Download Lenzs` Law - PhysicsAPB

Electromagnetic Induction
Induced EMF and Induced Currents
Note: This is an AP topic only, so if they ask which way
current will flow, they mean conventional current (+ with
RIGHT hand rules)
When there is no relative
motion between the coils
of wire and the magnet
there is no current
produces
Current is created in
the coil when the
magnet is moved
towards the coil
Note: This is an AP topic only, so if they
ask which way current will flow, they mean
conventional current (+ with RIGHT hand
rules)
Current also exists when
you pull it away from the
coil, just in the opposite
direction.
The current in the coil is
called an induced current.
The coil itself acts as a
source of emf known as
an induced emf.
Another way to look at it.
Changing the area of a coil, in
effect, reduces/increases the B field
that the coil is subject to. Changing
the B field strength experienced by
the coil. This will also create a
current.
Motional EMF
The EMF Induced in a Moving
Conductor
A rod is being pushed to the right with constant
speed v. Suddenly the bulb lights. Why?
Where is the current coming from ?
Where is this opposing force coming from?
We have been using the term emf, ε, or electro
motive force.
ε=BLv
Potential
Difference
Magnetic Flux
Motional EMF and Magnetic Flux
By definition

B
A
Of course the angle with
the field is important
therefore
  BA
  BA cos
It is convenient express emf in terms of area when using induction in motors and
generators.
 x  x0 
 xL  x0 L   A  A0 

BA   BA0 
 BL  
  
 B 
  
t  t0
 t  t0 
 t  t0   t  t0 
since
  BA
   0 


t  t0
t
Faradays Law actually reads

  N
t
Where N is the # of turns in the coil. But
what is the negative all about?
Consider the field created by the counterclockjwise
loop in our previous problem. What is the direction of
its field?
Lenzs’ Law
The induced emf resulting
from a changing magnetic
field will produce a current
in such a way that the
induced magnetic field will
oppose the original
change in flux.
Like “magnetic inertia”
Transformers
Later, I will ask you
why we need
ALTERNATING
CURRENT
to make this work!
Basically, this is a
transformer!
How many of those little Power Cube thingies do you have around your house?
Here's how many I found around mine:
The answering machine
The cordless phone
The cell phone
The electronic anti-flea ultrasonic noisemaker (we have 2 dogs)
The digital camera
The nicad battery recharger
The video camera battery replacer/recharger
The digital clock in the bedroom
The electric toothbrush
The electric razor
The electric screwdriver
The electric drill
Both sides of the baby monitor
The laptop computer
The office phone
The ink jet printer
The speaker system on the computer
The tape recorder that records notes in the office
The calculator recharger
The 9-volt battery replacer for the radio
The radio shack univeral power cube
So let's take one of these apart and see what's
inside. Here's the transformer we will be exploring
today:
Under the cover
• This transformer
came with a
rechargeable electric
screwdriver. This
particular transformer
is rated at 3 volts and
240 milliamps. Once
you convince the
cover to come off
here is what you find
inside:
• What you can see here are two windings. The purpose of
a transformer is to convert one AC voltage to another AC
voltage. In this case the transformer converts the normal
120 volt AC current in your house down to three volts.
Primary Winding
• The 120 volts comes in on the primary winding on the
left. Running down the middle of that winding (as well as
around the outside) is an iron core. The AC current in the
primary winding creates an alternating magnetic field in
the iron just as it would in an electromagnet.
Iron Core
Secondary Winding
• The other winding, known as the secondary winding
wraps around the same iron core. In the secondary winding
the magnetic field in the core creates current. The voltage
in the secondary is controlled by the ratio of the number of
turns in the two windings. So if
the primary and secondary
windings have the same
number of turns, the primary
and secondary voltage will
be the same. If the secondary
winding has half as many
turns as the primary then
the voltage in the secondary
will be half that of the voltage in the primary.
• You can see in the
following figure that
the primary in this
particular transformer
uses very fine wire
while the secondary
uses much thicker
wire. To drop down to
3 volts, there needs to
be 40 times more
turns in the primary
than in the secondary.
• On the other side of the
transformer you find
two diodes wrapped in
rubber insulation. The
diodes act as a
rectifier, turning the AC
current into DC current.
Most transformer cubes
that you find around the
house produce a lowvoltage DC current (3 to
12 volts, and less than
an amp of current).
Turning AC into DC
DC current is necessary
because rechargeable
batteries store DC current,
because most electronics
require low-voltage DC
current and because small
DC motors run directly
from batteries and are the
least expensive motors
available.
•On the other hand, the picture tube in your TV requires 15,000 V to
accelerate the electron beam, and a transformer is used to obtain this from a
120 V wall outlet.
The current coming into the house is 30 amps, find a way to
tell me the current in the High-voltage wire.