1. NATURAL
PHENOMENA

2.  Movement of electrons through a
conductor is called electricity.
 Electricity used in electrical
appliances is called current
electricity. It flows through wires.
 Electricity which does not flow is
called static electricity.

3.  Benjamin Franklin was an American
scientist who did his well known kite
experiment in the year 1749.
 He performed his experiment by
using a kite made of silk to which a
small metallic key was attached.
 He flew the kite into the clouds and
waited for lightning to strike.

4.  When lightning struck, the key was
found to have become hot.
 The key was also found to have got
electrically charged.
 Franklin concluded that the thunder
clouds must be electrically charged.

5.  Thus, in a way, in his experiment he
had drawn the electricity of the
atmosphere into the key.
 He also demonstrated directly that
lightning is associated with
electricity.

6.  Dr.Gilbert showed that amber
rubbing with cat’s skin and glass rod
rubbing with silk, it develops a
properties of attracting tiny bits of
dry paper, dry straw and also
develops similar properties when
rubbed against each other.

7.  When two such objects are rubbed
together and separated, they both
acquire the property of attracting
light objects. we call such objects
having this property, to attract or
repel as charged objects.
 Any object which does not have this
ability is referred to as an uncharged
objects.

8.  The process of charging up an object
is referred to as electrification.
 Therefore, Thales may be regarded
as having discovered that “electricity
can be produced by rubbing or by
friction”.

9.  A charged comb (rubbed against your
dry hair) can attract small pieces of
dry paper.
 A charged pen (rubbed against a dry
cotton T-shirt) can attract tiny bits
of dry paper.
 A charged plastic board (rubbed
against woollen cloth ) attracts
popcorns.

10.  Benjamin Franklin found that there
are two kinds of electrical charges,
A) Positive (+ve)
B) Negative (-ve)
 Like Charges repel each other unlike
charges attract each other.

11.  Two charged ebonite rods repel each
other.
 Two charged glass rods repel each
other.
 A charged ebonite rod and a charged
glass rod attract each other.

12.  Thunder is what you hear, lightning is
what you see, but both are due to
electrical discharge in the
atmosphere.

13.  They can seem to be separated in
time to an observer (you see the
lightening before you hear the
thunder) because of the large
difference in the speed of sound and
the speed of light.

14.  If you are significant distance away
(a few miles or kilometers), the light
from the discharge will reach you
almost instantly, but the sound can
take several seconds to arrive.

15.  In fact you can measure the distance
from you to the lightening in this
way: for each second delay between
seeing it and hearing it, the
lightening bolt is approximately 300
meters, or 1000 ft away.

16.  Sequence of steps are responsible
for these natural phenomena.
a) The +ve & -Ve charges present in
thunder cloud get separated. This
separation occurs in a way that is
similar to the separation that occurs
due to the rubbing of one object
against other.

17. The +ve charge move to the upper edge of
the cloud & the –ve charge move to the
lower edge of the cloud.
B) Because of this collection of –ve charges
near the bottom of the cloud, the earth
becomes +ve charged with respect to the
cloud by a potential of the order of as
much as 100 million volts.

18. c) This difference between the cloud and
earth ’breaks’ the air into + & -ve charges
or ions.
d) The huge –ve charges present near the
bottom of the clouds flows through the
conducting air down towards the earth.
If any tree or building comes in the path of
this flowing current, this object gets
burnt or seriously damaged. Then we say
that lightning has struck the object.

19. F) The lightning stroke through the air,
heats it to a very high temperature.
In fact, the air then momentarily
acquires temperature as high as
30,000 c.
 This rapid heating followed by a very
rapid cooling of air produces a shock
wave in it. It is this which results in a
thundering sound.

22.  Lightning is the result of the flow of
large amount of charges between two
(oppositely charged) clouds or a cloud
and the earth.
 A huge amount of electrical energy
get discharged during this flow.
 This heat up the air to very high
temperatures.

23.  This, in turn, produces the flashes of
lightning that we see in the sky and
accompanying thundering sound.
 It has been estimated that if this
interval is 30 seconds or less, ’the
lightning’ can pose a serious danger
to the area where it occurs.

24.  It has also been suggested that one
should continue to stay in one’s
shelter for almost 30 minutes after
observing the last flash of lightning.
 This is sometimes referred to as the
the 30-30 principle of safety against
Lightning.

27.  A metallic conductor fixed on high
rise buildings to protect them from
lightning by conducting the
atmospheric electricity into the
earth.
 The lower end of the metal is
connected to a large copper plate
which is buried inside the earth is
called earthing.