曝光台 注意防骗
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by one leaving), is balanced
immediately. An atom with one extra
is negatively charged, and with one
missing is positively charged. This
process is called ionisation, because an
atom wrongly charged is called an
ion, which we will come across latter
when we look at the ionosphere, or
what is now called the mesosphere.
Some components, like transistors,
depend on the movement of
electrons or holes (missing electrons)
one way or the other.
Electrons spinning round an atom
occupy energy levels, or shells, rather
like the orbits of the planets around
the Sun. The first shell can hold up
to 2 electrons, and the second up to
8, but it’s always the outer shell,
which contains valence electrons that is
important. Such atoms can be
dislodged easily by applying stress in
the form of heat or a magnetic field,
which is how an electrical current is
produced.
Electricity
There are three types of electricity:
· That which stays right where it
is, called static electricity.
· That which goes in one
direction only, usually at one
speed, called Direct Current.
· That which flip-flops back and
forth to form a wave pattern, or
Alternating Current.
The essential point is movement, since
nothing much happens when
everything is still, but it is the last
one, AC, with which we will mainly
be concerned.
Batteries
Certain chemicals when combined
with metals can cause electrons to
flow as direct current, until all the
electrons disappear from the metal,
causing it to eventually get eaten
away – since the atoms comprising it
lose electrons, they cease to be the
same atoms and therefore cease to
exist in their former state – if you
could contrive to put the electrons
Electricity & Radio 115
back, you would regain your metal
plate, and recharge the battery.
Actually, a battery is a collection of
cells, which typically have a charge of
about 2 volts each when it comes to
aircraft, hence the need to combine
them in order to do anything useful.
Knowing how to do this is handy
when you are out in the field with a
discharged battery and you are trying
to connect two car batteries together
to start your aircraft.
The polarities are positive and negative,
marked as plus (+) or minus (-), or red
and black. If you join batteries in
series, that is, one after the other, with
the positive of one connected to the
negative of the next:
you will get a voltage which is the
sum of the cells, but with the
amperage rating of one cell. If you
join them in parallel, with the positive
and negative terminals connected to
each other:
you would get the voltage of one
cell, but the amperage of all of them
(see above).
Since a typical aircraft runs on a 24-
volt system, you would therefore
connect two car batteries in series. Be
aware, though, that the terminals are
different sizes to stop them being
confused with each other, so you
need to carry an adapter in your
navbag to connect them up in the
middle (try Halfords). Be wary of
jumper cables, as they may open up
and spark when a load is applied.
Ensure that batteries have a load on
them before completing a circuit.
A battery cell is made up of electrodes
surrounded by electrolyte. Different
materials are better or worse at this
job, so you might get more or less
voltage out of one type of battery or
another, but the most common is
zinc-carbon (there are also alkaline,
mercury, NiCad, Lead-Acid,
Lithium, etc.). Electrons flow from
the negative (-) electrode, through
whatever circuit the battery is
connected to, back to the positive (+).
Primary cells are not rechargeable,
but secondary ones are.
A flat battery has maximum internal
resistance, which will generate lots of
heat when the aircraft alternator or
generator attempts to charge it (on a
bench, only a very small current is
used). It is therefore not a good idea
to continue flight if your battery gets
discharged – in any case, it should be
replaced before the next flight.
The battery itself will be rated in
terms of Amp/Hours, meaning that
it’s supposed to provide a certain
number of amps for a certain
number of hours when fully charged,
though it is never wise to rely on any
battery for more than about 20
116 Canadian Private Pilot Studies
minutes (officially, they should last
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