Transcript Psc CH-25 Electromagnetic Induction

Chapter 25
Electromagnetic
Induction
Do Now (2/8/12): True or False
1. Magnetic force is maximum when v and B are
perpendicular to each other.
2. A charge experiences magnetic force whether it
is moving or not.
3. When the magnetic field is directed to the north
while the positive charge is directed to the east,
the magnetic force is directed into this paper.
4. When a proton and an electron are projected
perpendicularly into a magnetic field, the electron
will experience a larger magnetic force than the
proton.
Review: Force on a single
particle
•F = qvB
What is centripetal force?
• Since the force is always perpendicular to
the velocity, it changes the particle's
direction, but not its speed.
mv
Fc 
r
2
Radius of Orbit
• the particle travels in a circle such that:
FB  Fc
mv
qvB 
r
2
mv
r
qB
Example:
• A proton orbits in a
magnetic field of 0.2
T at 30 m/sec. What is
the radius of its orbit?
Practice:
•Use the rest of class
to work on the
paper: Interaction
between E and B
matter
Do Now (2/9/12):
• A proton enters a magnetic
-8
field of 8.3 x 10 T with a
7
speed of 7.5 x 10 m/s.
What is the circumference
of its circular path?
Mass spectrometer
• a device for accurately determining atomic
mass.
• Atoms are ionized, accelerated to a known
velocity, and curved in a B-field until they
strike a photographic plate at P.
Mass spectrometer
q
2V
 2 2
m B r
• Charge to mass ratio
Example:
• A beam of singly ionized
oxygen atoms is sent through a
mass spectrometer. The values
are B=7.2x10-2T, r=0.085 m,
and V=110 V. Find the mass of
an oxygen atom.
E-Fields and B-Fields
Fb  Fe
qvB  FE
E
v
B
Example:
• A stream of singly ionized lithium atoms is
not deflected at it passes through a B-field
of 0.015 T that is perpendicular to an Efield of 650 N/C. What is the speed of the
atom as it passes through the fields?
Do Now (2/10/12):
• A beam of doubly ionized oxygen
atoms is sent through a magnetic
field of 1.85 T in mass spectrometer.
The radius of orbit is 0.95 m, and the
voltage is 30 V. Find the mass of an
oxygen atom.
Electromagnetic
Induction
•The process of generating
a current through a circuit
by passing it through a
magnetic field
Right-Hand Rule –
same one!!!
•B = pointer finger
•F = middle finger
•v = thumb-motion
Electromotive Force
(EMF or ε)
•Potential difference (V)
between two
substances needed to
produce a current
REMINDER….
THE TERM “ELECTROMOTIVE
FORCE” CAN BE CONFUSING…
BECAUSE IT IS NOT A FORCE
AT ALL!!! IT IS A
VOLTAGE
Electromotive Force
• Produced when a length
of wire is passed through
a magnetic field in a
direction  to the field
Electromotive Force
•EMF = BLv
•B = magnetic field
•L = lengthwire in field
•v = velocitywire in field
Example:
A straight wire, 0.2 m long, moves at a
constant speed of 7 m/s perpendicular
to a magnetic field of 0.08 T.
1. What EMF is induced in the wire?
2. The wire has a resistance of 0.5 Ω.
What is the current?
3. If a different metal (R=0.78Ω) were
used for the wire what would the new
current be?
Electromotive Force
•If the direction of the
wire is not  to the field,
its  component must be
determined to determine
the EMF
Electromotive Force
•EMF = BLvsinΘ
Practice:
• Use the rest of class to work on the paper
Electromagnetic Induction. It is due on
Tuesday!
Do Now (2/13/12):
A straight wire, 1.43 m long, moves
at a constant speed of 9 m/s
perpendicular to a magnetic field
of 0.18 T.
1. What EMF is induced in the
wire?
2. The wire has a resistance of 30 Ω.
What is the current?
Magnetic Flux
a measure of the number of magnetic
field lines passing through an area. If a
loop of wire with area A is in a field B,
the flux is:
  BA cos
Measured in Webers (Wb)
Faraday’s Law
•Any change in a magnetic
environment will cause a
voltage (emf) to be
"induced" in the coil.
Inducing EMF
• Changing area of coil
• Changing B-Field
• Changing B-flux
• Rotating coil (changing
time)
Faraday’s Law
n
 
t
n = # of turns in coil
Φ = flux
t = time
Example:
• http://farside.ph.utexas.edu/teaching/302l/le
ctures/node95.html
Do Now (2/14/12):
A circular loop of wire of radius 40 cm which
has 25 turns is placed in a uniform magnetic
field. The direction of the magnetic field
makes an angle of 60˚ with respect to the
normal of the loop. The magnetic field
strength is increased at a constant rate from
4T to 8T over 3 s.
1. What is the area of the loop?
2. What is the change in flux of the loop?
3. What is the EMF induced by the loop’s
motion?
Pop quiz:
A circular loop of wire (radius 0.3 m) which has
10 turns is placed in a uniform magnetic field.
The direction of the magnetic field makes an
angle of 60˚ with respect to the normal of the
loop. The magnetic field strength is increased
at a constant rate from 5T to 10T over 6 s.
1. What is the area of the loop?
2. What is the change in flux of the loop?
3. What is the EMF induced by the loop’s
motion?
Practice (2/14/12):
• Please use the rest of class to work on the
paper “Faraday’s Law.” It is due
tomorrow!!! Happy Valentine’s
Day!!!!!!!!!!
Do Now (2/15/12):
A circular loop of wire (radius 0.3 m) which has
10 turns is placed in a uniform magnetic field.
The direction of the magnetic field makes an
angle of 60˚ with respect to the normal of the
loop. The magnetic field strength is increased
at a constant rate from 5T to 10T over 6 s.
1. What is the area of the loop?
2. What is the change in flux of the loop?
3. What is the EMF induced by the loop’s
motion?
New Topic: Transformers!!!
2/15/12
Transformer
• A transformer is composed of two different
coils of wire around opposite sides of an
iron core.
• *passing current through wire that
surrounds a metal core induces a magnetic
field.
• This induced B-field will cause a
responding current flow in a secondary coil
wound around the opposite side.
Transformers
• Transformers are useful b/c if you change the
number of turns from one side to the other, you
change the voltage in the wire on the right!
Transformers can change a high voltage to a lower
one, or a low voltage to a higher one.
Step Up Transformers
• If you increase the number of turns on the
right, the voltage coming off the transformer
will increase in proportion.
Transformers
• Transformers do not change POWER:
Pp  Ps
V p I p  Vs I s
Transformer Equation
I s Vp N p


I p Vs N s
•
•
•
•
•
p=primary
s=secondary
N= # of turns
V=voltage
I=current
Example:
A step-up transformer has a primary coil
consisting of 200 turns and a secondary coil
consisting of 3000 turns. The primary coil is
supplied with an effective AC voltage of 90 V.
1. What is the voltage in the secondary circuit?
2. If the current in the secondary coil is 2 A,
what is the current in the primary circuit?
Practice (2/15/12):
•Please use the rest of
class to work on the
paper
“Transformers.” It is
due Friday!!!
Do Now (2/16/12):
• A step-down transformer has 7500 turns on its
primary coil and 125 turns on its secondary
coil. The voltage across the primary circuit 7.2
kV.
1. What voltage is being applied across the
secondary circuit?
2. If the current in the secondary circuit is 36 A,
what is the current in the primary circuit?
Practice (2/16/12):
•Please use the rest of
class to work on the
paper
“Transformers.” It is
due Friday!!!
Right-Hand Rule
•B = fingers-MF
•F = up from palm
•v = thumb-motion