Download N - Mr Bernabo at Affton High School

Note for next year:
I don’t get Ampere’s law
Clean up this unit- see AP objectives
Magnetism
This train does not roll on wheels.
All its weight floats on magnets.
Misc pictures to ebed later
Magnetism was discovered over 2,000 years ago.
The first magnets were natural rocks called lodestones.
Magnetism was named for the region of Greece
(Magnesia) where these rocks were found.
No matter the shape,
Magnets have two sides or “poles”
North pole
N
N
N
South pole
S
S
S
N
S
N
S
If you break a magnet
N
S
N
S
N
S
N
S
N
S
N
S
N
S
You could keep breaking the magnet
until you were down to a single atom
N
S
AND IT too would act as a magnet with
a north and south pole.
As before
LIKES POLES REPEL
OPPOSITE ATTRACT
S
N
N
N
S
S
REPEL
ATTRACT
REPEL
N
S
N
S
N
S
The earth is also a magnet.
It has North and South Magnetic Poles
(a little off from geographic poles)
Compasses use a magnetized needle which points
to the magnetic poles of the planet (a little off from geographic poles)
N
W
E
S
Notice a compass
would not work
too well here.
The North Pole
of a magnet is
the one that is
attracted to
the North Pole
of the earth
Which really means that the
north magnetic pole is by
definition the south pole of a
magnet since they attract.
(But this is just trivia).
Until 1820, people thought electricity and magnetism were
unrelated. Until Hans Christian Oersted made a discovery
during a class.
N
W
E
S
When the switch was closed and current flowed, the compass
needle moved!!!!
N
W
E
S
animation
ELECTRICITY AND MAGNETISM
are VERY RELATED
MAGNETIC Fields are created
by moving charged particles
Electricity & Magnetism
Similar because:
Likes repel
Opposites attract
Electricity & Magnetism
Different because:
Positive and negative charges exist independent of each other
P+
e-
+
N
S
N
S
S
N
YOU cannot have a N pole without a S pole t
ETC
Electricity & Magnetism
Similar because:
They both (like gravity) exert a
force without contact.
That really bugs me.
Electricity & Magnetism
Similar because:
THEY BOTH CREATE FORCE FIELDS
Field lines arrows run from N to S poles.
Is a north pole like a + or - charge?
Where is the field the strongest?
Magnetic Field Strength has units of :
TESLA (T)
Surface of a Neutron Star
Surface of a strong magnet
Near earth’s magnetic poles
Magnetic Field
100,000,000
10
.00005
gauss (G) is also used sometimes 1 G = 10-4 T
The variable for magnetic field is B.
Magnetic field lines are mapped out by using
another magnet like a compass
Phet magnet
simulation
A magnet will align itself with field lines
A magnet will align itself with field lines, when the torques
balance out around the needle pivot. Think back to torque, how
does the large red force balance the small blue force.
Why do magnetic field lines have to be mapped out with both a N and S
pole?
Why can’t we just put a N pole in the field and map out the net force on
it like we did with electric field with a + test charge?
Iron filings will trace out field lines because
they become temporarily magnetized.
Magnetic Fields are really 3D
Magnetism originates
in the motion of the
electrons in iron.
Spinning electrons
act like tiny magnets.
Almost 100 % cancellation
of this effect occurs in most
materials.
Iron, nickel, cobalt are
exceptions.
Last sub-level of
Fe- ferromagnetic
3d ___ ___ ___ ___ ___
Zn- Non magnetic
3d ___ ___ ___ ___ ___
Little regions of a ferro-magnetic material,
have the unpaired electrons align
image of domains (using a special method to show magnetic fields,
domains are not visible even with a microscope)
domains can be forced to
align by a magnetic field.
they will temporarily stay
aligned
heating or banging
tends to unalign them
again
This is why certain materials become magnetic
when touching a magnet.
Because the domains align in the field
(the coins must be made with Fe, Ni, or Co)
S
N
N
S
S
N
N
S
N
S
In order for the to be an electric force between 2 objects.
Both objects must have charges or induced charges.
