Electromagnetic Induction Notes

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Transcript Electromagnetic Induction Notes

Electromagnetic Induction
• When a magnet and a wire move
relative to each other, a voltage is
• Amount of voltage produced depends
– Speed: High speeds produce high voltages
– Magnetism: Strong magnets produce high
– Shape of Wire: Many coils in the wire
produce high voltages
Electromagnetic Induction
Electromagnetic Induction
Note: It is more difficult to push the magnet into a coil
with more loops because the high current generates a
stronger magnetic field which acts against the magnet.
Electromagnetic Induction
• Electromagnetic Induction: Inducing
voltage by changing the magnetic field
around a conductor
– ANY change in magnetic field will induce a
– i.e.) Traffic control signals
Faraday’s Law
The induced voltage in a coil is
proportional to the number of loops
multiplied by the magnetic field
changes within those loops.
Faraday’s Law
• When a magnet moves past different
materials, the voltage induced is the same
for each case
• The most current will be produced in the
material where the electrons are bound
most loosely
– i.e.) The magnet will produce a larger
current when moving past copper than
Bell Ringer
• To produce a current in a coil of wire with a
magnet, what must occur?
• What type of current will this produce?
Generators and AC
• As a wire moves back and forth past a
magnet, the resulting current changes
direction (AC)
• Recall: A motor converts electrical
energy (from the battery) into
mechanical energy (rotation of the
• A generator converts mechanical
energy into electrical energy
Generators and AC
Generators and AC
Generators and AC
Turbine: When the armature of a generator is
connected to a wheel which captures wind, water, or
steam in order to turn and produce electrical energy
Bell Ringer
• Explain what the difference is between a
generator and a motor in terms of energy.
• Consider two coils
side by side:
–Primary Coil:
Connected to a
voltage source
–Secondary Coil:
Connected to a
• When the voltage source is turned on:
– Current briefly surges through the secondary
• When the voltage source is turned off:
– Current briefly surges through the secondary
coil in the opposite direction
• The magnetic field building around the
primary extends to the secondary
– Changes in magnetic field intensity induce
voltage in the secondary
• Placing a core within the coils will
intensify the magnetic field
• The secondary will intercept more of
the field change
• Instead of switching a DC voltage
source off and on, the device is
connected to an AC voltage source
• The rate at which the magnetic field
changes = frequency of the AC
• This device is known as a transformer
• Used to step voltages up or down
–i.e. Used in AC adaptors
• Voltage is stepped up:
–When the number of coils (turns) in the
secondary > number of turns in primary
• Voltage is stepped down:
–When the number of coils (turns) in the
secondary < number of turns in primary
• Since voltages can be stepped up and
down so easily, electrical power is
primarily in the form of AC
Induction of Electric and
Magnetic Fields
• According to Faraday:
– Electric fields are created in any region of
space where a magnetic field is changing
with time.
• According to Maxwell:
– A magnetic field is created in any region of
space where an electric field is changing
with time.
• These laws are inverses of each other and
lead to the concept of electromagnetic waves
Electromagnetic Waves
• Composed of vibrating electric and
magnetic fields the regenerate each
–Waves move outward from a vibrating
–E.F. is always perpendicular to the
M.F. and both are perpendicular to the
direction of the moving wave
Electromagnetic Waves
Electromagnetic Waves
• All electromagnetic waves move at
the speed of light
–Discovered by Maxwell
• Changing electric fields constantly
induce changing magnetic fields and
vice versa
–If the waves traveled at less than the
speed of light, they would rapidly die
Electromagnetic Waves
• Maxwell found that light is simply
an electromagnetic wave vibrating at
a specific frequency
•4.3x1014 to 7x1014 vibrations per
•This frequency stimulates the retinas
in our eyes and allows us to see these