Post on 23-Dec-2015
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
Up Down
In Out
RightLeft
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
Source: Griffith, The Physics of Everyday Phenomena
N
S
dipole field
Solenoid Magnetic Field
Source: Griffith, The Physics of Everyday Phenomena
N S
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
Right-Hand Rule
F = qv B
Source: Griffith, The Physics of Everyday Phenomena
qv
Poll Question
What is the direction of the cross product A B?
A
B
A.
B.
C.
D.
E.
F.
Cross Product Review
A
B
a
b
AB = area of parallelogram
Cross Product Review
• Curl right-hand fingers in direction of
• Right-hand thumb points in direction of cross-product
• Not commutative
A
B
a
b
AB = –(BA)
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
paper square
Make an Origami Right Hand
fold over
magnetic
fieldcurrent
qv
For
ce
Lore
ntz
vectors
creases:in out
Think Question
What is the direction of the force on the object moving with velocity v through magnetic field B?
B
v
+
A.
B.
C.
D.
E.
F.
B
Group Poll Question
What is the magnitude of the force on object A compared to the magnitude of the force on object B?
v
q
v
2qA BA. FB = 4 FA
B. FB = 2 FA
C. FB = FA
D. FB = FA/2
E. FB = FA/4
B
Group Poll Question
How does the work done on object A compare to the work done on object B?
v
q
v
2qA B
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.
• 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.
Here’s where it is:
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