Download Magnets Induction 2017

9Name
Class
Date given
Date due
Magnets & Induction
Student Expectations/Self assessment
PHYS 5: know changes occur within a physical system and recognizes that energy
and momentum are conserved
I can:
 Be able to identify the source of all magnetism
o Ex. What makes iron magnetic?
o Initial understanding
A B C D F
Final understanding
A B C D F
 Name the factors that affect electromagnetism and how they affect the magnetic field strength
o Ex. Name at least two factors that affect the strength of a magnetic field around a wire
carrying a current.
o Initial understanding
Final understanding
A B C D F
A B C D F
 Describe the earth’s magnetic field, how a compass can be used to navigate, and give examples of the effects or
applications of magnetism in nature, industry, and science
o Ex. If a bar magnet were visible in the planet, which end of the magnet would be in the North
pole?
o Initial understanding
Final understanding
A B C D F
A B C D F
 Use a right-hand rule to predict the direction of a magnetic field around a current-carrying wire and the nature (attract or
repel) of the forces between 2 current-carrying wires
o Ex. If a wire is carrying a current from west to east, does the current go over the top towards
you or away from you?
o Initial understanding
Final understanding
A B C D F
A B C D F
 Identify the factors that affect the force on a current-carrying wire placed in a magnetic field, calculate that force, and
use a right-hand rule to predict its direction.
o Ex. What is the velocity of a proton if the magnetic field is 0.0003T South and the force is 3
x 10-16 down towards the Earth?
o Initial understanding
Final understanding
A B C D F
A B C D F
 Identify the factors that affect the force on a charge moving through a magnetic field, calculate that force, and use a
right-hand rule to predict its direction. Describe the path taken by such a charge
o Ex. What are the units for current, voltage and resistance? Give a proper formula to express
the relationship between them.
o Initial understanding
Final understanding
A B C D F
A B C D F
 Calculate the EMF (electromotive force) induced, calculate the current induced by that EMF, and use a right-hand rule
to predict their directions.
Ch 21 & 22: Magnets and Induction
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o Ex. If force is up and the magnetic field is away in front of you, which direction will a charge
move?
o Initial understanding
Final understanding
A B C D F
A B C D F
 Be able to compare and contrast motors and generators, tell how a galvanometer works
o Ex. How does a galvanometer work?
o Initial understanding
A B C D F
Final understanding
A B C D F
 Describe how and why a transformer works and calculate voltage, current, and power for transformer coils
o Ex. How does a transformer work?
o Initial understanding
A B C D F
Final understanding
A B C D F
Chapter 21&22 magnets & induction assignments.
Please check grade book for due dates.
Magnetic Explorations
p775 #1 – 5; p778 #1-4
test
p769 1 – 3, 5; p772 1,2
p800 #1 – 3
Magnetic material
There are three elements that can be magnetically induced:_________, _________,_________.
These elements are called ferromagnetic.
__________ and ____________ are magnetically hard, meaning they are hard to magnetize but
once they are, they stay that way easily.
____________ is magnetically soft – easy to induce, easy to lose magnetism by heating,
cooling, or hammering.
Make a magnet and compass:
http://adventure.howstuffworks.com/outdoor-activities/hiking/compass2.htm
Materials – needle, magnet, small piece of foam, weigh boat, water, compass
1. Stroke the needle with the SOUTH end of the magnet at least 50 times ALWAYS from
eye to point.
2. Carefully (remember, it is a needle!) push the needle through a small piece of foam so the
foam is in the middle of the needle and float the combo in the water.
3. Let the needle come to rest.
4. Does the point go towards the north or the south?
5. What happens when you bring the original magnet close to your compass?
Where does magnetism come from in the first place? _______________________________.
Ch 21 & 22: Magnets and Induction
2
A compass is a magnet suspended in the middle, usually different colors on different ends of
halves. A bar magnet is the same thing but bigger. Horseshoe magnets and flat magnet are
different formations of the same thing. Regardless of shape, like ends ____________ and
opposite ends _______________.
