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Quiz 6
1
Physics 7C Fall 2008
Lecture 6: Field model
Electric Force & Electric Field (review)
PEelectric & Electric Potential
Magnetic Force & Field
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
3
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
4
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
V
E
5
Reviewing what you’ve
previously studied…

Gravitational Potential Energy
3
g
2
1
6
Relationship between Potential
Energy and Force
0
r
Potential Energy
-
7
Relationship between Potential
Energy and Force
0
Potential Energy
-
r
1
2
3
Negative means decrease
of PE with decreasing r
8
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
9
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
10
Defining a new quantity

Gravitational Potential: How much Potential
Energy would a mass m have if placed (x,y)?
y
3
g
2
1
x
11
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
12
Electric Potential of a point charge:
Positive and Negative Charge.

Not all potentials are straight lines!
13
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)
15
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
17
18
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?
21
A little background

Compasses orient
in the same
direction as the
magnetic field.
22
A little background

Iron fillings also orient in the same direction
as the magnetic field.
23
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)
What does this mean for the magnetic field model??
25
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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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 _____
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Right Hand Rule # 1
30
Which direction is the
magnetic field at point A?
1) Into the screen
2) Out of the screen
A
B
3) Towards the wire
4) Away from the wire
C
5) Points down
6) Points up
7) Another direction
I
31
Which direction is the
magnetic field at point B?
1) Into the screen
2) Out of the screen
A
B
3) Towards the wire
4) Away from the wire
C
5) Points down
6) Points up
7) Another direction
I
32
Which direction is the
magnetic field at point C?
1) Into the screen
2) Out of the screen
A
B
3) Towards the wire
4) Away from the wire
C
5) Points down
6) Points up
7) Another direction
I
33
Magnetic Force
F into the screen
v
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 B
finger.
Very Bad Finger
x
B
q
v
F
F = qvBsinq,where qis the angle between B and v
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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
B
q
v
3) Left (along particle path)
4) Right (opposite path)
5) Down
6) Up
7) No Force
F = qvBsinq,
where qis the
angle between B
and v
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 out of the board, to the back
of the room? 1) Into the board
2) Out of the board
B
q
v
3) Left (along particle path)
4) Right (opposite path)
5) Down
6) Up
7) No Force
F = qvBsinq,
where qis the
angle between B
and v
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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
B
q
v
3) Left (along particle path)
4) Right (opposite path)
5) Down
6) Up
7) No Force
F = qvBsinq,
where qis the
angle between B
and v
37
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=qvBsinq
Direction of force given by RHR2
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