Field of an Electric Charge

In middle school you mapped magnetic field lines using iron filings.

In class, we saw a demonstration of electric field lines using grass seeds suspended in mineral oil.  When the grass seeds are exposed to a strong electric field, the grass seeds become polarized and line up along the field lines.  There is a video of this.

Using the PHET simulation or the PhysLet simulation you can draw the field diagrams for several geometries.

But the electric field will not always be uniform as it is between two large parallel plates.

Calculating the electric field for different geometries is an entire chapter in most upper level physics texts and often requires calculus.

The one geometry which is easy to calculate is for the field due to a point source, or a large distance from a small spherical source.

In this case, we can use E = F/q and substitute in Coulomb’s Law,


where Q is the charge on the object in question, and q is our test charge.  In this case, the test charge cancels and the field strength E,  a distance d from from a charge Q is


You will find a couple of workbook equations using this.  What is interesting to remember is that Electric Field is a vector.  If you wanted to find the field strength due to multiple charges, you would calculate each field separately and then add up the fields (remember it is a vector!).


Also, if you were to fire an electron parallel to a field, the field would accelerate the electron.

However, if you fired an electron perpendicular to a field, the field would exert a force perpendicular to the original motion, and it would follow a parabola.  You could think of this as a form of projectile motion.