Physics 7 C SS 1 Lecture 7 Field

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

Quiz 1 grades n n n 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! 2

Tesla Coil n n n 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/teslacoil 2. jpg 3

New Models: n n Electric Field and Forces Field, Forces, Potential Energy, and Potential § Magnetic Field and Forces 4

Field Model: Generic n A source n n n (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 n n Magnitude given by _____ Direction of force given by _____ What goes blanks for gravitation? Electricity? Magnetism? 5

Potential Energy Field Force 6

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

Field Model: Electric n A source charge q 1 creates an electric field Em 1. n n n + 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 q 2, placed in an electric field, experiences an electric force n n Magnitude given by: q 2 Etot. Direction of force is: n n for +q 2: same direction as Etot for -q 2: opposite Etot. - 8

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 9

Phenomenon: van de Graaff generator n 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 10

Phenomenon: van de Graaff generator n Next: Touch the conducting sphere to the generator… What will happen now? a) No affect b) Pulled toward c) Pulled away 11

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

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

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

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

Electric Field and Potential: Constant Electric Field n Slope of the potential n n n constant as a function of distance. negative Electric field is n n constant as a function of distance positive 16

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

Equipotential Surfaces: Lines where V is the same. n Equipotential surfaces for a point charge. n Circles differ by 0. 5 V. n Distance between circles is NOT uniform! n n Circles get closer and closer toward center Potential grows like 1/r 18

Putting it all together… n 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) 19

Depend on only source Depend on source and test Potential PE=q. V Potential Energy Field F=q. E Force Scalar Vector 20