Repel
-
-
so this is really an interaction
between 2 electric fields
-
-
-
-
Attract
+ + +
+
+
+
++
+
+
- - ++
NADA
electron
neutron
two permanent
magnets
repel
A permanent and
iron nail
(an induced magnet)
attract
A permanent and
copper wire
NADA
Magnet activity:
things to see or try
put a pencil through all 3 magnets, can you make them levitate?
how does a magnet affect a compass
does a compass affect another compass
Place the magnet flat under the magnetic viewer
Place the magnet on an edge under the magnetic viewer
(How does it differ, remember you are viewing field lines)
Next Topic -- Electric current produces a magnetic field
If a magnet and electron are both stationary,
there is no force between them.
because there is only: 1 magnetic field
&
1 electric field
e-
If the electron(s) are moving then…
moving charged particles (like electricity)
produce a magnetic field.
FORCE
eeeeeeeeeee-
Magnetic Field lines around a wire carrying current.
Magnetic Field Lines follow
the right hand rule
current
Which way is current flowing?
Which way would the magnetic field point inside a loop if the
current is traveling in a loop?
I
Common convention is to use x’s (into the page) and circles (out of the
page) to show magnetic fields etc…. Shouldn’t have to memorize this
I
Out of the “page”
I
Into the “page”
When the wire forms a loop the field
concentrates at the center.
We’ll come back to this.
vacuum permeability = 4p x 10-7 T m/A
current (A)
mo I
B=
2pr
distance from wire
magnetic field
strength around a
straight wire
15 A of current flows through a wire to the left, what is
the strength and direction of the magnetic field 15 cm
above the wire.
I
15 A of current flows through both wires below. If they are seperated
by a distance of 30 cm. What will the magnetic field strength be
midway between the?
I
I
The field is
strongest in the
middle of the
loop
the more the
loops the
stronger the
field
Coils of wire are used to intensify a
magnetic field within it.
Which would be the north pole
of the magnet?
If you are just given a wire there are two ways to
predict the north pole. Sorry This picture is not very clear
Use the 1st right hand rule at the top of a loop.
Or a very similar right hand rule for solenoids
Fingers follow current, thumb points to N pole
The greater the number of wraps the greater the internal
field strength
A better way of stating this is:
the field strength increases as the
density of the wraps increases.
Use thin wire with thin insulation
Within the coil the field strength
concentrated and pretty much uniform.
Outside the coil the field is weak and
divergent
B = m0nI
current (A)
field strength
in the coil (T)
permeability
of free space
# of wraps
meter
Charged particles which are moving in
a magnetic field will feel a force
What is the direction of the field between the ends of the magnet
N
S
If a stationary proton is placed between the poles what happens?
Nothing
If the proton is moving.
It experience a force to both the
field and its motion
N
S
Force
Field
Motion
Fingers following magnetic field.
Thumb following direction of movement
palm indicates direction of force
on a POSITIVE particle.
Force
Use left hand or opposite RH for
electron!
B
motion of particle
Applet 1
Fingers following magnetic field.
Thumb following direction of
movement
palm indicates direction of force
N
S
Force
Field
Motion
This is what steers the electrons to the right pixel in a CRT TV
applet TV electron beam
scan rate / persistence of vision (applet)
And also protects the earth from some solar radiation
The force on a single moving charged particle,
is it
orthe
outpage
of thehere.
page?
It
is in
into
B
F = qvB sin(q)
Force on
object (N)
Charge on
object (C)
Speed of
object (m/s)
q
v
1 if Perpendicular
0 if parallel
The amount of force felt by a charge particle is proportional to
RATE at which is “cuts” through magnetic field lines
Speed
Direction of movement
Location
A proton feels a force of 6x10-12 N pointed
to the left when it moves straight up at
45,000 m/s. What is the magnitude and
direction of the magnetic field?
If the magnetic field is into the page and the proton moves to the
right, what is the direction of the force on it?
F
F
F
v
Charged particle in a magnetic field applet
How much work will this magnetic force do on the particle?
This force can never do work on a charged particle because
it is always perpendicular to motion (right hand rule).
F
F
NONE
F
v
But remember everything is based on …….
