A pair of point charges with opposite sign create a dipole field.

Point Charge Electric Field Demo

The Point Charge Electric Field Demo model shows the electric field with multiple point charge configurations and vector field visualizations.  Users can select these configurations from a drop down menu or can create their own configurations.

The electric field is defined as the force per unit charge that would be experienced by a very small test charge at a given location. The Point Charge Electric Field Demo model shows an ideal test charge with an arrow that represents the field at the test location.  The numerical field magnitude is shown in a textbox near the bottom of the view as the test charge is dragged.

Because the electric field changes dramatically in the vicinity of a charged particle, it is difficult to visualize its global properties using only a single test charge.  A good way to visualize the field's geometric structure is to use a test-charge grid.  Test charges are placed uniformly and the force per unit charge is shown at each test charge location.  Users can select the number of locations and can use either arrow length or color to show the field's magnitude a the grid point. The color representation (with constant arrow length) uses light blue for weak fields and progresses through the spectrum of blue, green, red and black as the magnitude of the field increases.

The centers of the colored circles are the locations of the point particles used to compute the electrical fields and the magnitude of the electric field is undefined at these locations.  You can observe the point particle singularity if you drag a charge close to a test charge grid point.

Teaching Objectives 

Forces, Energy and Fields

Attractive and repulsive interactions at a distance (e.g., gravitational, magnetic, electrical and electromagnetic) can be described and explained using a field model.

Learning Goal

Students understand that the field model explains how objects exert attractive and repulsive forces on each other at a distance: their fields are the agents of the interaction.

Essential Knowledge

1. The field of a source (e.g., a particle with charge) depends only on the properties of the source and the position of an object relative to the source, not on any properties of objects placed in the field (e.g., a test charge).  The field of an object is always there, even if the object is not interacting with anything else.
   
2. The strength of the electric field at a certain location is given by the electric force per unit of charge experienced by a test object placed at that location.  If the electrical field at a certain position is known, then the electrical force exerted by the sources of that field on any object at that position can be calculated by multiplying the field strength (E) and the charge of the object. 

Credits:

The Point Charge Electric Field Demo model was created by Wolfgang Christian using the Easy Java Simulations (EJS) modeling tool version 4.3.1.  You can examine and modify the model for this simulation if you have EJS installed by right-clicking within the program and selecting "Open Ejs Model" from the pop-up menu. 

Information about EJS is available at: <http://www.um.es/fem/Ejs/> and in the OSP ComPADRE collection <http://www.compadre.org/OSP/>.