Transcript magnetism lesson - Red Hook Central Schools

Do Now: Copy and sketch the electric field
between the spheres below.
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• How is an electric field sketched?
• What give rise to an electric field?
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Pick up compass
Hold level
Which way does it point?
Why?
Needle is a magnet!
Forces:
Fnet on object causes acceleration.
• Forces – act at distance
• Fg– attractive btw masses.
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Field
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g = Fg/m. F on test mass.
• Fe – attractive/repulsive
between objects w net + or charge.
• E = Fe/q. + F on test charge.
• Fmag magnetic-
attractive/repulsive between
objects w net e- spin. Mag
Poles
• B ~ motion of charges.
Compass needle.
• Magnetism
What is
Magnetism?
Magnetism is the force of
attraction or repulsion of
a material due to the
motion of its electrons.
Like all forces Fmag can
be a Fnet that causes
acceleration & is
measured in N.
• All magnets have 2 Poles, N and S.
• Like Repel
• Opposite attract.
• Play
Magnetic Fields
regions of space around a
magnet where magnetic
materials feel a force.
Magnets create a force field which will
deflect a compass needle.
• Field lines point in
the direction of the
deflection of the
compass needle.
• Through Magnet.
The poles strongest force.
Field lines are close where field is strong.
Mag field lines come out of N pole, go into S pole.
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Lines never touch or cross
Only curves
Field lines called lines of flux.
Flux density shows field strength.
B is symbol
Ex: Make a rough sketch of the horseshoe
magnet below. Sketch the field lines near the
ends of the magnet.
More than 1 magnet – out of N, into S.
Earth has a mag. field. The north seeking end of a
compass points toward Earth’s N geographic pole.
Earth’s north pole acts like south pole of
bar magnet.
What causes magnetism?
Atoms have magnetic fields due to excess e- spin.
Groups of atoms join so that their magnetic fields
are all going in the same direction.
These areas of atoms are called “domains”
When an unmagnetized substance is
placed in a magnetic field, the substance
can become magnetized. This happens
when the spinning electrons line up in
the same direction.
An unmagnetized substance
looks like
this…
While a magnetized substance
looks
like this…
Magnetize Clip – Induced Magnetism
Poles of a magnet always come in pairs!
How to break a magnet:
1. Drop it
2. Heat it
This causes the
domains to become
random again!
Magnetic Polarization
N
S
Non-magnets with
magnetic properties
can be polarized in
the presence of a
magnetic field.
N
Temporary Magnets.
S
Hwk Intro Sheet.
Relationship of Electricity &
Magnetism
Similarities of Magnetic (B) to Electric
Fields (E)
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field lines also called lines of flux.
Both attract opposite. Repel like.
Drop off with d2.
Both Polarize objects.
Flux density shows intensity of field.
Both vector quantities.
Differences
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For B field, direction compass needle points.
E field direction + test charge would move
Magnet feels no force in static E field.
Charges feel no force in static B field.
Isolated poles do not exist. Isolated charges do.
Demos.
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Currents Cause Field (wrapped compass).
Solenoids form bar magnet– Mag N. Pole.
Mag Field puts force on charges.
Moving Magnets cause potential difference
and current flow.
• Bill Nye Magnets 19 min
• https://www.youtube.com/watch?v=rl9r2Ap9vOo
• Minute Physics How do magnets work 6.5 minutes.
Quantum
• http://www.youtube.com/watch?v=hFAOXdXZ5TM
• Good Explanation Electromagnetism 57 Min.
• https://www.youtube.com/watch?v=bht9AJ1eNYc
• Old Navy film How magnetism produce Electricity 7 min.
• http://www.youtube.com/watch?v=uoQelu7XRjk
Charge and magnetic fields.
• Stationary charges do not cause magnetic
fields, but do cause electric fields.
• Charges in motion….
• When a current flows charges move, B field
generated.
• B is perpendicular to wire.
• Electromagnet
• Current creates
magnet.
Magnetic fields induce pd voltage/current
flow.
• Drop magnet into wire loop – current flows in
loop.
• Spin wire coils through magnets – current flows in
magnet.
Sketching Fields
Use arrows to show the direction
of compass needle deflection.
Field into the page X like tail of
arrow.
Field out of page … like tip of
arrow.
Mag Field Direction around Wire.
2nd Hand Rule:
Wire loop – thumb in current
direction. Fingers in B direction.
Solenoid – Coils of wire (can be
wrapped around core of iron).
3rd hand rule: Fingers in direction of current.
Thumb points to magnetic N. Pole. Solenoids act
like bar magnets.
Charged particles moving in B
fields feel a force!!
Force direction determined by
hand rule three.
Fourth
Direction of mag force on q
perpendicular to v vector & to B field.
For +q place right hand fingers into
field, thumb points to v, palm points to
mag force.
For – q use left hand.
Electron in mag field B.
Force is proportional to amount
of charge on particle.
Force on current carrying wire.
The current in a wire also feels a force in mag
field.
Fifth hand rule:–force direction on wire: fingers
into field, thumb in current direction, palm points
toward force direction.
Currents in same direction exert
attractive forces.
Current in opposite directions,
create fields with repulsive forces.
By Newton’s 3rd
Law, F2 = -F1.
F1
F2
I1 I2
The Ampere (A) is a unit of rate of
current flow, but is based on the
force felt between 2 currentcarrying wires.
The Ampere, not the Coulomb, is
considered a fundamental unit!!
Very weird indeed!
For a straight length of wire the induced B
field drops off with distance (r).
The circles should get…
B = moI
2pr
mo constant find it.
r = dist fr wire m
I = current - A
B = field strength T
Minute Physics How do magnets
work 6.5 minutes.
• http://www.youtube.com/watch?v=hFAOXdXZ
5TM
Mag Field Strength around a solenoid:
B = moNI
l
N = number turns of wire
I = current
l = length of wire in field m
Force between two wires proportional to
current in wires:
F = moI1I2l
2pr
Ex 4: What is the magnitude & direction
of the force btw 5.00 m lengths of wire
each carrying 7.5 A of current in the
same direction if they are separated by
25.0 cm?
2.25 x 10-4 N toward each other.
Mag Field Between Current Carrying Wires.
These show neg charge flow.