A mass spectrometer zaps a substance into
fragment ions. These ions are then accelerated into
a magnetic field where they curve at different rates
based on the mass to charge ratio. It is used to
determine the elemental composition of a molecule
mass spec applet
mass spec video
Moving charged particle in a magnetic field spiral
video clip.
A proton is shot into a magnetic field.
In what direction with the magnetic force be?
In what direction should an electric field be set to keep it moving straight?
FB
FE
+
v
E
E
How about here with an electron?
Which way should the electric field point?
FB
FE
Next topic:
Force on a current carrying wire in a
magnetic field
If a wire with no current is placed in a
magnetic field, which way does it get pushed?
It doesn’t
If the current is flowing.
It experience a force to both the
field and current
N
S
Current
Field
Force
Fingers following magnetic field.
Thumb following direction of
movement or current
palm indicates direction of force
Fingers following electric field.
Thumb following direction of current
palm indicates direction of force
N
S
Current
Field
Force
If the moving charged particles are stuck in a wire.
The whole wire feels a force on it
Use the right hand rule to determine the poles of the magnet.
S
N
The force is greater if the motion (current) and field are perpendicular
for reasons explained before
current
current
less force
What factors will affect the Magnitude of Force the wire feels.
Amount of current
Angle of wire to field
Strength of magnetic Field
Force of a wire carrying current by a magnetic field is
B
q
L
I
F = BILSin(q)
Magnetic Field (T)
Current (A)
Length of wire (m)
1 if Perpendicular
0 if parallel
A wire is 50 cm long and carries 8.0 A through a magnetic
field strength of 10 T. What is the force on it assuming it is
perpendicular to the field.
If the wire has a mass of 35 g, what current would be
needed to levitate it?
Forces between two parallel wires
I
I
B
B
Two current carrying wires will either attract or repel
because of the interaction of their magnetic fields
I
I
B
B
But it is MUCH easier to think of 1 wire as creating the magnetic field.
The other wire is just a current carrying wire in a magnetic field.
Would the wires attract or repel?
I
I
Following the right
hand rule the force
will be towards the
other wire.
ATTRACT
A wire carrying 25 A of current is 5 cm from another wire carrying 35 A
of current. What is the force on the 35 A wire if it is 1.3 m long (include
its direction)?
25 A
35 A
F = B I L Sin(q)
mo I
B=
2pr
Who cares about forces on wires carrying current?
Which way does the magnetic field point?
What direction is the force on the left side of the loop?
What direction is the force on the right side of the loop?
S
N
current out
current in
Why doesn’t the current end of the loop generate a force?
S
N
current out
current in
rotation
A DC motor works very similarly but there is a problem
with this motor
S
N
current out
current in
Counter Clockwise
rotation
in
out
After the coil makes ½ a full rotation
S
N
current in
current out
rotation
out
in
Clockwise
How this is solved is by using a commutator
S
N
commutator
The two halves are separated by an insulator
each side connects to a wire.
but not current flows yet….
“Brushes” make the connection from the battery to the
commutator
S
N
dc motor animation
Stationary “brushes” make the connection from the battery to
the commutator and keep the current going in the right direction
S
N
The commutator and brushes keep everything turning
S
N
dc motor animation
What is the direction of the force on the right side of the loop?
Left side?
Top?
I
The loop will experience torque.
I
I
Torque on a loop in a magnetic field.
Works for any shaped flat coil.
t = N I A B Sin(q)
# of loops
Torque (Nm)
Magnetic
Field
Area within loop (m2)
Current (A)
I
I
view of loop from behind it
What is the direction of the forces on the top and bottom
What is the direction of the forces on the top and bottom
torque will be...
What is the direction of the forces on the top and bottom
What is the direction of the forces on the top and bottom
why a commutator is needed
no lever arm
sin(q) = 0
full lever arm
sin(q) = 1
t = N I A B Sin(q)
In terms of the coil and the magnetic field, you would
need to look at a right angle to the coil face.
no lever arm
sin(q) = 0
0o
full lever arm
sin(q) = 1
90o
t = N I A B Sin(q)
This is how a ammeter works
(measures current)also called a galvanometer
5
10
N
0
5
10
S
with no current a spring holds the needle at zero
A galvanometer or ammeter
A spring resists the turning of the coil
When current runs through the wire.
