1. Electric Charge, Energy and Capacitance Chapter 17 and 18

2. Essential Questions  What is electricity?  How can you shock someone by scuffing your feet on the carpet and then creating contact?

3. Objective(s): Students will be able to…  Understand the basic properties of electric charge.  Differentiate between conductors and insulators.  Distinguish between charging by contact, charging by induction, and charging by polarization.

4. Agenda:  Welcome back!  Discuss the rest of the year.  Term 3 Grades.  Intro to electricity.  Notes:  What is electric charge?  Electrical Conductors and Insulators  Charging Objects  Chapter 15 Tests back Tomorrow

5. Electricity at an Atomic Level  All matter has electric charge because it contains:  protons (positive charge), and  electrons (negative charge)  Usually these are stuck together, but in electrical conductors (like copper), electrons are free to move around.

6. Electric Charge  Like magnets: like charges repel each other and unlike charges attract.

7. Electric Charge  Electric charge is conserved.  If an object transfers electrons to another object, it becomes more positively charged while  the other object becomes negatively charged.  Losing electrons = more positive  Gaining electrons = more negative In the picture above, the balloon gains electrons from the woman’s hair.

8. Quantifying Electric Charge  Because charge comes from the transfer of electrons, all amounts of charge are multiples of the charge of an electron (e).  Charge (q) is measured in Coulombs (C).  electron: -1.60 x 10 -19 C  proton: 1.60 x 10 -19 C  neutron: 0 C  A charge of -1.0 C (1/e) contains 6.2 x 10 18 electrons.  Therefore, 1 Coulomb is a HUGE amount of charge.

9. Conductors and Insulators  Conductors allow electrons to flow freely through them.  Charge is easily distributed through the material evenly.  Most metals.  Insulators inhibit the flow of electrons.  Excess charge has nowhere to go, and remains on the surface.  Glass, rubber, silk, plastic…

10. Conductors and Insulators  Semiconductors are used to control the flow of electrons under specific conditions.  Shared properties of conductors and insulators.  Silicon and germanium.  Superconductors allow the flow of electrons perfectly (no “friction”).  But they only work at very low temperatures (~ - 250°C)

11. Charging by Contact  When two materials are rubbed together, like balloons and hair, electrons are actually knocked off the hair onto the balloon.  This is charging by contact.  Conductors that are charged quickly become neutral unless protected by an insulator.  Think copper rod with a rubber handle.

12. Charging by Induction  If an object is grounded, it can be charged by induction.  Excess electrons flee to the ground when a charged object is brought nearby, leaving a positive charge. This is induction. The charge is induced on the object.

13. Polarization  A surface charge can be induced in insulators.  The electrons can’t flow, but they can turn away. This is polarization.  This is why a charged balloon can stick to a wall (but not a metal doorknob).

14. Recap  Electric charge is created when there is an unequal number of electrons and protons.  Conductors allow the flow of charge easily, while insulators inhibit the flow of charge.  Objects can be charged by contact, induction, and polarization.

15. Homework  Due Wed:  p633 #1, 3-5

16. Electric Force and Electric Fields 17.2 and 17.3 pp 634 - 652

17. Essential Questions  How do electric charges exert forces on each other?  What do the forces from charges or acting on charges look like?

18. Objective(s): We will be able to…  Calculate electric force using Coulomb’s Law.  Calculate electric field strength.  Draw and interpret electric field lines.

19. Agenda:  New Seats!  Review Chapter 15 Test  Review 17.1 Homework  Notes  Electric Force and Coulomb’s Law  Electric Field Strength  Electric Field Lines  Start Homework

20. Electric Force  Because electric charges (electrons and protons) attract and repel each other, they exert forces on each other.  These forces are equal in magnitude, and opposite in direction.  These forces are field forces; no direct contact. Like gravity.  The forces between charges can be measured using Coulomb’s Law.

21. Coulomb’s Law  k is a constant relating force to the strength of the charges and the distance between them.  k ≈ 8.99 x 10 9 N  m 2 /C 2 (“Newton-meter squared per Coulomb squared”)

22. Coulomb’s Law Qualitatively  What happens to the force if one of the charges doubles?  What happens to the force if the distance between charges doubles?

23. Practice Problem  Calculate the force between two 0.001 C charges that are 3.00 meters apart.  ~1000 N  Stuff to watch out for:  metric conversions!  expect forces to be large and charges to be small.

24. What we are skipping.  Theory of superposition.  i.e. more than two charges.  at angles.  yuck.

25. The Electric Field  A charged object sets up an electric field in the space around it.  Kind of like the pull of gravity around a planet.  The strength of the field can be defined as the ratio of  the force it applies to charged object and  the charge of the object  Or…

26. The Electric Field  The units of E, the electric field strength, are [N/C] (Newtons of force per Coulomb of charge)  In other words, how much force acts on a charge this big?

27. Electric Field Strength  If we assume the charge is very very small (a point charge) we can calculate the general electric field strength: Here are some common electric field strengths.

28. Electric Field Lines  We can visualize what electric fields look like by drawing electric field lines.  These are drawn by following the simple mantra:  WWPCD?  What would a positive charge do?  The answer? Run away from positive and go toward negative.  Convention: The number of field lines is proportional to the size of the charge.

29. Field Lines of Equal Charges Same Charge Opposite Charges

30. Field Lines of Unequal Charges  Note that the positive charge is twice that of the negative, so it has twice the field lines.  It overpowers the negative charge.

31. Recap  Electric charges create forces that act on each other and can be measured with Coulomb’s Law.  Electric force is a field force, and the strength of that force acting on other charges can be measured.  Electric field lines help to visualize the electric force field from charges.

32. Homework  p 642 #1, 5  p 647 #3  p652 # 2