Lightning
most known and
powerful display of electrostatics in nature is a lightning storm. Lightning
storms are inescapable from humankind's attention. They are never invited,
never planned and never gone unnoticed. The rage of a lightning strike will
wake a person in the middle of the night. They send children rushing into
parent's bedrooms, crying for assurance that everything will be safe. The fury
of a lightning strike is capable of interrupting midday conversations and
activities. They're the frequent cause of canceled ball
games and golf outings. Children and adults alike crowd around windows to watch
the lightning displays in the sky, standing in awe of the power of static
discharges. Indeed, a lightning storm is the most powerful display of
electrostatics in nature.
In this part of Lesson 4, we will ponder two questions:
· What is
the cause and mechanism associated with lightning strikes?
· How do
lightning rods serve to protect buildings from the devastating effects of a
lightning strike?
Static
Charge Buildup in the Clouds
The scientific community has long pondered the cause of
lightning strikes. Even today, it is the subject of a good deal of scientific
research and theorizing. The details of how a cloud becomes statically charged
are not completely understood (as of this writing). Nonetheless there are
several theories that make a good deal of sense and that demonstrate many
concepts previously discussed in this unit of The Physics Classroom.
The precursor of any lightning strike is the polarization of
positive and negative charges within a storm cloud. The tops of the storm
clouds are known to acquire an excess of positive charge and the bottoms of the
storm clouds acquire an excess of negative charge. Two mechanisms seem
important to the polarization process. One mechanism involves a separation of
charge by a process that bears resemblance to frictional charging. Clouds are known to contain
countless millions of suspended water droplets and ice particles moving and
whirling about in turbulent fashion. Additional water from the ground
evaporates, rises upward and forms clusters of droplets as it approaches a
cloud. This upwardly rising moisture collides with water droplets within the
clouds. In the collisions, electrons are ripped off the rising droplets,
causing a separation of negative electrons from a positively charged water
droplet or a cluster of droplets.
The second mechanism that contributes to the polarization of
a storm cloud involves a freezing process. Rising moisture encounters cooler
temperatures at higher altitudes. These cooler temperatures cause the cluster
of water droplets to undergo freezing. The frozen particles tend to cluster
more tightly together and form the central regions of the cluster of droplets.
The frozen portion of the cluster of rising moisture becomes negatively charged
and the outer droplets acquire a positive charge. Air currents within the
clouds can rip the outer portions off the clusters and carry them upward toward
the top of the clouds. The frozen portion of the droplets with their negative
charge tends to gravitate towards the bottom of the storm clouds. Thus, the
clouds become further polarized.
These two mechanisms are believed to be the primary causes of the
polarization of storm clouds. In the end, a storm cloud becomes polarized with
positive charges carried to the upper portions of the clouds and negative
portions gravitating towards the bottom of the clouds. The polarization of the
clouds has an equally important effect on the surface of the Earth. The cloud's electric field stretches
through the space surrounding it and induces movement of electrons upon Earth.
Electrons on Earth's outer surface are repelled by the negatively charged
cloud's bottom surface. This creates an opposite charge on the Earth's surface.
Buildings, trees and even people can experience a buildup of
static charge as electrons are repelled by the cloud's bottom. With the cloud
polarized into opposites and with a positive charge induced upon Earth's
surface, the stage is set for Act 2 in the drama of a lightning strike.
The
Mechanics of a Lightning Strike
As the static charge buildup in
a storm cloud increases, the electric field surrounding the cloud becomes
stronger. Normally, the air surrounding a cloud would be a good enough insulator to prevent
a discharge of electrons to Earth. Yet, the strong electric fields surrounding
a cloud are capable of ionizing the surrounding air and making it more
conductive. The ionization involves the shredding of electrons from the outer
shells of gas molecules. The gas molecules that compose air are thus turned
into a soup of positive ions and free electrons. The insulating air is
transformed into a conductive plasma. The ability of a storm cloud's electric
fields to transform air into a conductor makes charge transfer (in the form of
a lightning bolt) from the cloud to the ground (or even to other clouds)
possible.
A lightning bolt begins with the development of a step leader. Excess electrons on the bottom of the cloud begin a journey through
the conducting air to the ground at speeds up to 60 miles per second. These
electrons follow zigzag paths towards the ground, branching at various
locations. The variables that affect the details of the actual pathway are not
well known. It is believed that the presence of impurities or dust particles in
various parts of the air might create regions between clouds and earth that are
more conductive than other regions. As the step leader grows, it might be
illuminated by the purplish glow that is characteristic of ionized air
molecules. Nonetheless, the step leader is not the actual lightning strike; it
merely provides the roadway between cloud and Earth along which the lightning
bolt will eventually travel.
