Transcript Electromagnetic Induction

Electromagnetic Induction
AP Physics
Chapter 21
Electromagnetic Induction
21.1 Induced EMF
Induced EMF
Michael Faraday – a changing magnetic field
can produce an electric current
This is an induced current
21.1
Induced EMF
A magnet moved quickly into a coil of wire
induces a current
When it is quickly moved the other way, a
current is induced in the opposite direction
21.1
Electromagnetic Induction
21.2 Faraday’s Law of Induction;
Lenz’s Law
Faraday’s Law of Induction; Lenz’s Law
From experiments the induced emf depends
on
1. Rate of change of the magnetic field
2. Loop’s area
3. Angle
Magnetic flux is the
total number of
magnetic field line
passing through
the are enclosed by the loop
21.2
Faraday’s Law of Induction; Lenz’s Law
Magnetic Flux
 B  BA cosq
When q is 90, no field lines pass through the
loop
The unit for magnetic flux
is a weber
21.2
Faraday’s Law of Induction; Lenz’s Law
Since the induced EMF is the rate of change
of flux
 B
 
t
This is Faraday’s Law of Induction
For multiple loops in a magnetic field
 B
  N
t
Where N is the number of loops in the field
21.2
Faraday’s Law of Induction; Lenz’s Law
Lenz’s Law – a current produced by an
induced emf moves in a direction so that its
magnetic field
(produced by
the current)
opposes the
original change
in flux
21.2
Faraday’s Law of Induction; Lenz’s Law
Steps in problem solving – Lenz’s Law
1. Determine if the magnetic flux inside the
loop is decreasing, increasing, or
unchanged
2. The magnetic field due to the induced
current
a. Points in the same direction as the
external field if the flux is decreasing
b. Points in the opposite direction if the
flux is increasing
21.2
Faraday’s Law of Induction; Lenz’s Law
Steps in problem solving – Lenz’s Law
3. Use the right hand rule-1 to find the
direction of the induced current
4. Always keep in mind that there are two
magnetic fields
a. An external field whose flux must be
changed if it is to induce an electric current
b. A magnetic field produced by the
induced current
21.2
Electromagnetic Induction
21.3 EMF Induced in a Moving
Conductor
Faraday’s Law of Induction; Lenz’s Law
If the rod above is made to move at a velocity v
The area of the rod increases A 
xlxvtlvt
So the change in area is
Blv
B


A
t

B
The induce EMF is
 Blv
t
21.3
Faraday’s Law of Induction; Lenz’s Law
Sometimes called motional emf
True as long as components are mutually
perpendicular
21.3
Electromagnetic Induction
21.4 Changing Magnetic Flux
Produces an Electric Field
Changing Magnetic Flux Produces Electric Field
Since there is an induced current when there is
an induced EMF, there must be an electric
field
A changing magnetic field induces an electric
field
F  qvB
So since
And
qvB
F
EE vB
qq
The electric field must be
21.4
Electromagnetic Induction
21.5 Electric Generators
Electric Generators
Generator – converts mechanical energy
(spinning) into AC current.
Generator Simulation
If we look at just one perpendicular arm of the
loop
  Blv sin q
Since we have two arms in each loop
  2Blv sin q
Now if we add loops
  2NBlv sin q
21.5