Which way is the current flowing?
5
10
N
0
5
10
S
the electromagnet twists against a spring
because of the torque
When the current increases so does the torque
on the magnet
5
10
N
0
5
10
S
so the spring stretches further.
Why does the needle line up like this?
(think field lines)
A circular coil of wire has a diameter of 20.0 cm.
The current running through the loops is 3.00 A,
and the coil is placed in a uniform 2.00 T magnetic
field. What is the minimum and maximum torque
on the loop?
minimum = 0
maximum = 1.88 N m
Electromagnets can turned on or off.
Just place a ferromagnetic material in a coil of wire
and flow current.
Which end would be the north pole?
electromagnet
bar magnet
A ferromagnetic material inside the coil of wire is
called a core.
It greatly increases the magnetic field strength.
You can think of all the domains lining up inside.
field from coil
alone
B0 = m0nI
field from core
BM = mMnI
magnetic permeability of
the substance
Btot = BM + B
0
material
mu-metal
mM
Tm
A
mM/m0
2.5×10−2
20,000
Steel
8.8×10−4
700
Nickel
1.3×10−4
100
vacuum
1.3×10-6
1
an alloy of Ni, Cu, Fe, Mo
Demo- air solenoid with cenco
low voltage power supply
map field using compasses
draw in a piece of metal
“core”
A solenoid valve
A sheet of metal is connected to a battery. Current flows
due to the difference in voltage between the two ends
-
A little more realistic
-
If the plate of metal is place in an magnetic field,
which way will the electron be pushed as if travels to
the right?
-
As electrons flow to the right, they get pushed
downward creating a negative side and a positive
side.
+ + + + + + + + +
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - - - - - - - - - -
If I connected a wire between the two sides, would
current flow?
Yes
+ + + + + + + + +
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - - - - - - - - - -
Because there is a voltage difference (electric
potential) between the two side
+ + + + + + + + +
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - - - - - - - - - -
The difference in voltage on the two sides is called
the HALL EFFECT.
+ + + + + + + + +
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - - - - - - - - - -
Why don’t all the current electrons all go to the
bottom as they make across?
They repel
+ + + + + + + + +
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - - - - - - - - - -
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
- -
-
- -
-
- -
- -
-
-
-
-
-
+
+
+
+
+
+
+
+
+
-
+
An electron traveling to the
right, is mostly
indistinguishable from a
proton traveling to the left.
Would the plate below be
charged the same as above?
The hall effect first
revealed that it was the
electrons travelling.
The voltage difference max’s out when the
electric force repelling the electrons is equal
to the magnetic force pushing them down.
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Felectric
-
F
Which way does the electric field point?
+ + + + + + + + +
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - - - - - - - - - -
Hall effect sensor can cheaply
measure magnetic fields
Moving charged particles makes a magnetic field.
(electricity makes magnetism)
Can the reverse be true?
ABSOLUTELY!!!
A wire and a magnet moving relative to each produces
voltage or current.
If the grey metal conductor is moved to the right,
which way will the electrons be pushed?
-
-
+
The motion of the conductor through the field
creates a potential / voltage difference / EMF.
Very much like the hall effect.
- +
+
+
+
-
+
+
A very crude representation of charge
distribution but you should get my drift
+
The imbalance of charge max’s out when the
magnetic push is balanced by the electrostatic
repulsion.
-
Fmag
Felectric
v
+
Magnetic force on a
moving charged particle
Electric Force on a
charge particle F, E, q?
F
E=
q
F=qvB
F=qE
q E = q vd B
-
Fmag
Felectric
v
+
E=vB
if we assume a uniform electric field
(a decent approximation here)
The potential difference
between the two ends is the
electric field x the length of
the rod.
V=Ed
V=vBl
Fmag
-
Felectric
l
v
+
e=vBl
Emf / voltage
difference due
to motion (V)
Speed of
motion (m/s)
Length of
metal in field
(m)
As far as I can tell, voltage and emf are the same thing except
that emf ignores any internal resistance.
