Electric Forces & Electric Fields

Electric Forces

Charges exert forces on one another.  Like charges repel one another and opposite charges attract one another.  Charles Coulomb discovered what variables determine how much force two charges exert on one another.

Coulomb's Law:

Electric Force  µ  q₁ q₂  / r²

F_electric  =  k · q₁ · q₂  / r²

OR

F_electric  = [ 1 / 4 π ε_o ] · q₁ · q₂  / r²

k = 8.99 x 10⁹ N m² / C²

ε_o = 8.854 x 10^-12 N m² / C²

q₁  →  point charge # 1

q₂  →  point charge # 2

r    → distance between the two point charges

You may have noticed that Coulomb's Law looks very similar to Newton's Law of Universal Gravitation where F_gravity = G · m₁ · m₂  / r²  

Electric Fields

WARNING:  This topic seems to create confusion among students.

What is the Gravitational Field Strength of the Earth on the Earth's surface?

What happens to the Earth's Gravitational Field above the Earth's surface?

 What is a Gravitational Field?

For a Gravitational Field to exist how many masses are needed?

An electric field is the electric force per unit charge.  An electric field of 7 Newtons per Coulomb means that an increase of 1 Coulomb of charge will increase the Electric Force by 7 Newtons.

How to Draw or Read Electric Field Lines

• Closer together electric field lines signifies a stronger electric field.
• The direction of the electric field is tangent to the electric field lines.
• Electric field lines start at positive charges and end at negative charges.
• The number of electric field lines leaving or entering a charge is proportional to the magnitude of the charge.

Field Lines Applet #1    Limited options

Field Lines Applet #2    Number, position, and magnitude of charges can be varied.  Unfortunately the field lines do not have arrows on them.

Field Lines Applet #3   

Field Lines Applet #4   

Field Lines Applet #5   

Field Lines Applet #6   

Additional Sites that are worth a look:

The Physics Zone

The Physics Classroom

Think Quest