Electric Charges 02 - Electric Fields

OPENING QUESTIONS: Brief your groupies on how charge is built up in a thunder cloud -- be prepared to present to/inquire from the class.

OBJECTIVE:  I will be able to caclulate the strength of an electric field on a test charge after today's class.



BOOK AUDIT - Show 'em if you got 'em and if you want to avoid $150 in fees



  • Test Charge: A mathematical construct-- a charge that does not exert any influence on surrounding particles but IS influenced by other electric fields.
  • Conductors - materials where electrons can roam
  • Insulators - materials that keep their electrons close to home
  • coulomb - a unit of electrical charge (see below)


  • e-mass = 9.1 x 10-31 kg
  • charge of an electron = 1.60 x 10-19 coulombs (C)
  • ke = 8.987 x 109 Nm2/C2


      • Electrical Force: Fe = (keq1q2)/r2
      • Electrical Field: E = Fe / qo where qo  = a positive test charge


Let's call up a couple of our colleagues to discuss *magnitude*


I wanted you to take a look at example 23.4 in DETAIL... note the very similar methodologies to our recent work on forces.

We are familiar with the concept of a gravitational FIELD. Such a field exists when an object containing mass exerts a gravitational attraction on another mass. It is important to note that such a field does not require a medium to transmit the field -- in otherwords, the Earth experiences the gravitational attraction of the sun, even though the sun is separated by a distance of 150,000,000 meters of vacuum.

Using gravity as an analogy

So...much like an object with mass attracts another object with mass through a gravitational field, objects with CHARGE attract objects of opposite charge AND repel objects of similar charge by means of an electric field.jj

We can mathematically test for the presence and size of an electrical field through the introduction of a "TEST CHARGE"

A test charge is a point charge with a positive or negative charge. However, in our usual and necessary ways of using simple examples to illustrate complex ideas, we dictate that the point charge itself exerts NO electric force-- in otherwords, the electric field of the test charge is so small that it exerts no influence on any neighboring particles whatsoever.


Work with your groupies to evaluate 23.5

Does anyone notice anything ODD about that problem....?


Problems 4, 10, 15, 26, 33