Transcript Slide 1

Draw the magnetic field between two opposite poles.
>
B
D
>
N
A
>
S
>
C
>
How will the north pole of a compass point if placed
at points A-D?
What dir. is the B-field at A-D?
How would the picture change if N became S and v.v.?
B
N
A
>
Draw the magnetic field between two N poles.
D
N
C
How will the north pole of a compass point if placed
at points A-D?
What dir. is the B-field at A-D?
How would the drawing change if it were two S poles?
III. Electromagnetic Induction.
force on a magnet
Oersted: Moving charge (I) exerts a _____________________.
N’s 3rd law: A magnet exerts a __________________________
force on a moving charge .
magnet
Dq
S
A: a wire
at rest
________
w/ current
N
v
v
q
moving
B: __________
charge
__________
C: wire with
no current
____________
but moving
____________
all 3 cases
The charges in ____________________
experience
magnetic
a _________________
force Fmag.
For any charge q moving with a velocity v through a B:
Magnitude (strength) of Fmag:
qvB sinq
Fmag = ________________,
q is angle between v and B.
0
Notice: If q = 0, then Fmag = _______
.
Direction of Fmag.
perpendicular
Fmag is _______________________
to v and B.
Ex: A charged particle moves at different angles to B.
Assume v is the direction of the charge's motion.
0
45
0
00
q
=
_____
90
q = _____
q = _____
z
Fmag
v
x
z
B
Fmag
y
max.
Fmag = _______
z
Fmag=0
B
B
y
x
v
med.
Fmag = _______
v
x
min.
Fmag = _______
y
Ex: The force between 2 current-carrying wires is used
ampere
to define the ________________:
I
wire 1
>
Fmag
wire 2
I
Fmag
>
ampere
1 ______________
= the amount of current in two infinite
wires of negligible diameter separated by a distance
1 meter
of _________________
that will produce a magnetic force of
-7 N
2
x
10
_______________________ per meter of length.
attractive
Note: The force is ___________________
when the current
is in same direction in both wires.
Case C (again): A wire conductor moved in a B field
the magnet could be moved
or __________________________________
S
N
E
Fmag causes the ______________________________________
electrons to move to one end of the wire.
induces (or sets up)
This ___________________________
an electric field in the
induced potential difference,
wire, which results in an _______________________________
induced voltage
or _________________________between
the ends of the wire.
Electromagnetic Induction:
Relative
“______________
motion between a conductor and a B field
potential difference in the conductor.”
induces (creates) a ___________________
magnetic force depends on
Because the strength of the _________________
v of the charge and the B field,
the angle between the __________________
potential difference also does.
the amount of induced ______________________
Ex 1. A wire moved between two magnets.
900
Its velocity v is perpendicular to B: q = ______
v
N
S
v
directly through
The wire "cuts" _____________________
the most B lines
max.
900 , so Fmag is ________.
The angle between v and B is _______
Maximum potential difference V is induced in the wire.
____________
In symbols: v
B  max. V
Ex 2. A wire moved at an angle:
00 < q < 900
v
N
S
v
some
The wire cuts ________________B
field lines.
not a max.,
Because the angle between v and B is ____________
less than max.
Fmag is ____________________
Less V is induced in the wire than in previous case.
_______
Ex 3. A wire moved parallel to B: q = 00
N
v
v
S
none of the
The wire cuts ________________B
field lines.
zero
Because the angle between v and B is ____________
zero
Fmag is _____________
No
_______
V is induced in the wire.
In symbols: v || B  min. V (=0)
In sum:
Case
angle between
v and B (0)
induced
potential
difference
1
90
max.
2
between 0-90
in between
3
0
min.
potential
Notice that in all three cases above, a ________________
difference
________________
was set up that is a result of the
relative motion
________________________
between the wire and B field.
no current
But unless the wire is attached to a circuit, _____________
will flow
________________
.
Electromagnetic devices use magnetism and electricity:
A
1. motors (based on Case _____
from first slide)
current
 A ___________
in wire experiences a force due to B.
turn a coil of wire
If that force can be made to ____________________,
then
you can transform:
electrical
mechanical energy
______________
energy  _______________
speaker
Another example of this is a ___________________
coil of wire
It is a ______________________
that is attached to a
paper cone. When current passes through the wire,
the magnetic force makes the paper cone move.
electrical
______________
energy  _______________
mechanical energy
 _______________
energy
sound
Simple DC motor
F
v
B
v
B
F
Instead of
permanent
magnets,
you can use
the current
from the
battery to
create
electromagnets
to provide the
B field.
