, such as its size.
  • The capacitance CC is the amount of charge stored per volt, or
  • The capacitance of a parallel plate capacitor is C=ε0AdC=ε0Ad, when the plates are separated by air or free space. ε 0 ε 0 is called the permittivity of free space.
  • A parallel plate capacitor with a dielectric between its plates has a capacitance given by
    where κ κ is the dielectric constant of the material.
  • The maximum electric field strength above which an insulating material begins to break down and conduct is called dielectric strength.

19.6 Capacitors in Series and Parallel

  • Total capacitance in series 1CS=1C1+1C2+1C3+...1CS=1C1+1C2+1C3+...
  • Total capacitance in parallel Cp=C1+C2+C3+...Cp=C1+C2+C3+...
  • If a circuit contains a combination of capacitors in series and parallel, identify series and parallel parts, compute their capacitances, and then find the total.

19.7 Energy Stored in Capacitors

  • Capacitors are used in a variety of devices, including defibrillators, microelectronics such as calculators, and flash lamps, to supply energy.
  • The energy stored in a capacitor can be expressed in three ways:
    where QQ is the charge, VV is the voltage, and CC is the capacitance of the capacitor. The energy is in joules when the charge is in coulombs, voltage is in volts, and capacitance is in farads.
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 OpenStax.

Attribution information Citation information

© Jan 19, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . 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.