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JOURNAL #15

2/22/12
Use what you learned in yesterday’s lab to
predict which picture shows magnetized atoms
and which shows non-magnetized atoms.
ELECTROMAGNETISM
Ch. 24-25
Quiz – Formulas and Vocabulary – Wednesday, 2/29
Test – Ch. 24-25 – Friday, March 2
MAGNETS
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Magnets have been around for more
than two thousand years.
One of the earliest uses for magnets was
in navigation by using compasses.
Other uses include electric motors,
generators, television sets,
computer hard drives, and
high speed trains.
BAR MAGNETS
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The north and south side represent to two poles of
the bar magnet.
Bar magnets are examples of permanent magnets.
All magnets are polarized, which mean they have
two distinct and opposite ends.
Opposite poles attract while similar poles repel.
Why?
MAGNETIC FIELDS
The reason magnets attract and repel each other
are due to the magnetic fields around them.
 Every magnet has a magnetic field around it.
 Magnetic field lines are similar to electric field
lines.
 Field lines start at the north pole and end at the
south pole.
 Magnetic field strength has units of Teslas (T).

MAGNETIC DOMAINS
What happens if you break a magnet in half?
 The two halves will still be polarized!!!
 A domain is a group of neighboring atoms that
have the magnetic fields of their electrons
aligned in the same direction.
 Magnetic materials have their domains aligned
in the same direction, while non-magnetic
materials have domains in random directions!

Non-magnetic
Magnetic
ELECTROMAGNETISM
Electromagnetism describes the relationship
between electricity and magnetism.
 We use electromagnets to generate electricity,
store memory on our computers, generate
pictures on a television screen, diagnose
illnesses, and in just about every other aspect of
our lives that depends on electricity.
 Electromagnetism works on the principle that an
electric current through a wire generates a
magnetic field.

ELECTROMAGNETS
When DC electricity is passed
through a wire, a magnetic field
rotates around the wire in a
specific direction.
 “Right Hand Rule”-If you take
your right hand and wrap it
around the wire, with your
thumb pointing in the direction
of the electrical current (positive
to negative), then your fingers
are pointing in the direction of
the magnetic field around the
wire.
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SOLENOIDS
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How would we model the
magnetic field in a
current loop?
A solenoid is a long coil
of wire consisting of
many loops.
The more loops a
solenoid has, the
stronger the magnetic
field.
MAGNETISM CONVENTIONS

Current:

Current is represented by the letter I
Left, right, up, and down are drawn by arrows
Into the page or away from you is represented by

Out of the page or towards you is represented by


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Magnetic Field:
Clockwise and counter-clockwise are drawn by
arrows
 Into the page or away from you is represented by


Out of the page or towards you is represented by
RIGHT HAND RULE PRACTICE

Use the right hand rule to draw the magnetic
field around the wire shown below
I
Current to the right
Magnetic field towards you above the wire
and away from you below the wire
RIGHT HAND RULE PRACTICE

Use the right hand rule to draw the magnetic
field around the wires shown below
I
I
Current out of the page
Current into the page
Counterclockwise magnetic
field
Clockwise magnetic field
RIGHT HAND RULE PRACTICE

Use the right hand rule to draw the magnetic
field through the wire loops as shown below
I
Current clockwise
Magnetic field towards you outside the
loop and away from you inside the loop
HOMEWORK

Textbook p.665 #59, 60, 61, 64, 65
JOURNAL #16

2/23/12
Draw the magnetic field around the following 3
wires:
A)
B)
I
I
C)
I
ELECTROMAGNETS
This magnetic field is the same force that makes
metal objects stick to permanent magnets.
 In a bar magnet, the magnetic field runs from the
north to the south pole.
 In a wire, the magnetic field forms around the
wire. If we wrap that wire around a metal object,
we can often magnetize that object. In this way,
we can create an electromagnet.

ELECTROMAGNETIC INDUCTION
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When a wire moves in a magnetic field, a force
acts on the charges in the wire.
Work is done on the charges, causing the
electrons to move.
Magnetic fields can induce a current in a wire!
For EM induction to occur, the wire or the
magnetic field needs to be moving or changing in
strength.
DC MOTORS
WHAT IS THE FUNCTION OF THE SPLITRING COMMUTATOR?
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If you look closely, you will see that it switches
the flow of current half-way through the rotation
of the armature.
By doing so, it reverses the magnetic field
produced around the armature. This reversal at
the exact right moment makes the armature
continue to be pushed/pulled by the forces of the
magnets.
The armature is a wire loop mounted on an axle
(it can be a single loop, or many repetitive loops).
GALVANOMETERS
A galvanometer is a device that uses electromagnetism
to measure very small amounts of current ( ~50 x 10-6 A)
 The current through the wire loop creates a magnetic
field
 The interaction between the loop’s magnetic field and the
permanent magnetic field causes the loop to rotate.