Magnetic fields
Magnetic Beards
Materials – magnet toy with iron shavings, various magnets
1. Put a magnet on a on top of or under a magnetic shavings toy
2. Gently shake the shavings around to see how the shavings respond to the magnet.
3. Try different shaped magnets but especially use a bar magnet.
4. Draw what you see when using the bar magnet.
List ways Magnetic fields are like electric fields.
The direction of a magnetic field at any location is the direction the NORTH end of a
compass needle points.
Since a compass is a bar magnet and opposites attract, when the north end of the compass points
to the geographic North, it’s really pointing to the ____________ magnetic pole of the Earth.
So field lines actually go from magnetically North to South – follow same rules as electrical
field lines (geographically south to north).
Drawing a magnetic field: +x, -x, +y, -y, +z, -z
Means the field is _______________
Means the field is _______________
Means the field is _______________
Electromagnetism:
When a current goes through a wire, a magnetic field is induced in concentric circles going
around the wire. Which direction does the circle go?
Ch 21 & 22: Magnets and Induction
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Right hand rule for field direction: Imagine you grab the wire with your right hand and your
thumb in the direction of the current. Your fingers will point in the direction of the magnetic
field.
Thumb points
direction of
current.
Fingers curl in
direction of
magnetic field
Draw fields around the wires. The dot means the current is coming out of the page. The X
means the current is going into the page.
As current goes up, the magnetic field strength goes _____.
As distance from the wire goes up, the magnetic field strength goes _______.
What about a coil of wire?
Ch 21 & 22: Magnets and Induction
4
Wire coils:
1. Bend a pipe cleaner into a one layer coil that just barely overlaps itself.
2. Using the right hand rule and assuming the current to go counterclockwise, determine the
magnetic field all the way around the coil.
3. Draw your results below.
Redraw the field around a bar
magnet
N
S
A solenoid or electromagnet, is based on the idea that a coil of wire carrying a current produces
a magnetic field going in one direction. The magnetic field increases with the number of coils
and when an iron rod is inserted in the center of the coil. Use the right hand rule and assume the
current to go counterclockwise.
Build an electromagnet:
Materials – wires, iron nails, batteries, battery holder, straight pins and compass for testing
1. Goal – prove you have created an electromagnet
2. Sketch what you made
Ch 21 & 22: Magnets and Induction
5
Magnetic Field Force
When a charge, q, moves through a magnetic field, it experiences a force.
A force is _________________________________
Fmagnetic = qvBsinθ
Variable
Fmagnetic
B
q
v
sinθ
Name
Unit
the magnetic force on a charged particle
magnetic field
T (tesla)
magnitude of the charge
speed of charge
(angle between v and B; sin90° = )
How do you know which direction the charged particle is being moved?
Right hand rule for magnetic force: USE LEFT HAND FOR ELECTRON MOVEMENT
hold right hand out flat
This is
palm faces direction of force, Fmagnetic
for
Fingers point direction of magnetic field, B
positive
Thumb points direction of velocity (direction) of charged particle, v of q
charges
only
(this includes a charge going through a wire)
Practice:
1. Which direction will a charged particle move if the magnetic field direction is straight
ahead of you / –z and the force is straight up / +y?
2. A proton moves under a force of 2 x 10-18 N down. The magnetic field is 5.2 x 10-5T
South at this point. How fast is the proton moving?
3. Which direction is the proton in #2 moving?
4. A 1.5T magnetic field points north. An electron moves vertically towards the ground at a
speed of 2.5 x 107m/s. What is the force causing this motion?
Ch 21 & 22: Magnets and Induction
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Wires experience a force when in a magnetic
field or when next to each other.
