16-7 The Electric Field

• The idea that forces act over a distance and don’t have to be in contact was hard to understand

• Michael Faraday developed the idea of a Field

• The electric field extends outward from every charge

• If there is a second electric field placed near the first it feels the force of the first field

• The electric field at a point, P, has to consider both electric fields

• To do this consider a small positive test charge and determine how the electric fields effect the test charge

• The electric field, E is defined in terms of the force, F exerted on a tiny positive test charge, +q

q

FE

Units of N/C

• The electric field at any point in space can be measured

22

2

4

1/

r

Q

r

Qk

q

rkqQ

q

FE

o

16-8 Field Lines

• The electric field is a vector, it is sometimes referred to as a vector field

• The electric field line or lines of force indicate the direction of the electric field at various points in space

• They are drawn indicating the direction of a force due to the given field on a positive test charge

• The number of lines starting on a positive charge or ending on a negative charge is proportional to the magnitude of the charge

• 1. Electric field lines indicate the direction of the electric field

• 2. The number of lines is proportional to the magnitude of the field

• 3. The electric field lines start on a positive charge and end on a negative charge

• The lines never cross, why not?

16-9 Electric Fields & Conductors

• The electric field inside a conductor is zero• The net charge on a conductor distributes

itself on the surface• The electric field is always perpendicular to

the surface outside of a conductor