1 Quiz 6

Physics 7C Fall 2008 Lecture 6: Field model Electric Force & Electric Field (review) PE electric & Electric Potential Magnetic Force & Field

3 Calendar Sections 1-4: DLM 17 on Tuesday Section 5: DLM 17 on Weds 8-10:20 Section 7 & 10: DLM 17 on Mon 8-10:20 or 2:10-4:30 Sections 8, 9, 11: DLM 17 either Monday OR Weds: Ask TA

4 Models of Electric Phenomena: Electric Field and Forces Each source charge Q generates an Electric Field E Q Direction convention shown at right The net Electric Field is the sum of all the source fields Charge q, placed in an electric field E tot, experiences a force F elec on q =qE Q. 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

5 Models of Electric Phenomena: Field, Forces, Potential Energy, and Potential PE & Forces started in 7A Similar relationship to V & E Potential--started in 7B PE & V have similar relationship as F & E PE F EV

6 Reviewing what you’ve previously studied… Gravitational Potential Energy g

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

8 Relationship between Potential Energy and Force Potential Energy r - 0 Negative means decrease of PE with decreasing r

9 Relationship between Potential Energy and Force Potential Energy r - 0 More slope closer to earth means F is greater there F = -  PE /  r, the - slope Force increases with greater slope

10 Reviewing what you’ve previously studied… Relationship between Potential Energy and Force Magnitude of Force = slope of PE vs. r graph.

11 Defining a new quantity Gravitational Potential: How much Potential Energy would a mass m have if placed (x,y)? y x g

12 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

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

14 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

15 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)

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

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

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19 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: _____

20 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 _____

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

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

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

24 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:

25 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) What does this mean for the magnetic field model??

26 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 _____

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

28 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?

29 Field Model: Magnetism A source moving charge creates a magnetic field in a direction given by RHR1. 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 _____

30 Right Hand Rule # 1

31 Which direction is the magnetic field at point A? 1)Into the screen 2)Out of the screen 3)Towards the wire 4)Away from the wire 5)Points down 6)Points up 7)Another direction I  B  A  C

32 Which direction is the magnetic field at point B? 1)Into the screen 2)Out of the screen 3)Towards the wire 4)Away from the wire 5)Points down 6)Points up 7)Another direction I  B  A  C

33 Which direction is the magnetic field at point C? 1)Into the screen 2)Out of the screen 3)Towards the wire 4)Away from the wire 5)Points down 6)Points up 7)Another direction I  B  A  C

34 Magnetic Force v B x RHR2 (for positive charge): your thumb points in the direction of the moving charge, B is along your index finger, and F is the middle finger. Very Bad Finger F into the screen F = qvBsin  where  is the angle between B and v  F v B

35 Magnetic Force Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling along the chalkboards, to your left? 1)Into the board 2)Out of the board 3)Left (along particle path) 4)Right (opposite path) 5)Down 6)Up 7)No Force  v B F = qvBsin  where  is the angle between B and v

36 Magnetic Force Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling out of the board, to the back of the room? 1)Into the board 2)Out of the board 3)Left (along particle path) 4)Right (opposite path) 5)Down 6)Up 7)No Force  v B F = qvBsin  where  is the angle between B and v

37 Magnetic Force Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling upward, toward the ceiling? 1)Into the board 2)Out of the board 3)Left (along particle path) 4)Right (opposite path) 5)Down 6)Up 7)No Force  v B F = qvBsin  where  is the angle between B and v

38 Field Model: Magnetism A source moving charge creates a magnetic fields in a direction given by RHR1. The net magnetic field is the sum fo all the source fields. A test moving charge, placed in a magnetic field, experiences a magnetic force Magnitude given by F=qvBsin  Direction of force given by RHR2