GALVANOMETER

Galvanometer - the historical name given to a moving coil
electric current detector. When a current is passed through
a coil in a magnetic field, the coil experiences a torque
proportional to the current. If the coil's movement is
opposed by a coil spring, then the amount of deflection of a
needle attached to the coil may be proportional to the
current passing through the coil. Such "meter movements"
were at the heart of the moving coil meters such as
voltmeters and ammeters until they were largely replaced
with solid state meters which have digital readouts.
ELECTRIC GENERATORS
The electric generator was invented by Michael
Faraday and converts mechanical energy into
electrical energy.
 The armature is often wrapped around an iron
core.
 The armature is free to rotate in the magnetic
field. As the wire rotates in the magnetic field, a
current is induced.
 As the loop rotates, the strength and direction of
the current change, producing an alternating
current.

DIFFERENCE IN AC/DC CURRENT
The difference between AC and DC has to do
with the direction in which the electrons flow.
 In DC, the electrons flow steadily in a single
direction, or "forward."
 In AC, electrons keep switching directions,
sometimes going "forward" and then going
"backward.”
 In the US, electric utilities use a 60-Hz
frequency, meaning that the current alternates
direction (forward to backward and back to
forward) 60 times in one second.

TRANSFORMERS
The primary voltage (on the
left) induces a magnetic field
in the core, which creates
the secondary voltage (on
the right).
 What makes transformers so
useful is that if you change
the number of turns from
one side to the other, you
change the voltage in the
wire on the right!
 Transformers can change a
high voltage to a lower one,
or a low voltage to a higher
one.
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STEP UP TRANSFORMER
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Step-up transformer converts a low voltage to a higher
one.
If you increase the number of turns on the right, the
voltage coming off the transformer will increase in
proportion.
The right side has 4 times more turns so the voltage on the
right has increased 4 times.
So the voltage has been stepped up by a factor of 4.
STEP DOWN TRANSFORMER
Step-down transformer reduces voltage.
 If you decrease the number of turns on the right, the
voltage coming off the transformer will decrease in
proportion.
 The right side has 1/5 the number of turns, so the voltage
is only 1/5 as large.
 So the voltage has been stepped down by 5.
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TRANSFORMER MATH
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The ratio of the number of turns is the same as the ratio
of the voltages
Vs N s

Vp N p
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An ideal transformer, which we will always assume,
dissipates no power (waste heat). The power of the
primary circuit is equal to the power of the secondary
current
Pp  Ps
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We use this assumption about power to find the current
in the secondary circuit
I pV p  I sVs
FINAL THOUGHTS ON TRANSFORMERS
Step-Up Transformer
Step-Down Transformer
Vp < Vs
Vp > Vs
Ip > I s
Ip < Is
Np < Ns
Np > Ns
WHY TRANSMIT WITH AC VS. DC
Advantages to using AC:
Transformers can only work using AC b/c the
magnetic field has to be constantly changing in
order to induce the secondary current. Due to the
constant change of direction of the electrons in
AC, the magnetic field is always changing as
well.

By using a step-up transformer to hike the voltage,
we are able to lower the current and by lowering
current, we are able to use thinner wires.
 Ultimately, we are able to transfer power farther
(higher voltage travels farther), for less money (cost
of wire), with better efficiency (less power lost to heat
caused by resistance).
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JOURNAL #17

2/27/12
The primary coil on a transformer has 100 turns
and the secondary coil has 500 turns. The
primary voltage is 110V and the current is 1.5 A.
What type of transformer is this?
 What power is being used on the primary coil?
 What are the resulting voltage and current in the
secondary coil?
 What is the resulting power on the secondary coil?
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PRE-LAB: ELECTRIC MOTORS
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Purpose of Lab:
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Create a functional motor and modify it’s components
to change it’s ability.
Today you will write down the function of each
piece of equipment so that you don’t have to ask
questions about them tomorrow.
EQUIPMENT
The commutator discs have
The
magnets
must be
different
numbers
of
arranged
in
the
rotor
in
a
alternating clear/black edges
specific pattern in order for
the electromagnet
to
A photogate
will be placed
in
Thebatteries
electromagnet
4 D-cell
effectively
“pushsoand
it
this position
thatpull”
it can
has potential
a sensor on the
provide
measurecontinuously
the frequency of color
end that
detects
difference
to the
changes. That information
or clear and
circuitdark
containing
will be used to calculate the
reverses the current
the electromagnet
rotational speed of the
alternating it’s
The rotor is freely spinning on ball bearings,
disc/motor
polarity
but the commutator discs must be firmly
attached with the washer so that they don’t
shift
JOURNAL #18

2/28/12
The ratio of turns on a transformer is 100 on the
primary to 1 on the secondary coil. The primary
voltage is 12000V.

What type of transformer is this?

If the resistance on the secondary coil is 100-ohms,
what current is induced on the coil?

If we consider this to be an ideal transformer, what
power is being used on the primary coil? How do you
know?