Mechanical

1
Mechanical Energy
Electrical Energy
generators
Mechanical Energy
Electrical Energy
motors
The first occurs in power plants. The second
in many appliances used in homes, factories
and cars. The way they work involves MAGNETIC as
well as ELECTRICAL effects
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Originally, in the days of Benjamin Franklin and
Alessandro Volta, electricity and magnetism were
studied as separate subjects. However in the
first half of the nineteenth century, workers
including Ampere, Faraday and Maxwell discovered
that electricity and magnetism are closely
related phenomena. First consider, as you are
doing in the lab this week, what happens to a
current (flow of electrical charge) near a magnet.
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Direction of the force on The charges moving
through the magnetic field
N
S
Moving electrical charges (whether in a wire
or not)
Magnetic field from north to south pole of a
magnet,
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N
S
Things to note The force due to the magnet Is
perpendicular to the direction of motion of the
charges AND perpendicular to the magnetic field
which in this case is along a straight
line between the north and south poles of the
magnet.
The thick arrow shows the direction of the charge
current. (If it is electrons, then the
electrons go the other way.)
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N
S

• The effect of the force on the moving charges
• in this picture will be to
• Cause the moving charges to accelerate along
• their line of motion.
• b. Cause the moving charges to decelerate
• along their line of motion.
• c. Cause the beam of moving charges to bend up
• d. Cause the beam of moving charges to bend down.
• e. Cause the beam of moving charges to bend
  toward
• the north pole.

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Fig. 11-4b, p. 362
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If you understand this peculiar property of the
magnetic force on moving charges then you can
understand the basic principle by which a motor
converts electrical to mechanical energy.
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force
I
I
force
Fig. 11-7, p. 364
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• In this picture what are the commutator rings
  and
• stationary brushes for?
• To keep the connections to the battery clean.
• To screen the magnetic field so it doesnt
  interfere
• with the battery
• c. To switch the direction of of the current
  when the
• armature turns ½ way around.
• d. To conserve electricity.

Fig. 11-7, p. 364
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These forces on currents when they are in the
presence of magnetic fields explain how motors
convert electrical into mechanical energy. To go
the other way, we can take off The battery and
put an appliance or other Dissipative (power
drawing) circuit element In its place. Now turn
the shaft mechanically, Putting in mechanical
energy
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crank
v
Motion (up) of charges in wire makes force, hence
current this way (for q0)
B
I
I
force
Resistive load (appliance)
Fig. 11-7, p. 364
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crank
Resistive load (appliance)

• In this generator, if the crank is turned
  steadily, how will the voltage at the device
  depend on the time?
• It will be a constant (DC) voltage, not changing
  in time
• It will be an AC voltage, varying from plus to
  minus and back
• It will be always of the same sign, but varying
  from zero to a maximum value and back as the
  crank is turned
• It will be a constant current by time varying
  voltage.

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Now mechanical energy goes in through the crank
shaft and electrical energy comes out of the coil
and is dissipated in the load. There is a force
on the charges in the wire because you force them
to move through the magnetic field as you turn
the crank.
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Magnetic field produced by current. In these
descriptions of motors and generators. the
magnetic field was presumed to be produced by a
magnet. It is also possible to produce a
magnetic field by use of a current through a
wire. This was first demonstrated by Ampere and
will be illustrated in class. In the generators
you will be using in the lab, the magnetic field
wil be supplied by a magnet. However in
commercial generators, the magnetic field may be
produced in part by a current through a wire
which is wrapped around a magnetizable material.
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Electrical energy delivery system (the
grid) To understand some details concerning
this You need to understand how transformers
work. Transformers change the voltage between
the pairs of wires (or the triplet of wires)
which carry the current in the grid. They only
work when the current is AC (switching direction,
usually every 1/120 th of a second. The
principle by which they work is Faradays Law,
which is illustrated by the following
experiment which you did in lab and we repeat in
the lecture
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Ammeter measures current
At the start of this experiment, the charges in
the circuit are not moving (no battery). When
you move the magnet into the coil, the charges
start to move (are accelerated) and a current is
read on the ammeter. The changing magnetic field
in the coil causes a force on the charges in the
wire. (Faraday)The current (and the force)
change direction as you pull the magnet out. No
current (and no force) if the magnet is not
moving.
Fig. 11-9, p. 365
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Transformers The fact that a magnetic field can
be produced by a coil of wire Is used to
advantage in transformers, which change the
voltage of electricity as it passes through the
grid from power plant to user. Why would you
want to change the voltage? Suppose you are
passing a fixed amount of power P along a power
line with voltage drop V. The Current I is P/V
and if the line has resistance R, the heating of
the line is (IR)II2R(P/V)2R. Therefore THE
HIGHER THE VOLTAGE, THE LOWER IS THE HEATING LOSS
IN THE LINE.
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7200V
120 V
Fig. 11-16a, p. 372
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• In the picture the voltages shown are typical.
• What is true of the heat losses in the high
  voltage
• transmission line?
• They are the same as they would be if there
• were no transformer between power plant and line.
• b. The transmission line losses are reduced by
• 25,000/345,000 by the transformer.
• c. The transmission line losses are increased by
• (25,000/345,000)2 by the transformer.
• d. The transmission line losses are reduced by
• (25,000/345,000)2 by the transformer.

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Fig. 11-12, p. 368
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Transformers Therefore, engineers design the
voltage in long distance power lines to be very
high. However such voltages would be very
dangerous in your home. Therefore, transformers
are needed to convert the voltage of the
electric power from low (at the power plant ) to
high in the line carrying the electricity to your
neighborhood and back down to a safe voltage for
delivery to homes and offices,
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How transformers work The high voltage creates
a magnetic field by passing the electrical
current through a coil. This magnetic field,
which is changing in time because the circuit is
AC, is passed through another coil. By Faradays
law (which says that changing magnetic fields
produce forces on charges) the charges in the
other coil start to move. By adjusting the
number of turns in the two coils, one can control
the voltage of the output in the second coil.
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Fig. 11-14, p. 369
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In the portion of grid shown, what would be the
ratio of turns in a transformer between the power
plant and the transmission line? a.345000/25000
more turns on the plant side. b.345000/25000 more
turns on the line side. c. (345000/25000)2 more
turns on the plant side d. (345000/25000)2 more
turns on the line side
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Fig. 11-15b, p. 371
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www.howstuffworks.com/power.htm
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Summary of relevant properties of magnetic fields
and moving charges (currents). A moving charge
or current of them experiences a force in a
magnetic field which is perpendicular to the
direction of the field and the charge
velocity. direction is determined by the right
hand rule. A moving charge or current PRODUCES a
magnetic field (Ampere). A changing magnetic
field produces a force on a stationary charge,
causing the charge to move. (Faraday)
Motors and Generators
Transformers