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better word), and the potential
energy it contains is called the
potential difference when it refers to a
difference in energy, or pressure,
between two points. You can look
on volts as the equivalent of water
pressure.
Another (older) name for volts is
electromagnetic force, or emf. It is
measured with a voltmeter, which you
might use to check the state of your
battery before you started a jet
engine.
Resistance
Even a good conductor slows
electrons down. The longer and
thinner the wire is, the more the
opposition, called resistance, expressed
in ohms. 1 ohm allows 1 amp to flow
when 1 volt is applied.
All this work causes heat, due to the
friction of electrons moving against
each other, and the more work you
make electricity do, the hotter things
get, which is how electric fires work.
If you make it work harder, you get
light as well, hence light bulbs.
When you start using AC, however,
the current flows on the outside of the
cable, increasing the resistance
(many times) because the effective
cross-sectional area is reduced,
which is called the skin effect.
Magnetism
A magnet is a substance, typically a
soft iron bar, that has lines of force
running through and around it (the
Earth is a magnet as well). All
magnets have a North and a South
pole, and two North poles will repel
each other. North and South poles
attract each other. If you therefore
had a bar magnet, its South Pole
(traditionally red) would point
towards the Earth's (magnetic)
North pole. This is what a compass
is all about, discussed more fully
under Instruments. The thing to
118 Canadian Private Pilot Studies
remember, though, is that the South
Pole is marked as North.
Magnetic reluctance is the ability of a
substance to pass lines of flux within
itself. Hard iron in this respect will
not pass flux easily, so it has a high
reluctance and is therefore not easily
magnetised. Magnetic permeability is the
opposite, characterised by soft iron,
which is easily magnetised.
A magnetic field moving round a
conductor will induce an electrical
field in the conductor – the more the
flow, the bigger the field, which
follows the direction of flow. You
can get the same effect by moving
the conductor in the field, or by
moving them both together at the
same time.
This works in reverse as well – a
flow of electricity in any conductor
produces a magnetic field around it.
If you put a coil of wire round a soft
iron bar, and run electricity through
it, the iron will become a magnet for
as long as the current flows.
During engine operation, the battery
will be recharged with either a
generator or an alternator, based on DC
or AC, respectively, which will put
out more voltage than the battery
(typically 28v in a 24v system, or 42v
in a 36v system) to make sure the
battery doesn't drive the generator.
The alternator will charge at low
RPM, but some helicopters, notably
the Bell 206, use a starter/generator to
save space, despite this advantage
(the same unit is used to spin the
engine on startup, and switched over
when it's running to become a
generator). If an alternator were
used, you would need yet another
item attached to the engine.
Generators therefore have the
disadvantage of not producing lots
of electricity at low engine RPM
(exam question).
Direct Current
As previously mentioned, this
electricity that goes in one direction
only. It can be produced in many
ways, such as friction, heat, pressure and
photoelectricity, but we are concerned
with magnetism and chemical action (see
Batteries, above)
The Generator
These use magnetism to create DC.
A coil of wire round soft iron core
(an armature) is spun in a magnetic
field to cause a current to flow in
them - in fact, the current starts off
as AC and is converted with a
rectifier. The lines of magnetic force
either come from a permanent
magnet or an electromagnet formed
from the generator’s own current (in
which case it is called self-excited,
which can happen in the first place
through residual magnetism). When the
generator's field is provided through
the Master Switch, generator voltage
will drop to zero when the switch is
turned off (Master Switches are
mainly there for alternators, which
are not allowed to produce current
without a battery in the circuit).
　
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