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Physics 7C SS1, Lecture 7: Field Model

Physics 7C SS1, Lecture 7: Field Model. Electric (and gravitational analogies) Magnetic?. Quiz 1 grades. 142 students, average of 8.4 (low B) 34 in the As (12 A+, 9 A, 15 A-) 59 in the Bs (11 B+, 19 B , 29 B-) 44 in the Cs (23 C+, 15 C, 6 C-) 5 Ds, 0 Fs

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Physics 7C SS1, Lecture 7: Field Model

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  1. Physics 7C SS1, Lecture 7: Field Model Electric (and gravitational analogies) Magnetic?

  2. Quiz 1 grades • 142 students, average of 8.4 (low B) • 34 in the As (12 A+, 9 A, 15 A-) • 59 in the Bs (11 B+, 19 B , 29 B-) • 44 in the Cs (23 C+, 15 C, 6 C-) • 5 Ds, 0 Fs • Submit Quiz 1 Regrade Requests by Thursday!

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

  5. Field Model: Generic • A source (A) creates a (B)field. • Direction given by _______. • The net (B) field is the vector sum of all the source fields. • A test (A) , placed in a (B)field, experiences a (B)force • Magnitude given by _____ • Direction of force given by _____ What goes blanks for gravitation? Electricity? Magnetism?

  6. Potential Potential Energy Field Force

  7. Field Model: Gravitation • A source mass m1creates a gravitational field gm1. • Direction is toward mass m1. • The net field gtot is the vector sum of all the source fields. • A test mass m2, placed in a gravitationalfield, experiences a gravitationalforce • Magnitude given by: m2gm1 • Direction of force is: attractive, along gm1. m1

  8. Field Model: Electric • A source charge q1creates an electric field Em1. • Direction is toward negative charge OR away from positive charge. • The net field Etot is the vector sum of all the source fields. • A test charge q2, placed in an electricfield, experiences an electricforce • Magnitude given by: q2Etot. • Direction of force is: • for +q2: same direction as Etot • for -q2: opposite Etot. + -

  9. Do Electric Field and Electric Force differ? What is the minimum number of object(s) required for each to exist? • Both require just one object • Field requires 1 object; force requires 2 • Force requires 1 object; field requires 2 • Both require at least two objects

  10. 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

  11. Phenomenon: van de Graaff generator • Next: Touch the conducting sphere to the generator… What will happen now? • No affect • Pulled toward • Pulled away

  12. Superposition of Fields The + and - charges have the same magnitude. Which direction does the net Electric Field point?  -

  13. Superposition of Fields The + and - charges have the same magnitude. Which direction does the net Electric Field point?  7 Etot = 0 8Not enough info 1 6 2 3 5 - 4

  14. Checking Understanding A charge is placed at the marked location. What direction is the force on that charge?  7 Etot = 0 8Not enough info 1 6 2 3 5 - 4

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

  16. 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

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

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

  19. 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)

  20. Depend on only source Depend on source and test Potential Potential Energy PE=qV Scalar Vector Field Force F=qE

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