Quiz 5

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# Quiz 5 - PowerPoint PPT Presentation

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 &amp; Electric Field, Magnetic Force &amp; Field If time permits, begin

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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 2008Lecture 6: Field model

Electric Force & Electric Field,

Magnetic Force & Field

If time permits, begin

Electric Potential Energy & Electric Potential

Dipole field

Tesla Coil
• 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.

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
• What do places with the same color have in common?
True or False
• Gravity is more on Earth than the moon.
True or False
• Gravity is more for a feather on Earth than for an elephant on the moon.
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
• Like gravitational force, the electrical interaction between objects acts at a distance.
• Electrical interaction depends on charges
New Models:
• Electric Field and Forces
• Field, Forces, Potential Energy, and Potential
• Magnetic Field and Forces
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
• 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
• 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 Generator
• Next: Touch the conducting sphere to the generator…what will happen now?
• No affect
• Pulled toward
• Pulled away
Understanding Fields & Forces
• True or False:
• The generator creates an electric field whether or not another object is placed near it.
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 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
• 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 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 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 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:
• 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
• How many objects are required to create a electrical field? At least…
• 0
• 1
• 2
• 3
Field vs. Force
• How many objects are required to create a electrical force? At least…
• 0
• 1
• 2
• 3
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:
• 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
• 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
• Compasses or bar magnets, if allowed, will always orient north-south

Why?

A little background
• Compasses orient in the same direction as the magnetic field.
A little background
• Iron fillings also orient in the same direction as the magnetic field.
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 wire
• If we place compasses around a long current-carrying wire, this is how they align

(view is looking down wire)

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
• The beam is composed of electrons--moving charges
• Observe the effects of a large magnet on the beam…
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…
• Gravitational Potential Energy

3

2

1

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 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 studied…
• Relationship between Potential Energy and Force.

1

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

4

3

2

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
• Slope of the potential
• constant as a function of distance.
• negative
• Electric field is
• constant as a function of distance
• positive
• Not all potentials are straight lines!
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…
• Which quantities depend only on source charge(s)?
• Electric Field (E)
• Electric Force (F)
• Electric Potential Energy (PE)
• Electric Potential (V)
Putting it all together…
• Which are vector quantities?
• Electric Field (E)
• Electric Force (F)
• Electric Potential Energy (PE)
• Electric Potential (V)
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