Quiz 5 Reminder Emilys Office Hours next week

Quiz 5 Reminder: Emily’s Office Hours next week are different: Wednesday 11/12 from 1 -2 pm in TB 114 Monday 11/10 OH are cancelled 1

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

Tesla Coil l 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. 3 Image: http: //tommcmahon. typepad. com/photos/uncategorized/2007/08/16/teslacoil 2. jpg

Field Model: What is a field? l …some physical quantity that has a value “everywhere, ” that can either change from location to location or stay the same. -Physics 7 C Course Notes l 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? 4

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

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

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

Field Model: Gravitation l A source mass m 1 creates a gravitational field in a direction toward the source mass with a value g=Gm 1/r 2 l l 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 l l Magnitude given by F=m 2 g Direction of force: Attractive 8

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

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

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

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

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

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

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

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

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

Superposition of Fields l l 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? - 18

Superposition of Fields l l 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 d - b c 19

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

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

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

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

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

Models of Electric Phenomena: l Electric Field and Forces l Each source charge Q generates an Electric Field EQ l l l 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=q. EQ. l l l Direction convention shown at right 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 l PE & Forces--started in 7 A Potential--started in 7 B We’ll develop more in DLM 13 & 14 25

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

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

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

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

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

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

Magnetic Field from a wire l If we place compasses around a long currentcarrying wire, this is how they align (view is looking down wire) 32

Field Model: Magnetism l A source moving charge creates a magnetic field in a direction given by _______. l l 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 l l Magnitude given by _____ Direction of force given by _____ 33

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

Phenomenon: jumping wires l Two wires l l Initially no current (observe wires) Connect both wires to a generator, making current flow. Observe: l l 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? 35

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

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

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

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

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

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

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

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

Equipotential Surfaces: Lines where V is the same. l Equipotential surfaces for a point charge. l l Circles are 0. 5 V apart. Distance between circles is NOT uniform! l Circles get closer and closer toward center l Potential grows like 1/r 44

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

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

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