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Fnet causes acceleration.
Forces – act at distance
• Fg– attractive btw masses.
g = Fg/m. Test mass.
• Fe – attractive/repulsive
between objects w net
• E = Fe/q. + Test charge.
• Fmag magnetic-
attractive/repulsive between
objects w net e- spin..
• B ~ currents in wire.
Compass needle.
• Magnetism
What is
Magnetism is the force of
attraction or repulsion of
a material due to the spin
of its electrons.
Like all forces Fmag can
be a Fnet that causes
acceleration & is
measured in N.
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 of a magnet are where the
magnetic effect is the strongest.
Each magnet has 2 poles –north, south.
Field lines are close where field is strong.
Field lines come out of N pole, go into S pole.
Lines never touch or cross
Only curves
Field lines called lines of flux.
Flux density shows field strength.
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.
Demo Magnetic Field Lines
• magnet on iron bars.
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
While a magnetized substance
like this…
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!
Non-magnets with magnetic properties can
be polarized in the presence of a magnetic
Minute Physics How do magnets
work 6.5 minutes.
Hwk Intro Sheet.
Relationship of Electricity &
Similarities of Magnetic (B) to Electric
Fields (E)
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.
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.
Electromagnetism 57 Min.
• Old Navy film How magnetism produce
Electricity 7 min.
-Moving charges cause magnetic fields.
When a current flows, a magnetic field is
*Stationary charges do not cause magnetic
fields, but do cause electric fields.
Sketching Fields
Use arrows to show the direction
of compass needle deflection.
Field into the page X like tail of
Field out of page … like tip of
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.
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
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.
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
mo constant find it.
r = dist fr wire m
I = current - A
B = field strength T
Mag Field Strength around a solenoid:
B = moNI
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
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.