Transcript Electromagnetic Induction

Ch 36 and 37
Magnetism &
Electromagnetic Induction
Characteristics of Magnets
• Each magnet has a north-seeking and
south-seeking pole
• Exert a magnetic force on the space
around them = magnetic field
– Lines closer together – stronger field
– Where is the field strength the greatest on a
magnet?
The
Earth
Nature of a Magnetic Field
• Produced by the motion of an electric
charge (current)
• Every spinning electron is a tiny magnet.
• If two electrons are spinning the same way, they
make a stronger magnet
• If two electrons are spinning in opposite
directions, their magnetic field cancels out (why
most substances are not magnets)
A current carrying wire can produce a
magnetic field
More about magnets
• Common magnets are made of iron,
nickel, and cobalt.
– The spin of their e- do not cancel
• When atoms are aligned, they have
magnetic domains.
• How can a magnet become weaker?
• How does a magnet attract a piece of
metal that is not magnetized?
What is electromagnetic induction?
• We learned about induction when we
studied how objects become charged.
• Inducing voltage by changing the
magnetic field around a conductor
How is voltage induced?
• Moving a magnet in a coil of wire will
induce voltage and therefore produce
a current
• Either the magnet must move through
coil of wire or a coil of wire must
move back and forth across a magnet
to induce voltage
Clarification
• Magnets do not produce voltage!!!
• Voltage is produced by the work done
when a magnet and closed loop of wire
are moved relative to each other.
Amount of voltage
• Depends on how quickly the magnetic
field lines are traversed by the wire
– Quick motion = more voltage
– Slow motion = less voltage
• Depends on how many coils
– 1 coil of wire = some amount of voltage (V)
– 2 coils of wire = 2 x V
– 4 coils of wire = 4 x V
Faraday’s Law
• Induced voltage in a coil is
proportional to the product of the
number of loops and the rate at which
the magnetic field changes within
those loops
Amount of Current
• Depends on the amount of voltage
induced
• Also depends on the resistance of the
coil and the circuit to which it is
connected
• What are some examples?
Practical applications of
electromagnets
• Maglev trains – page 569, http://www.okeating.com/hsr/maglev.htm
• Stereo speaker,
http://electronics.howstuffworks.com/speaker3.htm
• Alarm systems
• Junkyard crane
• MRI, http://electronics.howstuffworks.com/mri2.htm
Motor vs. Generators
• Motor- turns electrical energy into
mechanical energy
• Generator- turns mechanical energy
into electrical energy
• Simply opposites of one another
Motor
Generator
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Generators
• When the wire is rotated in a
magnetic field, the # of magnetic field
lines change, this causes induced
voltage to change.
– Causes alternating current  current
changes magnitude and direction
periodically
Generating electricity
• The faster you turn the coil the
greater the frequency of magnetic
field change.
• Greater frequency of AC
(N. America = 60 Hz)
• Greater induced voltage
Generators used in power plants
• Chemical or nuclear energy  mechanical
energy  electrical energy
Look
familiar?
Fuel: natural gas, oil
Motor vs. Generator Effect
• Battery involved
• No battery
• Initial moving
• Wire moved
charges = current
mechanically
• Wire forced upwards • Interaction between
due to magnetic field
electric field and
magnetic field
• Battery provides
causes charges to
voltage =
move = induce
mechanical energy
voltage = induce
results
current
Transformers
Coils
• Primary – connected to a power
source (input), opening or closing the
primary switch can induce voltage in
the secondary circuit
• Secondary – output, reacts to changes
in the magnetic field intensity of the
primary coil and voltage can be
induced
Transformers
• What is the point in using a
transformer?
– Voltages can be stepped up or down
• See figure 37.12
– Instead of opening and closing the switch
on the primary circuit – we just use an
alternating current to produce the same
effects.
Primary voltage
#primary turns
=
secondary voltage
# secondary turns
Do transformers disobey the law of
conservation of energy?
• No…..
– POWERIN = POWEROUT
– (VOLTAGE X CURRENT)PRIMARY =
(VOLTAGE X CURRENT)SECONDARY
• Power is delivered to your home at
high voltages but low currents
– (120 V and 1/10 amp)
Transformers
• Step-up transformers: the secondary
has more loops than the primary
• Step-down transformers: the
secondary has less loops than the
primary
• When would you want a step-up
transformer?
• When would you want a step-down
transformer?
Faraday
• An electric field will be created when you
change a magnetic field over time
Maxwell
• A magnetic field will be created when you
change an electric field over time
• Magnitude of the resulting field depends on
the rate at which the initial field is changed
• The resulting field is at right angles to the
initial field.
Electromagnetic Waves
• Composed of vibrating electric and magnetic
fields that regenerate each other, therefore
they are self-sustaining
– Conservation of energy
• No medium is required
• Electric field is perpendicular to the magnetic
field and both are perpendicular to the
direction of motion of the wave
• EM waves move at the speed of light