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Quiz 5

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

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Quiz 5

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  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 2008Lecture 6: Field model Electric Force & Electric Field, Magnetic Force & Field If time permits, begin Electric Potential Energy & Electric Potential Dipole field

  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? Takeaway message: the device is generating something electrical throughout space in the vicinity of the coil. Image: http://tommcmahon.typepad.com/photos/uncategorized/2007/08/16/teslacoil2.jpg

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

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

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

  8. Field Model: Gravitation • A source mass m1 creates a gravitational field in a direction toward the source mass with a value g=Gm1/r2 • The net Gravitational Field is the sum of all the source fields. • A test mass m2, placed in a gravitational field, experiences a gravitational force • Magnitude given by F=m2g • Direction of force: Attractive

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

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

  11. Applying Field Model to Electrical Phenomena • A charge Q generates an Electric FieldEQ • Charge q, placed in an electric field EQ, experiences a force Felec Q on q=qEQ. How might we draw the electric field?

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

  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? • No affect • Pulled toward • Pulled away

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

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

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

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

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

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

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

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

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

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

  25. Models of Electric Phenomena: • Electric Field and Forces • Each source charge Q generates an Electric FieldEQ • Direction convention shown at right • The net Electric Field is the sum of all the source fields • Charge q, placed in an electric field Etot, experiences a force Felec on q=qEQ. • 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. 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. 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. A little background • Compasses or bar magnets, if allowed, will always orient north-south Why?

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

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

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

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

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

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

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

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

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

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

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

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

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