UNIT 9 Electrostatics and Currents

1. UNIT 9Electrostatics and Currents

2. Tuesday March 13th Electrostatics and Currents

3. Tuesday, March 13 TODAY’S AGENDA • Electric Field • Hw: Practice D (all) p575 UPCOMING… • Wed: Problem Quiz #1 (Electric Forces) • Thurs: Electric Potential Energy • Fri: NO SCHOOL (Teacher In-Service)

4. Chapter 16-17 Electrostatics and Current

5. F2 = ? F1 = 3N Q Q ConcepTest 21.3aCoulomb’s Law I 1) 1.0 N 2) 1.5 N 3) 2.0 N 4) 3.0 N 5) 6.0 N What is the magnitude of the force F2?

6. F2 = ? F1 = 3N Q Q ConcepTest 21.3aCoulomb’s Law I 1) 1.0 N 2) 1.5 N 3) 2.0 N 4) 3.0 N 5) 6.0 N What is the magnitude of the force F2? The force F2 must have the same magnitude as F1. This is due to the fact that the form of Coulomb’s Law is totally symmetric with respect to the two charges involved. The force of one on the other of a pair is the same as the reverse. Note that this sounds suspiciously like Newton’s 3rd Law!!

7. F2 = ? F1 = 3N Q Q F2 = ? F1 = ? 4Q Q ConcepTest 21.3bCoulomb’s Law II 1) 3/4 N 2) 3.0 N 3) 12 N 4) 16 N 5) 48 N If we increase one charge to 4Q, what is the magnitude of F1?

8. F2 = ? F1 = 3N Q Q F2 = ? F1 = ? 4Q Q ConcepTest 21.3bCoulomb’s Law II 1) 3/4 N 2) 3.0 N 3) 12 N 4) 16 N 5) 48 N If we increase one charge to 4Q, what is the magnitude of F1? Originally we had: F1 = k(Q)(Q)/r2 = 3 N Now we have: F1 = k(4Q)(Q)/r2 which is 4 times bigger than before. Follow-up: Now what is the magnitude of F2?

9. The Electric Field Definition of the electric field: Here, q0 is a “test charge” – it serves to allow the electric force to be measured, but is not large enough to create a significant force on any other charges.

10. The Electric Field If we know the electric field, we can calculate the force on any charge: The direction of the force depends on the sign of the charge – in the direction of the field for a positive charge, opposite to it for a negative one.

11. Electric Field Lines The charge on the right is twice the magnitude of the charge on the left (and opposite in sign), so there are twice as many field lines, and they point towards the charge rather than away from it.

12. The Electric Field Just as electric forces can be superposed, electric fields can as well.

13. Electric Field Lines • Electric field lines are a convenient way of visualizing the electric field. • Electric field lines: • Point in the direction of the field vector at every point • Start at positive charges or infinity • End at negative charges or infinity • Are more dense where the field is stronger

14. Electric Field Lines Combinations of charges. Note that, while the lines are less dense where the field is weaker, the field is not necessarily zero where there are no lines. In fact, there is only one point within the figures below where the field is zero – can you find it?

15. Electric Field Lines A parallel-plate capacitor consists of two conducting plates with equal and opposite charges. Here is the electric field:

16. Shielding and Charge by Induction When electric charges are at rest, the electric field within a conductor is zero.

17. Shielding and Charge by Induction The electric field is always perpendicular to the surface of a conductor – if it weren’t, the charges would move along the surface.

18. Shielding and Charge by Induction The electric field is stronger where the surface is more sharply curved.

19. Electric Flux and Gauss’s Law Electric flux is a measure of the electric field perpendicular to a surface:

20. END