Side view of motor case:
Simple speaker:
House Speaker:
C
2. generators (based on Case _____
from first slide)
 When a wire with no current is moved, the Fmag
electrons to the ends of the wire, but
pushes the ____________
no charge flows
connected
___________________unless
the wire is ______________
to a circuit:
__________________
N
S
v
current
With a complete circuit, there is ______________,
and
mechanical energy  ______________
electrical energy
_____________
A
simple
AC
generator:
Instead of
permanent
magnets,
you can use
some of the
output current
to create
electromagnets
to provide the
B field.
B field has a constant direction, so Fmag depends
on the direction of v:
F
B
v
v
B
F
turn
counterclockwise
The electromagnetic
force pushes charges
out the "short" wire
Now coil has turned so that it is vertical:
v
B
B
v
turn
counterclockwise
No force on electrons
or induced voltage
because v and B
are parallel (or antiparallel)
Now coil has made another ¼ turn or so…
F
B
v
B
v
F
turn
counterclockwise
The electromagnetic
force pushes charges
out the long end –in
other words…in the
opposite direction!!!
Then you would
see something
like this:
And, as the coils
were turned, it
would look like:
Now imagine looking
straight down the
rings
B always left
This
Direction of induced current:
I
t
I
t
I
t
I
t
is
how
AC
is
m
a
d
e
!
same
Electrical motors and generators are basically the _______
devices—they have _________
wires that turn in _____________:
B fields
motor
generator
mech. energy
________________
out
electric energyout
______________
electric energy in
________________
mech. energy in
_______________
Instead of moving a
wire through a B field
to induce a voltage,
you can also change the
B field near the wire by
moving the magnet
______________________
magnetic strip.
3. Credit cards have a unique ____________
B field in the ATM.
Moving the card changes the __________
voltage in a coil in the ATM.
This induces a unique ____________
magnets
4. Guitar pickups are coils wrapped around ___________.
steel
When a ____________
guitar string vibrates, it changes the
B field of the magnet. This induces a _______________
voltage
__________
speaker
in the coil which is amplified and sent to a _____________
..
wire coils
5. Transformers  two ________________
wrapped
iron core
around the same ____________:
iron
core
primary
AC
A. ______current
in _______________
coil causes…
B field
secondary
B. …the __________
at ________________
coil to change…
potential difference (voltage)
C. …which induces a ______________________________
secondary
in the _________________
coil.
> N1, then voltage is ___________________________
stepped up (increased)
If N2 ___
.
stepped down (decreased) .
< N1, then voltage is ___________________________
If N2 ___
created
destroyed :
Energy is neither _______________
nor _______________
Power in =
Pin
=
VinIin =
Step up:
Power out
Pout
VoutIout
Iin = VoutI
Vin
Step down:
VinI
in
=
out
Iout
Vout
stepped up
Voltages are ____________________
for long-distance
I2R
transmission because less heat or ________
energy
is lost. Once they reach their destination, they are
stepped down
_____________________
for safe use.
More than
meets the
eye….
6. In antennas:
electrons
up and down
-________________are
accelerated _________________
electromagnetic
-As a result, ________________________
radiation
waves
is emitted from the antenna as ____________.
-The radiation is made up of:
electric
1/ an ____________
field and
magnetic
2/ a _____________
field
As they move, the fields are constantly
changing . As one field changes,
________________
the other field and vice versa .
it induces _______________________________
at the speed
These waves move out ________________
visible light
of light
_____________
. Radio waves, __________________
x-rays are all forms of ___________________
electromagnetic
and ___________
radiation
light
_________________
and travel at the speed of __________
.
This is an example of electromagnetic ________________
.
induction
energy
E&M
In this way, __________
waves can transmit ___________
by means of their _______________
through a vacuum.
fields
When these fields reach an antenna, the fields
force the _________________
in the antenna to accelerate
electrons
frequency
at the exact same _______________
.
Cell phones and microwave ovens also transmit
waves
energy by means of electromagnetic ____________.