Download File - Hondorf Physics

ANNOUNCEMENTS:
• Unit II-3 Exam and HW (8 assignments) due
Friday 2/26!
• Wednesday 2/24- Engineering Day
• Tuesday 3/1- Girls Field Trip.
• Thursday 3/3 Thermodynamics lab- $2
• Next Week: Testmageddon!
• Next weekend: Joseph! Extra Credit- put
your name and my name on the back of your
ticket!
Just in Case You’re Feeling
Down…………
Unit II-3:
Magnetism and EMI
Section 6: Generators, Motors and
Mutual Inductance
3rd Right Hand Rule
• A current carrying wire in a magnetic field
experiences a force.
We can make it work backwards:
• If the wire is moved relative to the magnetic
field a current is produced.
• Provide the velocity
• and the resulting force moves
the charges
• Alternatively, change the
magnetic field
Another way:
• Change the orientation of the circuit loop with
respect to the magnetic field.
• Practical means of generating electricity
• Generator = a device that converts mechanical
energy to electrical energy
Alternating current
Alternating Current
• The rate of rotation determines the maximum
emf
• In the US f = 60Hz
• In Europe, Asia and Africa f = 50Hz
Creating Direct Current
• Slip rings rotate
with the loop.
• Brushes provide
connections to the
external circuit.
• An AC generator
has a split slip ring,
called a
commutator.
Motors
• Exactly backwards of an AC generator
• A current is applied to a circuit loop in a
magnetic field and the magnetic force causes
the loop to rotate.
• Remember, Lenz’s law says that the induced
EMF must act to reduce the current.
• This is known as “back EMF”
• Faster rotations cause more back EMF and less
current.
Mutual Inductance
Sample Problem: What is the potential difference
across a 10 mH inductor if the current through the
inductor drops from 150 mA to 50 mA in 10μs?
Chapter 20
Transformers
• A transformer is a device that saves Earth
from the decepticons.
• …or zombies
Chapter 20
Transformers
• A transformer is a device that increases or
decreases the emf of alternating current.
Chapter 20
Transformers
• The relationship between the input and
output emf is given by the transformer
equation.
CARD!!
N2
V2 
V1
N1
induced emf in secondary =
 number of turns in secondary 
 number of turns in primary  applied emf in primary


Chapter 20
Transformers
• The transformer equation assumes that no power is lost
between the coils. Of course, this is NOT TRUE!
• Due to Flux Leakage (how embarrassing!)
• Real transformers typically have efficiencies ranging from
90% to 99%.
A transformer you all have: