boardwork indicate which pole of the magnet is north and which is south. (the blue lines represent...
TRANSCRIPT
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