Download Physics 7C SS1, Lecture 8: Field Model

Physics 7C SS1, Lecture 8:
Field Model
Finish Electric
Begin Magnetic
Depend on
only source
Potential
Scalar
Depend on
source and test
Potential
Energy
PE=qV
dV
E
dr
Vector

Field
dPE
F
dr
F=qE

Force
Field Model: Electric

A source charge q1 creates an
electric field Em1.



+
Direction is toward negative charge OR away
from positive charge.
The net field Etot is the vector sum of all the
source fields.
A test charge q2, placed in an electric
field, experiences an electric force


Magnitude given by: q2Etot.
Direction of force is:


for +q2: same direction as Etot
for -q2: opposite Etot.
-
Quiz 3
Electric Field and Potential:
Constant Electric Field

dV
1D : E  
dx

dV
dV
dV
3D : E  
xˆ 
yˆ 
zˆ
dx
dy
dz

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.

Not all potentials are straight lines!
Equipotential Surfaces: Lines
where V is the same.

Equipotential surfaces for
a point charge.
 Circles differ by 0.5 V.
 Distance between
circles is NOT uniform!


Circles get closer and
closer toward center
Potential grows like 1/r
Field Model: Magnetic

A source



creates a magnetic field B1.
Direction is
.
The net field Btot is the vector sum of all the
source fields.
A test
, placed in a magnetic field,
experiences an magnetic force


Magnitude given by:
Direction of force is:
.
.
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 currentcarrying 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)
Right Hand Rule # 1
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
B
A
C
I
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
B
A
C
I