Download Faradays Law of Electromagnetic Induction and Lenz`s Law

Michael Faraday (1791-1867)
An English Physicist and Chemist who discovered
that electricity can be generated by moving a
conductor through a magnetic field or by moving a
magnetic field pass a conductor.
Magnetic Flux
• A measure of the amount of magnetic field lines passing
a certain area.
• The SI system unit for magnetic flux is a Weber (Wb) or
Tm2 (Telsa times meters squared)
• Denoted by the capital Greek letter phi, Φ.
• Magnetic Flux is a scalar quantity.
• Φ=BA cosθ, θ is the angle between the magnetic field
and a normal to the plane of the area.
A magnetic field of 3.5 T passes through an area of 2.0 m2. How much flux passes
through the area given that the field lines passes perpendicular to the plane of the
area.
Φ = BAcos θ =3.5 T(2.0 m2)cos 0° = 7.0 Webers = 7.0 Wb
normal
Magnetic flux
(field lines)
passing thru
an area
Faraday’s Law of Electromagnetic
Induction
• A changing magnetic flux induces an EMF
(electromotive force/voltage/potential difference)
in the area surrounding the flux.

  EMF  voltage  N
t
N = number of turns (loops) of wire
Methods to Induce an EMF
1.
2.
3.
 (B)A
  EMF  voltage 

t
t
 B( A)
  EMF  voltage 

t
t
 B( A) B l w
  EMF  voltage 


 Blv
t
t
t
Continually changing the
magnetic field strength
Change the area exposed
to a constant magnetic
field.
Move a conductor with a
length, l, exposed to a
magnetic field at a velocity, v.
Changing the Strength of the Magnetic Field
S
N
Motion towards area
Magnet moves
closer to area
Increasing the
flux passing through
the conducting area
S
N
Motion away from area
Magnet moves
farther from area
Decreasing the flux
passing through the
conducting area
•A voltage/EMF results in the conducting area and as a result as a
current.
•The magnetic field change can be achieved by moving a magnetic
(steady magnetic field) closer and farther away from the conducting area.
Changing the Area Exposed to the Magnetic
Field
S
N
Increasing
Flux change
S
S
Induced
EMF/voltage
increased
N
N
Zero flux passing
through area
Maximum EMF/voltage
induced
Another Method of Changing the Area
Exposed to the Magnetic Field
All sides of the
wire are exposed
Moving the wire will change the flux
through the area thus inducing an
EMF/voltage in the wire.
Moveable
exposed wire
voltage
Another Method of Changing the
Magnetic Field Strength and Area
Exposed to the Magnetic Field
Moving a Conductor Through a
Uniform Magnetic Field
Δw through field
l
B
v
ε = EMF = voltage = Blv sin θ
B – magnetic field strength
l – length of wire exposed to the magnetic field
v – the speed of the area through the magnetic field
Θ = angle between velocity direction and magnetic field
How an Electrical Generator Works?
Generator – a device that converts mechanical energy to electrical energy.
Generator Animation
Power generation animation
Lenz’s Law
• The current induced in a conductor by an
external magnetic field will always have a
magnetic field from the induced current to
reduce the change in flux.
S
N
motion
Conducting
area
Which way will the current travel?
(clockwise or counterclockwise)
Lenz’s Law
• The current induced in a conductor by an
external magnetic field will always have a
magnetic field that opposes the change in
magnetic flux causing the induction.
I
T he magnetic field
from the magnetic
x
x
x
S
I
N
x
x
motion
x
x
x
x
x
I
The magnetic field in the wire
opposing the original magnetic
field causing the induction
Use the right hand rule on
the induced magnetic field
to determine the current
direction.
I
x – the opposing
magnetic field caused
from the induced
current.
Lenz’s Law
• Consider if the opposite occurred.
I
T he magnetic field
from the magnetic
x
S
I
N
motion
What would
x
I happen?
x x
x – the opposing
x
magnetic field cause
x
from the induced
x
current.
x x
x
I
Hypothetical magnetic field
induced in the wire
Use the right hand rule on
the induced magnetic field
to determine the current
direction.
Lenz’s Law Example
motion
View direction
N
S
N
clockwise
What is the pole induced on the right side of the metal bar in the solenoid?
What is the direction of the current as viewed towards the right?
Modification of Faraday’s Law Due
to Lenz’s Law

  EMF  voltage  - N
t
The negative sign takes into account
that the voltage opposes the change
In flux
Electromagnetic Wave
• A changing electric field will induce a
changing magnetic field.
• A changing magnetic field will induce a
changing electric field.
Electromagnetic field animation
Electromagnetic Animation 1
Electromagnetic field Animation 2