As the electrons of the step leader approach the Earth, there
is an additional repulsion of electrons
downward from
Earth's surface. The quantity of positive charge residing on the Earth's
surface becomes even greater. This charge begins to migrate upward through
buildings, trees and people into the air. This upward rising positive charge - known
as a streamer - approaches the step leader in the air above
the surface of the Earth. The streamer might meet the leader at an altitude
equivalent to the length of a football field. Once contact is made between the
streamer and the leader, a complete conducting pathway is mapped out and the
lightning begins. The contact point between ground charge and cloud charge
rapidly ascends upward at speeds as high as 50 000 miles per second. As many as
a billion trillion electrons can transverse this path in less than a
millisecond. This initial strike is followed by several secondary strikes or
charge surges in rapid succession. These secondary surges are spaced apart so
closely in time that may appear as a single strike. The enormous and rapid flow
of charge along this pathway between the cloud and Earth heats the surrounding
air, causing it to expand violently. The expansion of the air creates a
shockwave that we observe as thunder.
Lightning
Rods and Other Protective Measures
Tall buildings, farmhouses and other structures susceptible to lightning
strikes are often equipped with lightning rods. The attachment of a grounded lightning rod to
a building is a protective measure that is taken to protect the building in the
event of a lightning strike. The concept of a lightning rod was originally
developed by Ben Franklin. Franklin proposed that lightning rods should consist
of a pointed metal pole that extends upward above the building that it is
intended to protect. Franklin suggested that a lightning rod protects a
building by one of two methods. First, the rod serves to prevent a charged
cloud from releasing a bolt of lightning. And second, the lightning rod serves
to safely divert the lightning to the ground in event that the cloud does
discharge its lightning via a bolt. Franklin's theories on the operation of
lightning rods have endured for a couple of centuries. And not until the most
recent decades have scientific studies provided evidence to confirm the manner
in which they operate to protect buildings from lightning damage.
The first of Franklin's two proposed theories is often
referred to as the lightning dissipation theory. According to
the theory, the use of a lightning rod on a building protects the building by
preventing the lightning strike. The idea is based upon the principle that the electric field
strength is great around a pointed object. The intense
electric fields surrounding a pointed object serve to ionize the surrounding
air, thus enhancing its conductive ability. The dissipative theory states that
as a storm cloud approaches, there is a conductive pathway established between
the statically charged cloud and the lightning rod. According to the theory,
static charges gradually migrate along this pathway to the ground, thus
reducing the likelihood of a sudden and explosive discharge. Proponents of the
lightning dissipation theory argue that the primary role of a lightning rod is
to discharge the cloud over a longer length of time, thus preventing the
excessive charge buildup that is
characteristic of a lightning strike.
The second of Franklin's proposed theories on the operation
of the lightning rod is the basis of thelightning diversion theory. The lightning
diversion theory states that a lighting rod
protects a building by providing a conductive pathway of the charge to the
Earth. A lightning rod is typically attached by a thick copper cable to a
grounding rod that is buried in the Earth below. The sudden discharge from the
cloud would be drawn towards the elevated lightning rod but safely directed to
the Earth, thus preventing damage from occurring to the building. The lightning
rod and the attached cable and ground pole provide a low resistance pathway
from the region above the building to the ground below. By diverting the charge
through the lightning protection system, the building is spared of the damage
associated with a large quantity of electric charge passing through it.
Lightning researchers are now generally convinced that the
lightning dissipation theory provides an inaccurate model of how lightning rods
work. It is indeed true that the tip of a lightning rod is capable of ionizing
the surrounding air and making it more conductive. However, this effect only
extends for a few meters above the tip of the lightning rod. A few meters of
enhanced conductivity above the tip of the rod is not capable of discharging a
large cloud that stretches over several kilometers of
distance. Unfortunately, there are currently no scientifically verified methods
of lightning prevention. Furthermore, recent field studies have further shown
that the tip of the lightning rod does not need to be sharply pointed as Ben
Franklin suggested. Blunt-tipped lightning rods have been found to be more
receptive to lightning strikes and thus provide a more likely path of discharge
of a charged cloud. When installing a lightning rod on a building as a
lightning protection measure, it is imperative that the rod be elevated above
the building and connected by a low resistance wire to the ground.
Check Your Understanding
Use your understanding to answer the following questions.
When finished, click the button to view the answers.
1. TRUE or FALSE:
The
presence of lightning rods on top of buildings prevents a cloud with a static
charge buildup from releasing its charge to
the building.
Answer: FALSE
Contrary
to a commonly held belief, a lightning rod does not serve to prevent a
lightning bolt. The presence of the rod on the building can only serve to
divert the charge in the bolt to the ground through a low resistance pathway
and thus protect the building from the damage which would otherwise result.
2. TRUE or FALSE:
If
you place a lightning rod on top of your home but failed to ground it, then you
home would still be safe in the unlikely event of a lightning strike..
Answer: False
The presence of
an elevated lightning rod could serve to draw charge from the cloud to the
ground. In the event of a lightning strike, a bolt would likely select a path
from the cloud that ultimately connects to the rod. If the rod is not grounded,
then the charge would likely pass through the home during its journey to the
ground. The intense heat associated with the lightning bolt would cause severe
damage and possibly cause an explosion or a fire. In the end, it would have
been better to not even have installed a lightning rod than to have installed
one that is not grounded.