Earth’s magnetic field strength = 5 x 10-5 T
(roughly, but it varies with locationand it goes
from ______________________ to
_________________________
Fmagnetic = BIlsinθ
Variable
Fmagnetic
B
I
l
sinθ
Name
Unit
(angle between l and B; sin90° = )
Force between 2 current-carrying wires
The magnetic field produced by wires can interact with each other creating a force acting on
each other, too.
• When a current flows through a wire a magnetic field is produced around it.
• When 2 wires carry current near each other there will be an interaction (force) between
the magnetic fields produced by each individual wire.
Ch 21 & 22: Magnets and Induction
7
Practice:
5. A 12m wire carries a 30A current from North to South. The magnetic force is downward
towards the Earth and has a magnitude of 4 x 10-2N. What is the magnitude and direction
of the magnetic field at that point?
HW Magnetic Fields and Forces
PhET magnetic lab
So far we’ve looked at how a current can induce a magnetic field. What about the other way
around? Can a magnetic field induce a current? (remember the PhET activities?)
Induced EMF (Voltage)
• A conductor in a changing
magnetic field will have an
EMF (voltage) induced.
• Either the conductor can be
moving across field lines or
the magnetic field can itself
be changing.
• v in the right-hand rule is
still the motion of
something, in this case the
wire.
Ch 21 & 22: Magnets and Induction
8
EMF = V = Blv
Variable
EMF, aka V
B
l
v
Name
Unit
If something is induced, it is _____________________________
Induced EMF - stands for electromotive force, but it doesn’t really create a force, it really
creates a voltage.
The angle between the field and the circuit affects the strength of induction. Consider a loop of
wire in a magnetic field. The EMF is largest when the plane of the loop is perpendicular to the
magnetic field and decreases as the angle changes from 90°. EMF is the least when the plane of
the loop is parallel to the field.
θ is the angle between the magnetic field and the NORMAL to the wire loop; i.e.
“perpendicular” means the θ = 0.
Faraday’s Law of Magnetic Induction
emf = -NΔ[AB(cosθ)]
Δt
Variable
emf
-N
Ai and Af
Bi and Bf
Cosθi and f
t
Name
Electromagnetive force
-Number of loops in wire
Circuit loop area
Magnetic field
Angle
Time
Area of a circle = πr2
Unit
V
(no unit)
m2
T
Degree
s
Practice:
Ch 21 & 22: Magnets and Induction
9
6. A coil with 20 loops has an area of 0.8m2. A magnetic field at a 30 degree angle increases
from 0T to 0.8T in 0.6 seconds. What is the induced emf in the coil?
emf = -NΔ[AB(cosθ)]/Δt
7. If the coil has a resistance of 2.5Ω, what is the induced current?
8. What voltage is created by moving a 30m wire at 10m/s through a 4.0 x 10-2 T magnetic
field?
Induced Current
• Current flows when an EMF (voltage, or potential difference) is present in a closed loop
of conducting material. (Duh, isn’t this what a closed circuit with a voltage is?)
EMF = V = IR
Practice:
9. What is the current produced in #8 if there is a 2Ω bulb in the circuit?
10.What is the bulb’s wattage?
Induced EMF and Currents
Motor vs. Generator
• Motors
– Electric current is changed to motion.
– A coil of wire with a current through it will be forced to turn in a magnetic field.
• Generators
– Motion is changed to electric current.
– Turning a coil in a magnetic field will induce an EMF (voltage), thus current is
produced
Ch 21 & 22: Magnets and Induction
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– As the loop of wire is turned in the magnetic field, one side is moving up while the
other is moving down, therefore a current is induced in opposite directions in the
different sections of the loop.
– As the loop continues to turn, the sections of wire change places and so the current
switches direction.
– This causes the current to change constantly as shown in the graph.
Transformers (sorry, not the car kind)
• An alternating current flows through the primary coil creating an alternating magnetic
field.
• This changing magnetic field induces an EMF (Voltage) in the secondary coil and thus
current flows.
• In an ideal transformer, Power in = Power out
Ch 21 & 22: Magnets and Induction
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