Download barransclass

BoardWork
Indicate which pole of the magnet is north and
which is south. (The blue lines represent magnetic
field lines.)
Magnets and Currents
interacting and inseparable
Objectives
• Describe the magnetic field caused by an
electrical current.
• Determine the force on an electric charge
in a magnetic field.
What’s the Point?
• What is the force that underlies electric
motors?
Current Creates Magnetism
An electric current creates a magnetic field.
Look, Ma! No poles!
Vector Direction Conventions
Left
Right
Up
Down
In
Out
Poll Question
If a wire in front of you carries a current from
left to right, what is the direction of the
resulting magnetic field where you are?
I
A.
B.
C.
D.
E.
F.
Board Work
An electric current creates a magnetic field
whose lines circle right-handed around it.
Draw lines for the magnetic field created by
a ring of current.
I
Magnetic Field of Current Ring
N
dipole
field
S
Source: Griffith, The Physics of Everyday Phenomena
Solenoid Magnetic Field
N
S
Source: Griffith, The Physics of Everyday Phenomena
Electrons are Magnets!
spin
Electrons are Magnets!
current
Electrons are Magnets!
magnetic
field
Electrons are Magnets!
N
S
magnetic
dipole
Types of Magnets
• Electromagnets
– currents travel through conducting coils
• Permanent Magnets
– materials whose electrons have aligned spins
or orbits
Moving charges create the fields!
The Lorentz Force
making electrons work for us
Magnetic Force on a Charge
•
•
•
•
•
Currents create magnetic fields.
Currents are made of moving charges.
Moving charges are magnets.
Magnets apply forces to each other.
Magnets apply forces to moving charges.
How do those forces behave?
Lorentz Force
F = qv  B
qv
Source: Griffith, The Physics of Everyday Phenomena
Right-Hand Rule
Poll Question
What is the direction of the cross product
A  B?
A
A.
D.
B.
E.
C.
F.
B
Cross Product Review
a
q
A
B
b
AB = area of parallelogram
Cross Product Review
a
q
A
B
b
• Curl right-hand fingers in
direction of q
• Right-hand thumb points
in direction of crossproduct
• Not commutative
AB = –(BA)
Lorentz Force Properties
•
•
•
•
•
F = 0 unless charge is moving
F = 0 if velocity is  to field
F = maximum if velocity is  to field
F  0 only if charge crosses B field lines
If q, v or B reverse, direction of F reverses
Make an Origami Right Hand
vectors
fold over
qv
current
Force
creases:
in out
Lorentz
paper square
magnetic
field
Think Question
What is the direction of the force on the
object moving with velocity v through
magnetic field B?
B
A.
D.
B.
E.
C.
F.
+
v
Group Poll Question
What is the magnitude of the force on object
A compared to the magnitude of the force on
object B?
B
A
B
q
v
2q
v
A.
B.
C.
D.
E.
FB = 4 FA
FB = 2 FA
FB = FA
FB = FA/2
FB = FA/4
Group Poll Question
How does the work done on object A
compare to the work done on object B?
B
A
B
q
v
2q
v
A. wB > wA.
B. wB = wA.
C. wB < wA.
Lorentz and Newton’s third law
• F = qv  B does not explicitly include a
reaction force. Here’s where it is:
• Magnetic fields (B) are always created by
moving charges.
• Moving charges (qv) always create
magnetic fields.
• The moving charge creating B “feels” the
field of qv. So F = qv  B goes both ways.
Board Work
From F = qv × B, find the SI unit of magnetic
field B.
Challenge Question
A current runs through one wire of a pair of
parallel wires. What is the direction of the
resulting magnetic field at the location of the
other wire?
I
?
Force between parallel currents
What is
the force
on this
current?
IB
Definition of Ampere
• If two parallel wires are held 1 m apart,
• with currents of 1 A through each wire,
• the attractive force between the wires is
2  10–7 N for each meter of length of the
wires.
Reading for Next Time
• Faraday’s law
• Big Ideas
– A changing magnetic field can create an
electric potential
– AC Transformers can be understood using
Faraday’s law and conservation of energy
• Very abstract
– You are ready
– It is very cool