Download Welcome to Physics 7C

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
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Physics 7C Fall 2008
Lecture 6: Field model
Electric Force & Electric Field,
Magnetic Force & Field
Dipole field
If time permits, begin
Electric Potential Energy & Electric
Potential
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.
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Image: http://tommcmahon.typepad.com/photos/uncategorized/2007/08/16/teslacoil2.jpg
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?
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Temperature Field

What do
places with
the same
color have in
common?
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True or False

Gravity is more on Earth than the moon.
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True or False

Gravity is more for a feather on Earth than for
an elephant on the moon.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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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
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Electric Phenomenon


Like gravitational force, the electrical
interaction between objects acts at a
distance.
Electrical interaction depends on charges
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New Models:

Electric Field and Forces
Field, Forces, Potential Energy, and Potential

Magnetic Field and Forces

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Applying Field Model to
Electrical Phenomena


A charge Q generates an Electric Field EQ
Charge q, placed in an electric field EQ,
experiences a force Felec Q on q=qEQ.
How might we draw the electric field?
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Applying Field to Electrical
Phenomena


A charge Q generates an
Electric Field EQ
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
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Phenomenon: Van de Graaff
Generator

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
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Phenomenon: Van de Graaff
Generator

Next: Touch the conducting
sphere to the generator…what
will happen now?
a) No affect
b) Pulled toward
c) Pulled away
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Understanding Fields & Forces

True or False:

The generator creates an
electric field whether or not
another object is placed near it.
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Understanding Fields & Forces

True or False:

The generator creates an
electric force whether or not
another object is placed near it.
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Applying Field to Electrical
Phenomena


A charge Q generates an
Electric Field EQ
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
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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?
-
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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
d
-
b
c
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Superposition of Fields


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
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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
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Checking Understanding:

If I put a charge at the marked location, which
way will the force be?

a
e Insufficient
Information
b
-
d
c
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Field vs. Force

How many objects are required to create a
electrical field? At least…
a)
b)
c)
d)
0
1
2
3
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Field vs. Force

How many objects are required to create a
electrical force? At least…
a)
b)
c)
d)
0
1
2
3
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Models of Electric
Phenomena:

Electric Field and Forces

Each source charge Q generates an
Electric Field EQ



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.



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



PE & Forces--started in 7A
Potential--started in 7B
We’ll develop more in DLM 13 & 14
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Field Model:

A source
(A)
field in a direction


The net
(B)
source fields.
creates a
.

_
_
_
field is the sum of all the
A test
(A)
, placed in a
field, experiences a
(B)

(B)
_
(B)
force
_
Magnitude given by _____
Direction of force: _____
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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 _____
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A little background

Compasses or bar magnets, if allowed, will
always orient north-south
Why?
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A little background

Compasses orient
in the same
direction as the
magnetic field.
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A little background

Iron fillings also orient in the same direction
as the magnetic field.
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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:
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Magnetic Field from a wire

If we place compasses
around a long currentcarrying wire, this is
how they align
(view is looking down wire)
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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 _____
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Phenomenon: Magnet near an
electron beam


The beam is composed of electrons--moving
charges
Observe the effects of a large magnet on the
beam…
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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?
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Reviewing what you’ve
previously studied…

Gravitational Potential Energy
3
2
1
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Relationship between Potential
Energy and Force
0
r
Potential Energy
-
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Relationship between Potential
Energy and Force
0
Potential Energy
-
r
1
2
3
Negative means decrease
of PE with decreasing r
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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
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Reviewing what you’ve
previously studied…

Relationship between Potential Energy and
Force.
• Magnitude of
Force = slope of
PE vs. r graph.
1
4
3
dPE
• F 
dr
2
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Defining a new quantity

Gravitational Potential: How much Potential
Energy would a mass m have if placed (x,y)?
y
3
2
1
x
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Electric Field and Potential:
Constant Electric Field

dV
dx

dV
dV
dV
3D : E  
xˆ 
yˆ 
zˆ
dx
dy
dz
1D : E  

Slope of the potential



constant as a function of
distance.
negative
Electric field is


constant as a function of
distance
positive
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Electric Potential of a point charge:
Positive and Negative Charge.

Not all potentials are straight lines!
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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
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Putting it all together…

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)
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Putting it all together…

Which are vector quantities?
a)
b)
c)
d)
Electric Field (E)
Electric Force (F)
Electric Potential Energy (PE)
Electric Potential (V)
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Putting it all together…

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
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