Download Honors Physics Chapter 21electromagnetic induction

Warm-up
• Why do loops of wire in a motor rotate?
• What is similar between Electric motor,
and Electric meter?
• What is electromagnetic induction?
• Name 4 objects that apply
electromagnetic induction.
Chapter 37:
Electromagnetic Induction
•
1.
Faraday discovered:
Magnetic and
Electric fields
interact to produce a
Force.
(3rd right hand rule)
2. Magnetism can
produce electric
current in a wire!!
• How so?
• Michael Faraday
Electromagnetic Induction
• The phenomenon of inducing voltage
in a conductor by changing the
magnetic field near a conductor is
known as Electromagnetic Induction
Magnetic field changes
when either the magnet
or the coil moves or
when they both move.
Faradays experiment
• Molecular Expressions: Electricity
and Magnetism - Interactive Java
Tutorials: Faraday's Magnetic Field
Induction Experiment
Electromagnetic induction
• video
• http://glencoe.com/sec/science/phys
ics/ppp_09/animation/Chapter%202
5/Electromagnetic%20Induction.swf
Induced Emf (Voltage)?
• why is the phrase Induced voltage used
instead of induced current?
• Answer:
– In order for current to flow between two
points of a conductor, there must be a
difference of potential between those points.
That potential difference is the voltage.
Therefore, Voltage (Emf) is induced first in
order for current to flow.
Application of electromagnetism
• Generator
• The coil is rotated
mechanically using
one’s hand,
• Other ways of rotating
the coil include
– Water-hydro power
– Wind- wind mills
– Steam• Geothermal,
• Nuclear Reactors
Lenz’s Law
Question;
• Why is it
hard to
move the
magnet into
the coil if
the coil has
many loops?
Lenz’s Law
• The law notes that;
– an induced voltage always produces a
magnetic field that opposes the field
that originally produced it.
Lenz’s law
Magnitude of the induced Emf ()
•
•
What factors affect the magnitude
of the induced Emf () /voltage?
Answer:
1. The rate at which magnetic flux is
changing,
2. The number of coils/turns (N) of the
conductor
Magnetic flux
• Magnetic flux is the product of the
average magnetic field (B) and the
perpendicular area that it
penetrates.
• If the magnetic field penetrates the area
at an angle, magnetic flux is determined
using the formula:
 B  B  A  cos
Example
• A square loop of wire 10.0 cm on a
side is in a 1.25 T magnetic field at
an angle of 350. Calculate the flux
through the loop.
 B  B  A  cos
Homework
• Textbook pg 609 # 1-8
The Faradays law.
• Induced Emf () or (Voltage) in a coil is
proportional to the product of;
– the rate at which the magnetic flux
changes within the loops, and
– Number (N) of loops (turns /coils)
 B
  N
t
The negative sign indicates
the direction of induced
emf. The direction of
induced current is such that
its magnetic field opposes
the original change in flux
Example;
•
A square coil of length 5 cm contains 100
loops and is positioned perpendicular to a
uniform 0.6 T magnetic field. If the coil has a
resistance of 100Ω and is pulled across the
field in 0.1 seconds, calculate
a) The rate of change of flux
b) Induced emf and induced current
Applying Lenz’s law
• Increasing the
external magnetic
field.
– The conductor gets
induced with
current whose
magnetic field is in
the opposite
direction to that of
the external field.
S
Magnet with its North
Pole facing the coil is
moved towards the
coil (increasing B)
What is the direction of induced
current? (use 1st Right hand rule)
Applying Lenz’s law
• Decreasing the
external magnetic
field.
– The conductor gets
induced with current
whose magnetic field
is in the same
direction as that of
the external field.
N
Magnet with its South
Pole facing the coil is
moved away from the
coil (decreasing B)
What is the direction of induced current?
Exercise B, page 589
I decreasing
I increasing
I constant
I increasing
What is the direction of the induced current in
the circular loop due to the current shown in
each part of this figure?
Emf induced in a moving
Conductor
• When a conductor of length l moves
through a magnetic field B at a
velocity v, Induced emf is given by
the formula;
  Bl v
If the angle between the direction of conductor’s
velocity and the magnetic field is changing from being
perpendicular to any other angle, The induced Emf on
the conductor at that angle becomes;
  B  l  v sin 
Example
• An air plane travels at 1000km/h speed in
a region where the earth’s magnetic field
of 5.0 x 10-5 T is perpendicular to the
plane. What is the induced Emf on the
planes wings of width 70 m?
Class work
• Textbook page 610 # 1-10
Generator Equation
• For an rectangular
coil with two side
lengths rotating
within a magnetic
field like in a
Generator, the
induced emf is
given by;
  2B  l  v sin
The transformers
A simple transformer
• A simple transformer is a set of
induction coils as shown below.
Primary coil induces voltage
onto the secondary coil
Primary and secondary
coils
• A common transformer
is made up of an iron
core and two coils of
wires as shown below.
• The Primary coil is
connected to the power
source.
• The secondary coil
gets induced with
current.
What does it do?
• Molecular Expressions: Electricity and
Magnetism - Interactive Java Tutorials:
Transformer
Induction process (Mutual induction)
• Happens when current in the primary is changing
(increasing or decreasing).
• Changing current induces a magnetic field that is
changing continuously on the core, a condition
necessary for induction of voltage continuously on to
the secondary coil.
• When current is not changing (increasing or
decreasing), there is no induction taking place.
Why is the AC preferred to DC?
• We can increase and decrease current flow
by opening and closing (Switching off and on)
the circuit continuously when using a DC
current, or just use AC current. The Ac is
changing continuously
Why is an Iron core preferred
to two coils apart?
• Answer:
Iron core concentrates the magnetic
field within itself by forming loops that
get into contact with the secondary coil.
These loops of magnetic field induce
voltage in the secondary coil.
Step up Transformers
• The number of coils
in the secondary and
primary determines
whether a
transformer is a
step up or step
down transformer.
less number
of coils in
primary
more number
of coils in
secondary
Step down Transformers
Notice that power transferred is the
same.
More number
of turns in
primary coil
Less
number of
turns in
secondary
• What is the relationship between primary
voltage, secondary voltage and the number
of turns?
• Answer:
Vp
Vs

Np
Ns
•
The relationship between in-put
and out-put power is:
Power input = power output.
I p  V p = Is  Vs
Class work & Homework
• Textbook Page 612, # 30-36