2. 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

3. 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?

4. 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!)

5. 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

6. 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

7. 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

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

9. 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

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

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

13. 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

14. 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)