(I remember being annoyed with this in college physics)
A metal bar slides over a wire,
Why is the direction of current (I) in the wire?
CW or CCW
+
How fast do you need to move 15 meters of
wire perpendicular to a 8.0 tesla field to
generate 120 volts.
If a proton is shot through the field as shown
it experiences an upward force following the right hand rule
N
S
Force
Field
Motion
If a wire is moved so that it moves only in the same direction as before
the charged particles inside feel a force and the mobile ones move
N
S
Force (current)
Field
Motion
The result is electric current
Fingers point to the field,
Thumb points in the direction of the wire’s movement
Your palm gives you the direction of conventional current
If the wire is moved up, what direction is current
N
S
Motion
current
Field
Fingers point to the field,
Thumb points in the direction of the wire’s movement
Your palm gives you the direction of conventional current
If the wire is moved down, what direction is current
N
S
Field
current
Motion
A generator is A LOT like a motor, but you
put work in and get electricity out!!!
Which way will current flow, if the loop is rotated as shown.
S
N
left side of the loop
For the right side of the loop
Motion
Field
Motion
current
current
Field
Which way will current flow, if the loop is rotated as shown.
S
N
The current is picked up by brushes and sent to your home
As the wires trade places….. the current direction will flip
S
N
current
+A
0A
-A
+A
0A
-A
When the wire is moving parallel to the magnetic field, the current is the ZERO
OR the faster a wire “cuts through” magnetic
field lines the greater the current
An AC
generator
Animation of AC or DC
generator
Faraday’s law of induction
(the math behind the scenes of what we have already seen)
Michal Faraday
(1791 – 1867)
The 3rd child of a blacksmith was “given”
the most basic education…. the rest he
worked for having been born lower class.
He discovered relationships the really made
electricity useful on a wide scale basis as
well as many discoveries in chemistry.
The unit of capacitance is named for him.
1st off the concept of flux.
It is the rate of flow
through a boundary.
Think of the arrows as
water…
Which ring would get
the most water flowing
through it
Phi is the variable for flux
(the greek f )
F
Magnetic flux is the amount of magnetic field “flowing”
through something (like a loop wire)
FB
Magnetic flux
B
FB = B A Cos (q)
Magnetic
flux
Magnetic
field (T)
Cos (q) = 1
B
Area of
“boundary”
(m2)
Field to loop
orientation
Cos (q) = 0
B
What is the magnetic flux through a loop of wire forming a
radius of 10 cm in an magnetic field of 3 T if it is at an angle
of 45o to the field?
B
Don’t write this equation down yet
e=
DFB
Dt
Voltage is proportional to the rate of change in
magnetic flux vs. time. (the rate at which the wire cuts through
field lines)_
B
B
Flux
Rotated
by 10o
Flux
Where during the spin is the greatest
rate of change of flux
Rotated
by 10o
As the ring rotates around at a constant
rate, the rate of change of flux is not
constant.
The easiest way is again, the
faster the wire cuts through
field lines the more current /
voltage is produced.
Faraday’s law of induction
e=
Induced EMF
(Volts)
DFB
-N
Dt
Number
of loops
Rate of
flux
change
The negative sign has to do with Lenz’s law which we will see coming up
A loop with a radius of 7.0 cm is initially
perpendicular to a 4.0 tesla field. It is rotated
such that it is parallel with the field in 0.01
seconds. Determine the average emf in the coil.
If there had been 10 loops?
If it had rotated in 0.1 s?
remember
e = v B l sin (q)
Emf produced by a wire
moving in a magnetic field
e = 2NBlv sin(q)
distance
from a to b
number
of loops
v
q
b
NN
a
S
v
Instead of moving the wire, you can move the magnet.
Which direction of motion would generate the most current
or
N
S
What kind of motion of the magnet would generate the most
current
N
S
A magnet is brought closer to a loop of wire
which way does the current flow in the loop?
1st draw the magnetic field
2nd pick a point that is easy to visualize direction of the field
S
N
We haven’t learned a right hand rule for magnets moving but we
have learned one for moving wires/charged particles….
