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? Image: Takeaway message: the device is generating something electrical throughout space in the vicinity of the coil.

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 Energy FieldForce

7 Field Model: Gravitation A source mass m 1 creates a gravitational field g m1. Direction is toward mass m 1. The net field g tot is the vector sum of all the source fields. A test mass m 2, placed in a gravitational  field, experiences a gravitational  force Magnitude given by: m 2 g m1 Direction of force is: attractive, along g m1. m1m1

8 Field Model: Electric A source charge q 1 creates an electric field E m1. Direction is toward negative charge OR away from positive charge. The net field E tot is the vector sum of all the source fields. A test charge q 2, placed in an electric field, experiences an electric  force Magnitude given by: q 2 E tot. Direction of force is: for +q 2 : same direction as E tot for -q 2 : opposite E tot. - +

9 Do Electric Field and Electric Force differ? What is the minimum number of object(s) required for each to exist? a)Both require just one object b)Field requires 1 object; force requires 2 c)Force requires 1 object; field requires 2 d)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? a)No affect b)Pulled toward c)Pulled away

11 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

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

13 Superposition of Fields  - 7 E tot = 0 8 Not enough info The + and - charges have the same magnitude. Which direction does the net Electric Field point?

14 Checking Understanding  - 7 E tot = 0 8 Not enough info A charge is placed at the marked location. What direction is the force on that charge?

15 Defining a new quantity Gravitational Potential: How much Potential Energy would a mass m have if placed (x,y)? y 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)? a) Electric Field (E) b) Electric Force (F) c) Electric Potential Energy (PE) d) Electric Potential (V)

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