Electric Charge, Energy and Capacitance

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 1018 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 109 Nm2/C2 (“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 Opposite Charges Same Charge

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