1. 1 Quiz 5 Reminder: Emily’s Office Hours next week are different: Wednesday 11/12 from 1-2pm in TB114 Monday 11/10 OH are cancelled

2. Physics 7C Fall 2008 Lecture 6: Field model Electric Force & Electric Field, Magnetic Force & Field If time permits, begin Electric Potential Energy & Electric Potential Dipole field

3. 3 Tesla Coil Powered by electricity Observe the top--what do you see? Observe as a fluorescent lamp is brought near--what do you see? Image: http://tommcmahon.typepad.com/photos/uncategorized/2007/08/16/teslacoil2.jpg Takeaway message: the device is generating something electrical throughout space in the vicinity of the coil.

4. 4 Field Model: What is a field? …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same. -Physics 7C Course Notes In physics, a “field” refers to a quantity that has a value for every point in space.” -homework from DLM 10 Is temperature a field?

5. 5 Temperature Field What do places with the same color have in common?

6. 6 True or False Gravity is more on Earth than the moon.

7. 7 True or False Gravity is more for a feather on Earth than for an elephant on the moon.

8. 8 Field Model: Gravitation A source mass m 1 creates a gravitational field in a direction toward the source mass with a value g=Gm 1 /r 2 The net Gravitational Field is the sum of all the source fields. A test mass m 2, placed in a gravitational field, experiences a gravitational force Magnitude given by F=m 2 g Direction of force: Attractive

9. 9 Electric Phenomenon Like gravitational force, the electrical interaction between objects acts at a distance. Electrical interaction depends on charges

10. 10 New Models: Electric Field and Forces Field, Forces, Potential Energy, and Potential Magnetic Field and Forces

11. 11 Applying Field Model to Electrical Phenomena A charge Q generates an Electric Field E Q Charge q, placed in an electric field E Q, experiences a force F elec Q on q =qE Q. How might we draw the electric field?

12. 12 Applying Field to Electrical Phenomena A charge Q generates an Electric Field E Q Charge q, placed in an electric field E Q, experiences a force F elec Q on q =qE Q. For + test charge, force points in the same direction as field. For - test charge, force points in opposite direction of field

13. 13 Phenomenon: Van de Graaff Generator Observe what happens as the generator builds charge. What happens when a neutral conductor is brought near the generator? a)No affect b)Pulled toward c)Pulled away

14. 14 Phenomenon: Van de Graaff Generator Next: Touch the conducting sphere to the generator…what will happen now? a)No affect b)Pulled toward c)Pulled away

15. 15 Understanding Fields & Forces True or False: The generator creates an electric field whether or not another object is placed near it.

16. 16 Understanding Fields & Forces True or False: The generator creates an electric force whether or not another object is placed near it.

17. 17 Applying Field to Electrical Phenomena A charge Q generates an Electric Field E Q Charge q, placed in an electric field E Q, experiences a force F elec Q on q =qE Q. For + test charge, force points in the same direction as field. For - test charge, force points in opposite direction of field

18. 18 Superposition of Fields The electric field is a vector field To find the field from multiple sources, add the vectors! Which way is the electric field at the marked spot? - 

19. 19 Superposition of Fields The electric field is a vector field To find the field from multiple sources, add the vectors! ab cd 1) Which vector might be the electric field from the top charge? - 

20. 20 Superposition of Fields The electric field is a vector field To find the field from multiple sources, add the vectors! ab cd 2) Which vector might be the electric field from the bottom charge? - 

21. 21 Superposition of Fields Which direction does the net Electric Field point?  - E+E+ E-E- c E tot = 0 d Neither a nor b (but not 0) b a

22. 22 Checking Understanding: If I put a charge at the marked location, which way will the force be?  - d b c a e Insufficient Information

23. 23 Field vs. Force How many objects are required to create a electrical field? At least… a)0 b)1 c)2 d)3

24. 24 Field vs. Force How many objects are required to create a electrical force? At least… a)0 b)1 c)2 d)3

