The full explanation of
sparking
(from Van de Graaff generator, hair and comb or 'lightning' - its
all the same!)
You cannot get a visible spark in a
vacuum, because there are no particles to excite to the point of emitting
light.
Ignoring the uncharged neutrons, atoms
contain positive nuclear protons and surrounding negative electrons.
At room temperature, unless a chemical
reaction is taking place, everything is electrically balanced, and atoms and
molecules are stable, including those of air.
Under these circumstances,
air is an
electrical insulator BUT ....
However, if there is a powerful
electrical field (more
on fields in previous section), the very high potential
difference causes the atoms and molecules in air to
ionise and break up into
positive ions and negative electrons.
The electric field is so powerful it
removes an outer electron from the atom or molecule - the process
of ionisation.
The formation of ions increases the
electrical conductivity of air, so the discharge - flow of current,
rapidly accelerates.
The charged particles come together
discharging the electrical energy, heating the air and exciting the air
particles so much they emit visible light.
The electrons of the excited atoms or
molecules fall back down to their original energy levels emitting
visible photons of light in the process - so we see a spark of light.
You cannot see a visible spark in a
vacuum because there are no atoms or molecules to be excited and ionised to
the extent that they will emit energy as visible light photons.
Charge movement or not?
Under normal circumstance the potential
difference between the earth and any object is 0V (p.d. of zero volts). However,
If electric charge increases on an object the p.d. between the object and the
earth rises above 0V. If the potential difference is great enough,
electrons can leap across the gap between the charged object and the earth.
The electrons, effectively an electrical
current, can jump the gap to any conductor in the vicinity that is earthed (in
contact with the ground), though for small p.d. voltages the gap must be small.
e.g. (i) If static charge has built up on
an article of clothing, as you remove it the electrons can move through your
body to earth via your hands and you feel a 'crackling' shock and maybe see
some sparks of light!
(ii) As a car is moving along the
friction between the car body and the air can cause the build up of static
charge because the car body is insulated from the earth by the rubber tyres.
As you get out of the car and touch the metal body the static electricity
can be discharged through your body to earth and you experience a small
electrical shock!
(iii) The Van de Graaff generator
experiments have already been described.
In the case of
lightning you are dealing with
a massive build up of static charge in clouds, and the potential difference
between the earth and atmosphere is very large. Consequently lightning strikes
('huge sparks') occur across some pretty big gaps between the earth and clouds
with spectacular visual results!
The static charge in clouds is caused by
ice particles bumping against each other and becoming charged by friction.
Lighter positive ice particles accumulate at the top of the cloud and heaver
negative ice particles collect lower down.
When the number of charged
particles increases, at some point, you get a massive discharge of
electricity as the oppositely charged particles come together to give a
giant spark - a flash of lightning.
Sometimes the clouds of negative particles induce a positive charge on the
ground and then discharge takes place from cloud (-ve) to ground (+ve), and
this is a lightning strike (diagram on the right).
If a negatively charged thunder cloud
passes over, it induces a positive charge on the nearest object on the
ground e.g. a church spire lightning conductor, and, on the bottom end
of the lightning conductor, a negative charge on the copper earth plate.
The electrons travelling down the
copper lightning conductor and rapidly dissipated safely into the
ground.
Unfortunately this positive charge
builds up on any tall objects nearer the cloud's charge e.g. like tall buildings or trees,
and that's why in a thunderstorm you should not take refuge under a tall
tree - its nearer than you to the charge cloud!
Being inside a car does protect you because the electrical discharge
can run through the car body to earth, but avoid touching the car body until
the event has passed!
So, sparks occur when there is a big enough
potential difference between a statically electrically charged object and any
object that is connected to earth (an 'earthed object').
This high p.d. creates
a strong electric field between the two objects ('charged' and 'earthed').
In industry, and the home to, you can use of insulating mats
and using
shoes with insulating soles if there is a danger of an electric discharge
through your body.
If the electric field effect is strong
enough, that is if the potential difference is great enough, electrons can be
remove from particles in the air, a process called ionisation and
positive particles are formed (+ve ions).
Air is a good electrical
insulator but the presence of these ions makes it a better conductor so an
electrical current can flow.
This can be sometimes be seen as a spark because
some of the electrical energy is converted to heat and light energy.
I've written more about these examples and
how to counter the effects of static electricity in the final section of the page.
If any object can be connected to the
'earth', by e.g. a conductor such as a copper wire or strip, then any build up
of static electricity can be safely discharged.
This is called earthing
the object.
You can get an electrostatic shock if you
are electrically 'charged' yourself and you touch something that is earthed,
so the static electricity runs through you.
Similarly if you yourself are
earthed and you touch something that is charged, you can also get an
electric shock.
That's why household circuits and electrical appliances
should be earthed for your safety and protection.