The magnet moving to the right would be JUST like
the coil moving to the….
S
N
current
Field
Motion
S
Current is into the page
N
As the magnet move back to the left it would be
like the coil moving to the…..
S
N
And current will flow…
Current reverses!!!
Field
Current is out of the page
Motion
current
S
N
The greater the length of wire in the changing magnetic field, the
greater the current.
A simple way to do this is to coil the wire.
The more coils, the more effective the “generator” is.
This is how a “shaker” light works. A decent one will have a
lot of coils
The field lines “cut” through the wire as the magnet moves
A CHANGING MAGNETIC FIELD
PRODUCES CURRENT
This is called electromagnetic induction
A stationary magnet and wire produce NO Voltage or CURRENT
“Shaker” light.
A generator produces electric voltage &
current which means it makes energy?
NO of course,
Energy is not created or destroyed
it just changes forms.
A generator converts energy usually
supplied by coal/gasoline into electrical
energy.
Phet generator applet
It makes sense that moving the magnet faster
(more KE), produces more electrical energy.
Slower
Faster
More Energy IN = More Energy OUT
Moving a magnet at the same speed through more coils also
produces more energy
HOW does that make sense with conservation of energy?
It is harder to push a magnet through the middle of more coils
Work (energy) = Force x Distance
Pushing & Pulling the magnet, produces an electric current.
A current flowing through a
wire produces a…..
Magnetic Field
LENZ’s Law:
When a moving magnetic field creates a current,
that current produces a magnetic field which
opposes the motion of the magnet that created it.
Lenz’s law demo’s
Magnetic Aluminum coins?
Slowing down gravity?
Lenz’s Pendulum
A wire moves down through the magnetic field.
In what direction will current flow?
What direction will the magnetic field be around the wire?
B
The more current produced, the harder you have to push
the magnetic to move it because the lenz magnetic field is
stronger
velocity
force from eddy current magnetic field
Magnet
A magnet produces eddy currents when
moving near a conductor.
These eddy currents produce a
magnet field which slows the magnet
Applications of Lenz’s law
What slows you down on
Six Flags Superman Ride
You know how long it takes a triple
beam balance to stop moving?
Not if….
A vacuum motor consists of just a lot wire coiled and
with each end connected to 120 V outlet.
What do you think would happen if you took lets say
100 feet of wire and put the ends into an outlet?
How does the wire in a motor not short out the circuit
or burn up the wire
Even 200 or 300 feet of copper would only have a
resistance of a few ohms meaning ~ 60 Amps of
current.
Needed for 60 A
Motor loop wire
The current through the loops is impeded
not by the resistance of the wire but by a
backwards voltage/emf created by the
rotation of the loop.
Explanation follows……
Picture a simple motor (commutator omitted for clarity)
S
N
current out
current in
The magnetic field will create torque on the current carrying
loop of wire as before
BUT Don’t forget the wire is rotating
For a second ignore the direction that the “protons” are moving in the
wire due to current flow. Look at the direction a “proton” is moving
movement of the now rotating wire with the wire.
On the right side
S
Motion
N
Force
Field
On the left side
Field
Force
Motion
What direction would the force
be on the proton?
The rotation of the wire, creates a force a force
which opposes the current in the wire.
Motion
S
Force
N
current
current out
current in
current
Field
Force
Motion
Field
The force pushing back current is called Back EMF.
It acts like resistance in the wire (since it is slows current).
Motion
S
Force
N
current
current out
current in
current
Field
Force
Motion
Field
S
N
When the motor first start
up, the coil is not rotating.
There is no back EMF and
current is briefly very
BIG.
Motion
Back
EMF
current
S
N
As the coil speeds up,
back EMF increases and
current slows.
Motion
Back
EMF
current
S
N
Eventually the motor
reaches a speed where the
back EMF balances out
with the voltage driving
current and friction
etc…and the motor
maintains a constant
speed.
Motion
Back
EMF
current
S
N
Now the motor starts to
lift something heavy,
which causes its
Speed to …
Back EMF…
Current to…
Motion
Back
EMF
current
Why will a motor that gets
jammed trip a breaker?
