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1. Outline
1. Electric Motor
PHY 101 Lecture #11: 2. Magnetic Induction
Electromagnetic 3. Electrical Generator
Direct Current or DC generator
Motors and Generators 4. Alternating Current
Prof. Peter R. Saulson
AC generator
saulson@physics.syr.edu
http://physics.syr.edu/courses/PHY101/
Off. Hrs: Tue 9:30 –11:00, Physics 263-4
Prof. Schwarz’s Problem Sessions:
Mon and Tues, 5:15 – 6:15, Physics 202/204
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
1. Electric Motor Real Electric Motor
Force on current in B field means that electric Practical device: electric motor.
energy can be turned into mechanical energy. Key parts:
– magnet (permanent or electromagnet)
– rotatable coil (rotor)
Current goes around coil, in B field of fixed
magnet. – connections to ensure current goes to coil always
in the “right” way
Right hand rule says one side of coil forced up,
– electrical energy input (battery)
other side forced down.
– mechanical energy output (shaft)
ROTATION!
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
Making current go
Energy in, energy out
the “right” way
If coil just hooked to battery, coil would just It takes electrical power to keep current flowing
twist to “no-force” position. in coil.
Clever parts: Mechanical power can be extracted from
rotating shaft.
Brushes
Examples:
Split-ring commutator
Brushes make sliding contact (no twisting up!) – Let string wind up around shaft, lifting weight.
– Turn the wheels of a car.
Commutator changes direction of current just as
coil lines up in “happiest” orientation. Motor is a device whose job is energy
transformation.
Coil needs to flip, but then current reverses
again, ...
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
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2. Observation of
2. Magnetic induction
magnetic induction
Electric motor converts electrical energy into Consider circuit with just coil of wire and
mechanical energy. ammeter. (Coil has some resistance.)
Can we make electrical energy from mechanical Move a magnet near the coil.
energy, using magnets and wire? Yes! Current flows whenever magnet moves.
Key idea: (No current when magnet is still.)
Induction of current from changing magnetic flux. Big current when:
Governing equation: Magnet moves fast.
Faraday’s Law Magnet is close.
Coil has many turns.
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
Strength of voltage induced by
Magnetic flux
moving magnet
Magnetic flux is the key quantity for induction. Voltage in circuit comes from change in flux
through the coil.
Consider coil with area A and N turns of wire.
The strength of the voltage (“induced
If magnetic field B fills the coil, then the
electromotive force” or “emf”) is given by
flux ΦΒ is
Faraday’s Law:
ΦB = NBA. ΔΦ B
Vinduced = ,
(Note: B must be perpendicular to coil. If not,
Δt
use strength of B that is perpendicular to
coil.) or in words, “Induced emf equals rate of change
of magnetic flux.”
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
What has to move? 3. Electrical Generators
Induced emf depends only on the rate of change Magnetic induction means that mechanical
of magnetic flux. energy can be turned into electrical energy.
Can equally well Practical device: electrical generator.
move magnet, Key parts:
move coil, – magnet (permanent or electromagnet)
turn magnet, – rotatable coil (rotor)
or turn coil. – connections to ensure current comes from coil
always in the “right” way
– mechanical energy input
– electrical energy output
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
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3. Generator is motor Investigations with an
run backwards oscilloscope
Motor, generator parts are almost all the same. An oscilloscope is a way to study voltages that
may change with time.
Input and output are reversed.
It plots a graph of voltage vs. time, V(t).
Apply mechanical energy to rotor (e.g. turn it
by hand), and voltage is generated in coil. The voltage scales and time scales are
adjustable, to enable the graph to be clearly
Flux grows, then shrinks, as coil turns in B field.
read.
Just as flux would grow with opposite sign,
commutator switches contacts -- voltage out
always has the same sign. We can use it to look at V(t) out of a DC
generator.
“Direct current”, or DC.
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
“DC” from a generator Capacitor helps make V(t)
isn’t steady! more constant
Δ ΦB Put a large capacitor between the output
Vinduced =
.
terminals of the DC generator.
Δt
The bumps in V(t) smooth out. Why?
As the coil turns between the poles, there are
points in the cycle when flux is changing
rapidly, other places where it doesn’t change Charge is stored on the plates of the capacitor.
at all. It doesn’t all get to leak off before another cycle
Like a sine function, but “folded” so that it is starts. Hence, variations get smoothed out.
always positive.
This is an example of a filter.
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
Generator transforms mechanical
4. AC generator is even simpler
power into electrical power
Attach electrical load to generator, to use Split-ring commutator means voltage always
electrical energy. has the same sign (current always same
direction.)
Light a bulb, turn a motor, …
We could replace commutator with slip rings.
Electrical energy comes from mechanical energy
Each brush always touches same end of coil.
that turns the generator.
As coil turns, flux through coil changes sign.
You can feel the power you supply.
Induced voltage alternates sign.
Compare how hard it is to turn generator,
with light bulb attached vs. without bulb.
“Alternating current” or AC.
Generator needs mechanical power input.
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PHY 101 Lecture #11 Electromagnetic Generators and PHY 101 Lecture #11 Electromagnetic Generators and
Transformers Transformers
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