Electric Fields The space around a magnet is different than it would be if the magnet werent there. A paperclip will move without you ever touching it! Similarly, the space around a concentration of electric charge is different than it would be if the charge werent there. Your hair might stand on end as you walk close to a Van de Graaff generator! The space that surrounds a magnet or electric charge is altered. The space is said to contain a force field. Field forces can act through space, producing an effect even without contact.

An electric field has both a magnitude and a direction. Its magnitude (strength) can be measured by its effect on charges located in the field. Imagine a small test charge placed in a field. Where the force is greatest on the test charge, the field is strongest. Where the force on test charge is weak, the field is small. When they place a test charge in a field to test its magnitude, the test charge must be small enough so that it doesnt push the original charge around and alter the field were measuring.

Calculating Electric Fields If we put a test charge in an electric field, we could define the strength of the electric field at the position of the test charge as the force per unit of charge. E = F/q E=Electric Field strength F=Forceq=test charge Newtons/Coulomb Newtons Coulombs The direction of the electric field is the direction of the force on a positive test charge. So the direction of E depends on the sign of the charge producing the field.

Another look at calculating electric field strength We really have two charges here: Charge setting up the field Small, positive test charge If we know how far apart these charges are, we can use Coulombs Law to find the force on the test charge. F e = k q 1 q 2 d 2 q 1 =charge setting up the field q 2 =test charge Substitute this force into our previous equation: E=kq 1 q 2 d 2 q 2 This simplifies to: E=kq 1 d 2 Direction of the field is the same