The amount of back EMF is proportional to the speed at
which the motor rotates
Motion
back EMF
Field
Motor starts up, no movement
initially. No motion or back EMF.
Large current.
current
Motion
back EMF
Field
current
Motion
back EMF
Motor spins up. Speed and back
EMF Increase. Current Decreases
Field
current
Under no load the motor will
reach a maximum speed, when
the Back EMF reduces the
current to just overcome Friction
and Heat Loss
Motion
Max Speed no load (no work)
back EMF
Field
current
Motion
back EMF
Field
current
When the motor does work
against a load. It slows down.
Back EMF drops, current goes
up and it does work.
This is how a motor “knows” how hard to push (within limits)
When a the current is shut off (current is cut)
Motion
back EMF
Field
Back EMF will flow current the
backwards as it continues to
rotate.
current
When a motor is shut “off” its rotational momentum keeps it
moving for a time and it acts like a generator.
The current produced can back feed current to any device
connected. This can be bad.
Motors and generators are the same
device just used differently
BMW regenerative
braking video clip
Faraday’s switch applet
Current is produced in a wire only when the
magnetic field around the wire CHANGES!
Field lines cut through it
Faraday applet
Transformer change the voltage of electricity
but not for free
Current
is fed to
this side
A current is
produced on
this side
When a current flows through the primary coil
wrapped around a core it acts like an electromagnet.
This causes the entire core to become magnetized.
Current in
Which way will the field lines point?
If the current is constant, they core will
remain strongly magnetized
Current in
NONE!
Will the secondary coil have more, less,
or the same amount of current as the
primary?
Remember:
a stationary magnet (constant magnetic
field) and wire produce NO Voltage or
CURRENT
In order to make current, the magnetic field
has to change. Like moving the magnet in
and out of the coil
This is like holding a stationary magnet
in the secondary coil.
Current in
Even a strong magnet
makes no current
IF it does not move
This VEXED early scientists trying to
produce current in the other coil.
UNTIL…...
Current in
No current
They noticed right when the switch shut off the primary current.
xxx
A light would light!
xxx
But only for a split second.
xxx
But when the switch was opened again…..
xxx
the light lit again.
xxx
The light would only light RIGHT
when the switch was opened or closed
xxx
To keep it lit, the switch must be
OPEN
xxx
To keep it lit, the switch must be
CLOSED
xxx
To keep it lit, the switch must be
OPEN
xxx
To keep it lit, the switch must be
CLOSED
xxx
To keep it lit, the switch must be
OPEN
xxx
To keep it lit, the switch must be
CLOSED
xxx
Because it is not a magnetic field that makes current in a wire
but a magnetic field that is….
Moving or changing.
Important:
A magnet and wire only make electricity if they
are moving relative to each other.
a magnet sitting on top of a wire doesn’t
make your toaster work. DUH...
IT WORKS
xxx
If only there was a way to get electricity that
moved back and forth on its own!!!
xxx
Like alternating current!!!!!!!!!!!!!!!!!!!!!
+A
0A
-A
The current jiggles back and forth automatically!!!
Early inventors only had batteries, which made DC
xxx
Steady current, constant magnet = No light
DRATS!@!
xxx
From AC Generator
AC does the work for
me this is great!!
I’m getting paid for nothing.
But who is paying me?
NO ONE…..
I’m sorry do you want fries with that?
You are getting ready to
go to college where you
will be FRESHMEN
again. Keep working…..
And don’t do anything
stupid with your new
found freedom!
e = BLv
The magnetic field strength is the same
More wire means….
e = BLv
The magnetic field strength is the same
More wire means….
More wire = More Voltage
More coils in the secondary means the voltage is increased
Less coils in the secondary means the voltage is decreased
A typical power “grid” why bother with the voltage increase????
240,000 V
6,000 V
8000 V
120V
Steps down from
240,000 V to 8000 V
Steps down from
8000 V to 240 V
Nin Nout
=
Vin Vout
A primary coil of 500 loops fed from household current is
connected via a transformer to 38 loops. What is the
output voltage?
