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Magnetism
AP Physics
Chapter 20
Magnetism
20.1 Mangets and Magnetic Fields
Magnets and Magnetic Fields
Objects that align with the magnetic field of
the earth.
The north pole is
magnetic south
pole
Like poles repel and
unlike poles attract
Unlike charges, poles
can not be isolate
If a magnet is cut, new poles appear
20.1
Magnets and Magnetic Fields
Magnetic field lines – look similar to electric
field lines
Point in the direction of a force on a North
monopole
Interaction is also similar to
electric fields
20.1
Magnets and Magnetic Fields
The earths magnetic
field –
Caused by the
iron core
Does not point to
geographic
North
Provides protection
against solar wind
20.1
Magnetism
20.2 Electric Currents Produce
Magnetic Fields
Electric Currents Produce Magnetic Fields
Hans Christien Oerseted – 1820 a compass
needle deflects when a current is first
produced in a wire
The field is in a circle around the
wire
We can determine
the direction
of a magnetic
field using the
right hand rule
20.2
Electric Currents Produce Magnetic Fields
Right hand rule #1
1. the thumb points in the
direction of the current
2. the fingers curl in the
direction of the magnetic
field
A magnetic field due to a loop
20.2
Magnetism
20.3 Force on an Electric Current
in a Magnetic Field
Force on an Electric Current in a Magnetic Field
Magnets exert force on a current-carrying wire
(Newton’s Third Law)
The force is
1. perpendicular to the direction of current
2. perpendicular to the direction of the
magnetic field
20.3
Force on an Electric Current in a Magnetic Field
Right Hand rule #2
1. fingers, direction of
magnetic field
2. thumb, direction of current
3. palm, direction of force
The force is defined as
F  IlB sin 
B – magnetic field measure in tesla (T) –
vector
l – length of wire in the field
20.3
Force on an Electric Current in a Magnetic Field
A tesla is also called a weber per meter
squared
1T  1Wb / m
2
The units in cgs is called a Gauss
4
1G  10 T
20.3
Magnetism
20.4 Force on Electric Charge
Moving in a Magnetic Field
Force on Electric Charge Moving in a Mag Field
The equation can be determine by an analysis
of the current
I is the amount of charge that passes a point
in a given time
q
I
t
l can be defined as l  vt
Putting those in the force equation
q

 
vtsin
B sin
F FFqvB
IlB
t
20.4
Force on Electric Charge Moving in a Mag Field
The force is maximum when the charged
particle moves perpendicular to the field
Zero when it moves parallel to the field
The angle is the angle between the velocity
and the field
The right hand rule will give you the correct
direction for the force on a positive charge
F  qvB sin 
20.4
Force on Electric Charge Moving in a Mag Field
If we establish a uniform magnetic field and
the charge moves perpendicular to the field
In our example
-Field (fingers) is out of the
slide
-Velocity (thumb) is in the
direction of the blue arrow
-Force (palm) – points toward middle (assume
positive charge)
The force is centripetal
20.4
Force on Electric Charge Moving in a Mag Field
A very common type of problem
The electromagnetic force is the centripetal
force
2
v
FqvB
F qvB
qvB
msin 
r
Charged particles from the sun, concentrate at
the poles because of this – called the Aurora
Borealis
20.4
Magnetism
20.5 Magnetic Field Due to a Long
Straight Wire
Magnetic Field Due to a Long Straight Wire
As discussed earlier, a current in a wire
produces a magnetic field
This can be demonstrated by moving a
compass around a current carrying wire.
20.5
Magnetic Field Due to a Long Straight Wire
The field around the wire is directly
proportional to the current and inversely to the
distance from the wire
m0 I
B
2r
m0 is called the permeability of free space
m0  4x10 7 T  m / A
20.5
Magnetism
20.6 Force Between Two Parallel
Wires
Force Between Two Parallel Wires
Assuming that two wires are parallel to each
other
m 0 I1
B1 
2dr
m 0 I1 I 2
I 2sin
l2 B1l2
FF2F
IlB
2 
2d
20.6
Magnetism
20.9 Torque on a Current Loop;
Magnetic Moment
Torque on a Current Loop
Electric Motor
20.9
Magnetism
20.10 Applications
Applications
Galvanometer
1. Current flows through
the wire
2. Produces a magnetic
field
3. The magnetic field is
attracted or repelled by the permanent
magnet
Found in analog voltmeters, ammeters,
and ohmeters
20.10
Applications
Electric Motor
1. Current flows through
the wire
2. Produces a magnetic
field
3. The magnetic field is
attracted or repelled by the permanent
magnet
4. Brushes allow the current to switch
20.10
Applications
Loudspeaker
1. Current from the amp
flows through the wire
2. Produces a magnetic
field
3. This pushes the coil
away, causing the cone to move outward
4. Signal turns off, or switches, the cone
moves back
20.10