Download The Motor Effect.

Moving Charges
Produce Magnetic Fields
Magnetic Materials
Magnetic Materials
Ferromagnetic materials (ferrum – iron) have strong magnetic
properties. Loadstone contains magnetite which contains iron.
Magnetite is an oxide of iron and has the composition Fe3O4.
Magnetizing Materials
Magnetic Domains become aligned in External Magnetic Field.
B
Magnets
• a magnet is electrically neutral
• a magnet has two poles:
north & south
• like poles repel
• unlike poles attract
• No Magnetic Monopoles!
Smallest Magnet:
Electron
Atomic Sources of Magnetic Fields
B: The Magnetic Field
Magnetic Field Lines point from North (+) to South (-)
Use a compass to map out the field lines.
How does a compass work?
A compass is a small magnet.
Outside Field lines point from North to South.
A compass is a small magnet.
Inside the Magnet, the Field points from South to North.
Which way will the magnetic rotate?
N
N
S
S
External B Field
Like Poles repel, Unlike Poles Attract.
Which way will the magnetic rotate?
S
N
S
N
External B Field
Magnets align themselves so that their internal field is
aligned with the external B field.
Magnetic Compass
North seeking compass points to Earth’s Magnetic North which
is really a South Pole. A compass’ arrow head is the north pole.
Some typical B fields:
surface of the Earth
small bar magnet
MRI
magnet lab
surface of neutron star
10-4 T
10-2 T
2-3 T
30 T
108 T
Currents
Produce Magnetic Fields
Currents
Produce Magnetic Fields
Electromagnets:
Making Magnetic Fields with Electricity
Multiple wraps of a wire is a solenoid.
It produces a strong magnetic field inside the coil.
Magnetic Force
A charged particle moving in an EXTERNAL magnetic field
experiences a force that is perpendicular to both the velocity and field.
Only the perpendicular components give rise to a force.
Electric & Magnetic Force
The Electric Force acts
parallel to the Electric Field.
(Electric Field goes from + to -)
The Magnetic Force acts
perpendicular to the Magnetic
Field and the velocity.
(Magnetic Field goes from N to S)
Cathode Ray Tube
Electric Force makes the Electron gun.
Magnetic Force directs the beam.
Cathode TV has 3
electron guns, one
for each color
RGB which scan
525 times in 1/30
of a second.
Magnetic Force: Centripetal Force
The magnetic force is perpendicular to BOTH the
velocity of the charged particle and the magnetic
field, producing circular motion!
F
F
E
Magnetic Force
The Mass Spectrometer
The mass spectrometer is a very important instrument which can
measure the masses and relative concentrations of atoms and molecules.
Question: Three particles with the same charge
move through a constant Magnetic Field.
Which particle has the greatest mass? Least mass?
1>3>2
Charged Particle in a Magnetic Field
Magnetic Force: Centripetal Force
Earth’s Magnetic Field
The Earth’s magnetic field is caused by
movements in Earth’s outer core
The Earth’s magnetic field shield us
from the stream of charged particles
from the Sun called the solar wind
Auroras
The colors are caused by energetic electrons colliding with
oxygen and nitrogen molecules in the atmosphere. This excites
the molecules, and when they decay from the excited states they
emit the light that we see in the aurora.
Aurora
surrounding
the north
geomagnetic
pole as seen
from space
Aurora as seen from space
Magnetic north
(actually the location
of the south pole of
the magnet) is in
Hudson Bay some
1300 km from
geographic north. It
has moved about
700 km since 1904!!
The Motor Effect
An electric current in a
magnetic field
experiences a force that
is perpendicular to both
the direction of current
and the direction of the
field. If that current is
bent into the shape of a
loop, the magnetic force
produces a torque that
causes the loop to rotate.
This is the basis of
The Motor Effect.
The Motor Effect: Meters
Galvanometer: measuring currents and voltage
Electromagnetic Induction
A Changing Magnetic Flux Induces a Voltage &
Current in a Coil
The greater the number of turns, the greater the voltage.
The faster the CHANGE in flux the greater the voltage.
?Magnetic Flux?
Flux: how much Magnetic Field flows through loop.
B
Maximum
Flux
θ=0
Minimum
Flux
θ = 90
Maximum
Flux
θ = 180
TRY THIS!
What Happens?
•Move the north (red) end in and out of the coil. As you
move it in, does the meter read + or -? As you move it out?
•Now move the south end in and out of the coil? How do
the signs on the meter read?
•Move the magnet into the coil and leave it there. ????
•Now do the above over again but this time move the coil
instead of the magnet! What happens? Rotate the coil. Can
you induce a current as you rotate the coil around?
Electromagnetic Induction
The Generator Effect
A magnet moving in a
stationary coil induces a
voltage and current.
You could also move the coil in a
stationary magnet instead! This will also
generate an ac current.This is the basis
of an ac generator!
Electromagnetic Induction
The Generator Effect
A changing magnetic flux induces a voltage and an ac current.
In the US the ac current has a frequency of 60 hertz.
Question:
If the current is going on and off, why don’t we see the lights flicker?
Sources of Electrical Energy
One of these is not like the others?
One of these things doesn’t belong!
Which one and why?
Solar doesn’t use a EM induction!!
Polluting Power Production:
Coal, Nuclear, Oil or Gas
It all comes down to making
HOT WATER & STEAM
to turn a turbine!
The Power Grid
Typical power plant can generate MegaWatts of Energy.
US consumption of electricity is 90 Quadrillion BTUs/year.
1 quad = 1015 Btu = 2.931 x 1011 kW-hrs
Hetch Hetchy: 2 billion kW-hrs per year
Wind in CA: 4.5 million kW-hrs per year
Power Transmission: Transformers
Step up Voltage & Step Down Current to reduce Power loss due to
heat loss from resistance in the wires.
Transformers: Change the Voltage
9V Battery
Recharger
Step Down
Decrease Voltage
Increase Current
Step UP
Increase Voltage
Decrease Current
120V
15,000 V
Transformers:
Auto Ignition
Converting 12V to 40,000V
EM Inductance:Transformers
The AC current is changing in Coil 1, producing a changing Magnetic
Flux in Coil 2, which induces a voltage and current. Since Coil 2 has
fewer wraps than Coil 1 it will have a smaller Voltage than Coil 1.
Primary Voltage
Secondary Voltage
=
Number of Primary Turns Number of Secondary Turns
EM Inductance:Transformers
Primary Voltage
Secondary Voltage
=
Number of Primary Turns Number of Secondary Turns
If 100 V of dc are put across a 100-turn transformer primary, what will
be the voltage output if the secondary has 200 turns?
a) 200 V
b) 100 V
c) 50 V It has to be AC current to work so there is a changing
magnetic flux through the coil!!!
d) 0V
If 100 V of ac are put across a 100-turn transformer primary, what will
be the voltage output if the secondary has 200 turns?
a) 200 V
N2
200
V2 =
xV1 =
b) 100 V
x100V = 200V
N1
100
c) 50 V
d) 0V
Motors & Generators
Inverse Effects!
Motor Effect
Magnetic Force on
Moving Charges in
Loop: Motor Effect
Generator Effect
Magnetic Field
Changing through
Loop: Generator Effect