BUT no escaping the :
Law of conservation of energy
Which means:
energy in = energy out
Time
Time
power in = power out
For electrical energy:
Power = current x voltage
P = IV
P = IV
power in = power out
IinVin = IoutVout
When voltage is decreased….
current…..
power…..
High voltage = less current for the same energy transferred.
-minus the energy lost in the tranformers
A 12V transformer delivers 1.5 W, how
much current is drawn from the 120V
outlet it is connected to?
How does this power-stealing device work?
Next set of slides are already done
delete next year.
15 A of current flows through both wires below. If they are seperated
by a distance of 30 cm. What will the magnetic field strength be
midway between the?
I
I
Faraday’s law of induction
e=
Induced EMF
(Volts)
DFB
-N
Dt
Number
of loops
Rate of
flux
change
The negative sign has to do with Lenz’s law which we will see coming up
A loop with a radius of 7.0 cm is initially
perpendicular to a 4.0 Tesla field. It is rotated
such that it is parallel with the field in 0.01
seconds. Determine the average emf in the coil.
If there had been 10 loops?
If it had rotated in 0.1 s?
A proton is shot into a magnetic field.
In what direction with the magnetic force be?
In what direction should an electric field be set to keep it moving straight?
FB
FE
+
v
E
E
How about here with an electron?
Which way should the electric field point?
FB
FE
A charge like an electron
produces an electric field
e-
Moving an electron produces changing electric field.
( only 1 field line shown for clarity)
Moving charge applet
animation
This moving ELECTRIC field
also moves which creates a...
e-
Magnetic FIELD….
This moving magnetic field also moves which creates a.
e-
ELECTRIC FIELD….
Which makes a….
magnetic field which makes an
electric field which makes a
magnetic field which makes an
electric field which makes a
magnetic field which makes an
electric field which makes a
magnetic field which makes an
electric field which makes a etc………...
An oscillating charge produces and electromagnetic wave
Electromagnetic wave applet
Electromagnetic wave applet
All that is needed to produce an EM wave is some electrons
moving back in forth in a conductor
AC
SOURCE
This type of device is called an…
Antenna
If another antenna is near
AC
SOURCE
It electrons will tend to oscillate too...
AC
SOURCE
oscillating electric charge applet
If a meter is attached, AC current would be detected.
AC
SOURCE
oscillating electric charge applet
Some people in the late 1800’s people thought that the wires
that delivered electricity to houses would be gone soon…
That didn’t work out
Nikola Tesla had this crazy
idea in the early 1900’s that
this could be used to send
information without wires.
"As soon as [the Wardenclyffe plant is] completed, it will be
possible for a business man in New York to dictate instructions,
and have them instantly appear in type at his office in London or
elsewhere. He will be able to call up, from his desk, and talk to
any telephone subscriber on the globe, without any change
whatever in the existing equipment.. . . ." — Nikola Tesla
Sending information in the EM waves did however catch on
a bit...
An antenna is used for sending and receiving a signal.
The optimum length of the antenna depends on the
wavelength being sent
How information is sent in a wave.
Miscellaneous
not used stuff
Placing ferromagnetic material in the center of the coils makes it
even stronger. Iron is called a CORE.
Soft iron is used because its domains don’t stay aligned after
the current is turned off. Steel would remain magnetized.
The direction of the magnetic field can be found by curling your fingers
(right hand) following the current.
Your thumb shows the direction of the field (pointing to N)
If the current was reversed. The poles would switch.
What are the poles?
S
N
The magnet on the boom of this crane can be turned on and off.
A quick note about
“conventional current”
Vs
“actual current”
the rules we deal with upcoming are based
on conventional current
The magnetic force is due to the electron moving through
a magnetic field. so
F = qvB sin(q)
Fmag = qe vd B
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-V - drift
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F
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F
E=
q
F=qE
Fe = q e E
Fmag = qe vd B
qe E = q e vd B
E = vd B
Kind of like parallel plates so we can use…
V= Ed
+ + + + + + + + +
d
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E = vd B
V=Ed
The book uses “l”
instead of d
VHall = vd B l
+ + + + + + + + +
d
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