25. 25 Models of Electric Phenomena: Electric Field and Forces Each source charge Q generates an Electric Field E Q Direction convention shown at right The net Electric Field is the sum of all the source fields Charge q, placed in an electric field E tot, experiences a force F elec on q =qE Q. For + test charge, force points in the same direction as field. For - test charge, force points in opposite direction of field Field, Forces, Potential Energy, and Potential PE & Forces--started in 7A Potential--started in 7B We’ll develop more in DLM 13 & 14

26. 26 Field Model: A source (A) creates a (B) _ field in a direction _. The net (B) _ field is the sum of all the source fields. A test (A), placed in a (B) _ field, experiences a (B) _ force Magnitude given by _____ Direction of force: _____

27. 27 Field Model: Magnetism A source ____________ creates a magnetic field in a direction given by _______. The net magnetic field is the sum of all the source fields. ___________, placed in a magnetic field, experiences a magnetic force Magnitude given by _____ Direction of force given by _____

28. 28 A little background Compasses or bar magnets, if allowed, will always orient north-south Why?

29. 29 A little background Compasses orient in the same direction as the magnetic field.

30. 30 A little background Iron fillings also orient in the same direction as the magnetic field.

31. 31 Magnetic Field from a wire If we allow iron fillings freedom to rotate, and put them near a current-carrying wire, this is how they align:

32. 32 Magnetic Field from a wire If we place compasses around a long current- carrying wire, this is how they align (view is looking down wire)

33. 33 Field Model: Magnetism A source moving charge creates a magnetic field in a direction given by _______. The net magnetic field is the sum of all the source fields. A test moving charge, placed in a magnetic field, experiences a magnetic force Magnitude given by _____ Direction of force given by _____

34. 34 Phenomenon: Magnet near an electron beam The beam is composed of electrons--moving charges Observe the effects of a large magnet on the beam…

35. 35 Phenomenon: jumping wires Two wires Initially no current (observe wires) Connect both wires to a generator, making current flow. Observe: What happens to the wires? What happens if I reverse the direction of the current in one wire (compared to first time)? What happens if I reverse the direction of the current in both wires (compared to the first time)? What would happen if I could put current in only one wire?

36. 36 Reviewing what you’ve previously studied… Gravitational Potential Energy 1 2 3

37. 37 Relationship between Potential Energy and Force Potential Energy r - 0

38. 38 Relationship between Potential Energy and Force 1 2 3 Potential Energy r - 0 Negative means decrease of PE with decreasing r

39. 39 Relationship between Potential Energy and Force 1 2 3 Potential Energy r - 0 More slope closer to earth means F is greater there F = -  PE /  r, the - slope Force increases with greater slope

40. 40 Reviewing what you’ve previously studied… Relationship between Potential Energy and Force. 1 2 3 4 Magnitude of Force = slope of PE vs. r graph.

41. 41 Defining a new quantity Gravitational Potential: How much Potential Energy would a mass m have if placed (x,y)? 1 2 3 y x

42. 42 Electric Field and Potential: Constant Electric Field Slope of the potential constant as a function of distance. negative Electric field is constant as a function of distance positive

43. 43 Electric Potential of a point charge: Positive and Negative Charge. Not all potentials are straight lines!

44. 44 Equipotential Surfaces: Lines where V is the same. Equipotential surfaces for a point charge. Circles are 0.5 V apart. Distance between circles is NOT uniform! Circles get closer and closer toward center Potential grows like 1/r

45. 45 Putting it all together… Which quantities depend only on source charge(s)? a)Electric Field (E) b)Electric Force (F) c)Electric Potential Energy (PE) d)Electric Potential (V)

46. 46 Putting it all together… Which are vector quantities? a)Electric Field (E) b)Electric Force (F) c)Electric Potential Energy (PE) d)Electric Potential (V)

47. 47 Putting it all together… Which quantities are related by slopes (that is, if you take the slope of one, you get the other) a)Electric Field & Electric Force b)Electric Potential Energy & Electric Potential c)Electric Force & Potential Energy d)Electric Field & Electric Potential