Electric Forces and Fields

1. Electric Forces and Fields

2. Electric Charge • Let’s review… • Atoms consist of: • Protons, neutrons, electrons • Protons have a charge of: • +e • Electrons have a charge of • -e • Neutrons have a charge of 0 What is e? e=1.6 x 10-19C Unit is the coulomb (C) This is the smallest charge yet discovered.

3. Electric Charge • Intrinsic property of electrons and protons • Most objects have balanced charges- in order to carry a charge, there must be an imbalance btwn p+ and e- (ionization) • Can be positive or negative • Likes repel • Opposites attract

4. Electric Charge is… • Charge is conserved- net charge cannot be created or destroyed • Charge is quantized- must be an integer multiple of e • q= the charge of an object (unit is C)

5. Creating Electric Charge • Usually charge is created through transfer of electrons • Gain electrons = gain negative charge • Lose electrons= gain positive charge • You can do this in 3 ways: • Friction • Conduction • Induction

6. Asbestos Fur (rabbit) Glass Mica Wool Quartz Fur (cat) Lead Silk Human skin, Aluminum Cotton Wood Amber Copper, Brass Rubber Sulfur Celluloid India rubber Charging by friction • Different objects have different propensities for gaining or losing electrons • Triboelectric sequence ranks objects-if you rub any 2 objects together, the one on top will lose elecrons and the one on the bottom will gain • The farther they are apart, the more they will charge

7. Charging by friction • Using the triboelectric sequence, predict what will happen when a glass rod is rubbed on a piece of silk • Will a charge develop? • How big will it be? • Which object will be negative?

8. Charging by Conduction • Conduction= TOUCHING • If you touch a charged object to another object, electrons can actually flow from one to the other

9. Showing Conduction • We can use an electroscope to show charging by conduction • Take a positively charged rod and touch it to the ball on the top of the electroscope • Which way would electrons flow? • What would be the residual charge on the electroscope? Electroscope shows SAME charge after conduction

10. Charging by Induction • Charged object is brought near but never touches another object • Because electric charge does not require contact (electric field), you can induce a charge just by proximity with a charged object

11. Showing Induction • If you bring your + rod near the electroscope, electrons will be attracted to it and will gather at the ball- this is temporary- once you move it away, the charge is gone • However, if the electroscope is grounded (attached to the earth which is a pretty good conductor) you could get electrons flowing from the earth and keep a net charge- opposite to charging rod!

12. Induction in insulators • In an insulator, electrons can’t move freely • You can still induce a charge by polarizing the molecules close to the charged object In this insulator the top surface becomes - and the bottom surface becomes +

13. Induction in insulators • Try this with a charged rod and tiny scraps of paper (an insulator) • What happens to the paper? Why? • What processes are happening?

14. Coulomb’s Law • Electrostatic force, Fe depends on • The charge on the objects: q1 and q2 • The distance between them, r

15. Coulomb’s Law • For point charges • F=k q1q2 r2 • On your green sheet, k is written as: • k= 1/40 but in the section on constants, you can find k- look it up now • Easiest to deal with magnitudes of q and then note that likes repel, opposites attract

16. Coulomb’s Law, visually

17. Example: Coulomb’s Law • Consider 2 small spheres, one carrying a charge of +1.5nC and the other a charge of -2.0nC, separated by a distance of 1.5cm. Find the electric force between them. • (reminder: nano=10-9)

18. Solution: Coulomb’s Law • FE=kq1q2/r2 • =(9x109Nm2/C2) (1.5x10-9C)(-2.0x10-9C) (0.015m)2 • =-1.2x10-4N • When F is -, it means attraction…but you knew that already by the opposite charges

19. Worksheet: Coulomb’s Law

20. Superposition and Coulomb • If there are more than 2 point charges, the FE is the sum of all FE acting on a point

21. Worksheet: Coulomb Beyond the Fundamentalsand Suspended spheres

22. The Electric Field

23. Electric Field • The presence of a charge created an electric field in the space that surrounds it • Other charges will be affected by this field • Directional so it is a vector and can be added/subtracted as a vector E=Fon q/q The Electric field equals the electric force on a small test charge, q, divided by that small test charge. Unit: N/C Direction: same as force The electric field leads to the F

24. FE=kq1q2/r2 E=Fon q/q Putting it together: FE and E E=Fon q/q E=kqQ r2q E=kQ/r2 Note- not on green sheet. You will need to be able to derive this if you need it!

25. Electric Field Lines Begin at +, end at -, do not start or stop in midspace, number of lines is proportional to the charge. • Point in for -source • Point out for +source • Density of lines shows strength of field

26. Distance and Strength of E

27. Vector Nature of E At any point, you can add the EB+EA

28. Vector Addition of E

29. Common E interactions • Note direction of arrows • 2 equal but opposite charges (above right) are called an electric dipole • NOTICE- fields lines NEVER CROSS

30. Electric Field Example • A charge of q=+3.0nC is placed at a location at which the electric field has a strength of 400N/C. Find the force felt by the charge q.

31. Electric Field Solution • Fon q=qE • F=(3x10-9C)(400N/C)=1.2x10-6N

32. Worksheet: Electrostatic Forces and Fields: Point Charges

33. Conductors and Insulators • Conductors permit the flow of excess charge • Insulators don’t let electrons flow • Semiconductors- kind of in between • Superconductors- no resistance to flow of electrons (many metals act this way at low T)

34. Conductors- a closer look • Any excess charge resides solely on the outer surface of a conductor • The charge inside is zero! • Any LOST fans? Michael Faraday built a “room within a room” to demonstrate this- the man in the picture is safe- no charge inside his cage • We use this to “shield” our sensitive electronics by enclosing in a metal box

35. Faraday Cage

36. Conductors- a closer look • For points outside the conductor, the electric field acts as if it is concentrated at the center of the conductor • Electric field is always perpendicular to the surface • No matter what the shape

37. Special Situations: Parallel Plates • Above you see 2 parallel plates attached to a battery • The symbol on the left is the battery- the longer line is the + terminal and the shorter line is the – • Thus the top plate is + and bottom is – • The electric field then is the the direction a + charge would move- thus the arrows

38. Special Situations: Parallel Plates • This type of field is uniform • In parallel plates, E=V/d where V is voltage supplied by battery and d is distance between plates • (units of V/m= N/C) for E • Thus a test charge would experience the same force regardless of where it is located in the field • F=qE ANYWHERE between plates

39. Example: Parallel Plates • V=28V and d=0.14 m • Find the F on a 2nCcharge inserted anywhere between the plates

40. Plates problem solution • 1st find E: • E=28V/0.14m • E=200V/m • Or 200 N/C • 2nd find F • F=qE • F=(2x10-9C)(200N/C) • F=4 x 10-7N • TOWARD which plate?