Electric Fields

1. Electric Fields A paperclip will move without you ever touching it! The space around a magnet is different than it would be if the magnet weren’t there. Your hair might stand on end as you walk close to a Van de Graaff generator! Similarly, the space around a concentration of electric charge is different than it would be if the charge weren’t there. 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.

2. 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. 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 doesn’t push the original charge around and alter the field we’re measuring. Imagine a small test charge placed in a field.

3. 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. Newtons/Coulomb E=Electric Field strength E = F/q Newtons F=Force q=test charge Coulombs So the direction of E depends on the sign of the charge producing the field. The direction of the electric field is the direction of the force on a positive test charge.

4. Another look at calculating electric field strength Small, positive test charge Charge setting up the field We really have two charges here: Fe = k q1 q2 d2 If we know how far apart these charges are, we can use Coulomb’s Law to find the force on the test charge. q2=test charge q1=charge setting up the field E=kq1q2 d2q2 Substitute this force into our previous equation: E=kq1 d2 Direction of the field is the same This simplifies to: