Download Magnetic Fields ch 20

Magnetic Fields
Ch 20 is about Magnetism, magnetic fields,
and interactions between moving charges and
magnetic fields
Magnetic materials
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Magnetic materials: what are they?
How do you make a magnet?
What is a compass and how does it align with
a magnetic field?
Why does a magnet “stick” to certain metals?
Magnetics
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Magnetic materials: typically magnets are
made up of iron, nickel, and/or cobalt
Materials which are “magnetic” allow the little
magnetic “dipoles” to align together, creating
a magneti
Magnetic materials
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Magnetic materials with dipoles aligned
Magnets
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Magnet showing field lines
Magnetism
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Magnetic fields are not electric fields
A magnetic field does not mess with a
stationary charge, but an electric field does.
For every magnet, there is a North and South
pole which can never be “separated”. Ain’t no
thing as a North by itself.
Magnetism
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Magnetic field lines point in the direction that
a north side of a magnet would align
A North on a compass aligns itself with
magnetic field lines.
The North pole is the south pole
For a magnet, field lines point away from N
and into the S.
Moving charges create Bmagnetic Fields
(B fields)
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A wire carrying a current creates a circular magnetic
field around it (Use right hand rule #2 for orientation)
RHR#2: point thumb in direction of current, the
fingers wrap in the direction of the magnetic field.
If you coil a wire or wrap wire around a cylinder, the
magnetic field from each wrap adds, creating a
North at one end of the cylinder and a South at the
other end. This is an electro-magnet, sometime
called a solenoid.
Magnetic field around a current carrying
wire
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B (magnetic) field around a wire, this is what I
call Right Hand Rule #2
Force on an Electric Current (or a moving
charge) in a magnetic field
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If a charge moves across a magnetic field, it
experiences a “strange” magnetic force which is
oriented Perpendicular to its motion and the
direction of B.
For a current in a wire, the Force = ILBsinө, I =
current, L = Length of wire crossing the magnetic
field, B = magnetic field strength (Teslas) and ө =
angle between the wire and the magnetic field lines.
Notice when the angle = 90, the Force is a
maximum.
F = BILSinө, (force = bill(nye)Sin(guy) )
Magnetic Force on a Current carrying wire
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Magnetic Force, use Right Hand Rule #1
Force on a Moving charge crossing a
Magnetic Field
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The same idea applies to a charged object crossing
a magnetic field (like a charged duck flying across
the earth’s magnetic field)
F = qvBsinө, q = charge (coulombs), v = velocity, B
= magnetic field (Tesla). Again, when the angle =
90, the charge is crossing the magnetic field lines
and is a maximum.
If the angle = 0, the charge is moving parallel to the
magnetic field lines and there ain’t not no Force no
mo.
Magnetic Force on a moving charge
Magnetic Force on a moving charge
Right Hand Rule #1
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For the Force on a moving charge you must use
RHR #1 to determine the direction of the force
1) Point fingers in direction of the moving charge or
current
2) Orient your Balm so it points in the direction of the
magnetic Field (B)
3) Extend you Fumb and it points in the direction of
the Force.
4) the above orientation is for a + (positive) charge,
for a – (negative) charge, the direction of the force is
opposite.
Motion of a charged particle moving
across a magnetic field
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The Force on a charge particle moving across a
uniform magnetic field is always perpendicular to its
motion.
This force causes the particle to move in a circle (as
in circular motion, dude)
F=qvB = ma =mv2/r which can be rearranged to
yield r = mv/qB.
This is a fairly common equation which shows up,
it’s a derived equation and not on the green
equation sheet.
Charged particle moving perpendicular to
magnetic field
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Particle moves in a cirlce
Magnetic Field Due to a Long Straight
Wire
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For a long straight wire, a circular magnetic
field exists (use RHR#2 for its direction)
B = (μ0I)/(2лr)
μ= permeability of free space
Magnetic field around a wire
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Magnetic field strength, B = (μ0I)/(2лr)