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Quiz 5. Reminder: Emily’s Office Hours next week are different: Wednesday 11/12 from 1-2pm in TB114 Monday 11/10 OH are cancelled. Physics 7C Fall 2008 Lecture 6: Field model. Electric Force & Electric Field, Magnetic Force & Field If time permits, begin

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quiz 5
Quiz 5

Reminder: Emily’s Office Hours next week are different:

Wednesday 11/12 from 1-2pm in TB114

Monday 11/10 OH are cancelled

physics 7c fall 2008 lecture 6 field model

Physics 7C Fall 2008Lecture 6: Field model

Electric Force & Electric Field,

Magnetic Force & Field

If time permits, begin

Electric Potential Energy & Electric Potential

Dipole field

tesla coil
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

field model what is a field
Field Model: What is a field?
  • …some physical quantity that has a value “everywhere,” that can either change from location to location or stay the same.

-Physics 7C Course Notes

  • 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?

temperature field
Temperature Field
  • What do places with the same color have in common?
true or false
True or False
  • Gravity is more on Earth than the moon.
true or false1
True or False
  • Gravity is more for a feather on Earth than for an elephant on the moon.
field model gravitation
Field Model: Gravitation
  • A source mass m1 creates a gravitational field in a direction toward the source mass with a value g=Gm1/r2
    • The net Gravitational Field is the sum of all the source fields.
  • A test mass m2, placed in a gravitational field, experiences a gravitational force
    • Magnitude given by F=m2g
    • Direction of force: Attractive
electric phenomenon
Electric Phenomenon
  • Like gravitational force, the electrical interaction between objects acts at a distance.
  • Electrical interaction depends on charges
new models
New Models:
  • Electric Field and Forces
  • Field, Forces, Potential Energy, and Potential
  • Magnetic Field and Forces
applying field model to electrical phenomena
Applying Field Model to Electrical Phenomena
  • A charge Q generates an Electric FieldEQ
  • Charge q, placed in an electric field EQ, experiences a force Felec Q on q=qEQ.

How might we draw the electric field?

applying field to electrical phenomena
Applying Field to Electrical Phenomena
  • A charge Q generates an Electric FieldEQ
  • Charge q, placed in an electric field EQ, experiences a force Felec Q on q=qEQ.
    • For + test charge, force points in the same direction as field.
    • For - test charge, force points in opposite direction of field
phenomenon van de graaff generator
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
phenomenon van de graaff generator1
Phenomenon: Van de Graaff Generator
  • Next: Touch the conducting sphere to the generator…what will happen now?
    • No affect
    • Pulled toward
    • Pulled away
understanding fields forces
Understanding Fields & Forces
  • True or False:
    • The generator creates an electric field whether or not another object is placed near it.
understanding fields forces1
Understanding Fields & Forces
  • True or False:
    • The generator creates an electric force whether or not another object is placed near it.
applying field to electrical phenomena1
Applying Field to Electrical Phenomena
  • A charge Q generates an Electric FieldEQ
  • Charge q, placed in an electric field EQ, experiences a force Felec Q on q=qEQ.
    • For + test charge, force points in the same direction as field.
    • For - test charge, force points in opposite direction of field
superposition of fields
Superposition of Fields
  • 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?

-

superposition of fields1
Superposition of Fields
  • 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

b

d

c

-

superposition of fields2
Superposition of Fields
  • The electric field is a vector field
  • To find the field from multiple sources, add the vectors!

a

b

2) Which vector might be the electric field from the bottom charge?

d

c

-

superposition of fields3
Superposition of Fields
  • Which direction does the net Electric Field point?

c Etot = 0

d Neither a nor b (but not 0)

a

E-

E+

b

-

checking understanding
Checking Understanding:
  • If I put a charge at the marked location, which way will the force be?

a

e Insufficient Information

b

d

c

-

field vs force
Field vs. Force
  • How many objects are required to create a electrical field? At least…
    • 0
    • 1
    • 2
    • 3
field vs force1
Field vs. Force
  • How many objects are required to create a electrical force? At least…
    • 0
    • 1
    • 2
    • 3
models of electric phenomena
Models of Electric Phenomena:
  • Electric Field and Forces
    • Each source charge Q generates an Electric FieldEQ
      • Direction convention shown at right
    • 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=qEQ.
      • 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
    • PE & Forces--started in 7A
    • Potential--started in 7B
    • We’ll develop more in DLM 13 & 14
field model
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: _____
field model magnetism
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 _____
a little background
A little background
  • Compasses or bar magnets, if allowed, will always orient north-south

Why?

a little background1
A little background
  • Compasses orient in the same direction as the magnetic field.
a little background2
A little background
  • Iron fillings also orient in the same direction as the magnetic field.
magnetic field from a wire
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:
magnetic field from a wire1
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)

field model magnetism1
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 _____
phenomenon magnet near an electron beam
Phenomenon: Magnet near an electron beam
  • The beam is composed of electrons--moving charges
  • Observe the effects of a large magnet on the beam…
phenomenon jumping wires
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?
reviewing what you ve previously studied
Reviewing what you’ve previously studied…
  • Gravitational Potential Energy

3

2

1

relationship between potential energy and force1
Relationship between Potential Energy and Force

0

r

1

2

Potential Energy

3

Negative means decrease of PE with decreasing r

-

relationship between potential energy and force2
Relationship between Potential Energy and Force

0

r

1

2

F = - DPE/Dr, the - slope

Potential Energy

3

Force increases with greater slope

More slope closer to earth means F is greater there

-

reviewing what you ve previously studied1
Reviewing what you’ve previously studied…
  • Relationship between Potential Energy and Force.

1

  • Magnitude of Force = slope of PE vs. r graph.

4

3

2

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

y

3

2

1

x

electric field and potential constant electric field
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
electric potential of a point charge positive and negative charge
Electric Potential of a point charge:Positive and Negative Charge.
  • Not all potentials are straight lines!
equipotential surfaces lines where v is the same
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
putting it all together
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)
putting it all together1
Putting it all together…
  • Which are vector quantities?
    • Electric Field (E)
    • Electric Force (F)
    • Electric Potential Energy (PE)
    • Electric Potential (V)
putting it all together2
Putting it all together…
  • Which quantities are related by slopes (that is, if you take the slope of one, you get the other)
    • Electric Field & Electric Force
    • Electric Potential Energy & Electric Potential
    • Electric Force & Potential Energy
    • Electric Field & Electric Potential