Coulomb’s Law n The force between a charge q and another charge Q a distance r apart is given by n Where k = 9 x 10 9 N  m 2 /C 2

Action-at-a-distance n Coulomb’s Law describes the force between two charges. What happens to the force on q if Q is suddenly moved? How quickly does q realize the change in force? What is it that q and Q interact with?

Michael Faraday (1830s) n Faraday proposed the concept of the “Electric Field” n Charges create a “force field” surrounding themselves n Charge q would interact directly with the field created by charge Q – it wouldn’t interact with Q directly n Charge Q would likewise interact with the field created by q – not q directly n If Q were suddenly moved, q would still react to the field that already exists in that region… The change would move at the speed of light (the change in the field actually IS light!)

Mapping a mine-field n Send disposable robots to different points in the field – if they blow up, put a marker at that spot BOOM! Mapping an E-field n Place positive test charges at different points in the field – measure the force (magnitude and direction) acting on the charge at that point

E-field around positive point charge n E-field around positive charge points outward n Spacing of lines represents strength of field n Arrows point in the direction of the force that would act on a positive charge placed in the field

E-field around negative point charge n E-field around negative charge points inward n Spacing of lines represents strength of field n Arrows point in the direction of the force that would act on a positive charge placed in the field

Definition of E-field E-field is measured in N/C

E-field around a point charge n Combine E = F/q with Coulomb’s Law to get E-field around a charge follows an inverse-square law

Field around multiple charges n Field around multiple charges is the vector sum of the field at each point around the charges

The electric field obeys a superposition principle. The total field at any point is the vector sum of all fields at that point.

E-field between parallel plates n The E-field between two oppositely charged plates is uniform (constant) n A charge placed anywhere between the plates will feel the same magnitude of force

E-field inside a conductor n When an E-field passes though a conductor, charges will move around until the E-field inside the conductor is zero (an equilibrium is established)