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Chapter 15 : Some Natural Phenomena
In Class VII you read about winds, storms and cyclones. You learnt that cyclones can cause a lot of damage to human life and property. You also learnt that we can protect ourselves from these destructive phenomena to some extent. In this chapter we shall discuss two other destructive natural phenomena. These are lightning and earthquakes. We shall also discuss what steps we can take to minimise destruction caused by these phenomena.
15.1 Lightning
You might have seen sparks on a electric pole when wires become loose. This phenomenon is quite common when a wind is blowing and shacking the wires. You might also have seen sparks when a plug is loose in its socket. Lightning is also an electric spark, but on a huge scale.
15.2 Charging By Rubbing
Activity 15.1
Take a used ballpen refill and rub
it vigorously with a piece of
polythene. Bring it close to small
pieces of paper. Take care not to
touch the rubbed end of the refill
with your hand or with a metallic
object. Repeat the activity with small
pieces of dry leaf, husk and mustard
seeds. Record your observations.
15.3 Types Of Charges And Their Interaction
We will select some objects from Table
15.1 for the next activity.
Activity 15.3
a) Inflate two balloons. Hang them
in such a way that they do not touch
each other (Fig. 15.1). Rub both the
balloons with a woollen cloth and
release them. What do you observe?
15.4 transfer Of Charge
Activity 15.4
Take an empty jam bottle. Take a
piece of cardboard slightly bigger in
size than the mouth of the bottle.
Pierce a hole in it so that a metal
paper clip could be inserted. Open
out paper clip as shown in Fig. 15.4.
Cut two strips of aluminium foil
about 4 cm × 1 cm each. Hang them
on the paper clip as shown. Insert
the paper clip in the cardboard lid
so that it is perpendicular to it (Fig.
15.4). Charge a refill and touch it with the end of the paper clip.
Observe what happens. Is there any
effect on the foil strips? Do they
repel each other or attract each
other? Touch now, other charged
bodies with the end of the paper clip.
Do foil strips behave in the same
way in all cases? Can this apparatus
be used to detect whether a body is
charged or not? Can you explain
why the foil strips repel each other?
15.5 The Story Of Lightning
It is now possible to explain lightning
in terms of the charges produced by
rubbing.
You learnt in Class VII that during
the development of a thunderstorm, the
air currents move upward while the
water droplets move downward. These
vigorous movements cause separation
of charges. By a process, not yet
completely understood, the positive
charges collect near the upper edges of
the clouds and the negative charges
accumulate near the lower edges. There
is accumulation of positive charges near
the ground also. When the magnitude
of the accumulated charges becomes very
large, the air which is normally a poor
conductor of electricity, is no longer able
to resist their flow. Negative and positive
charges meet, producing streaks of
bright light and sound. We see streaks
as lightning (Fig. 15.5). The process is
called an electric discharge.
15.6 Lightning Safety
During lightning and thunderstorm no
open place is safe.
1. Hearing thunder is an alert to rush
to a safer place.
2. After hearing the last thunder, wait
for some time before coming out of
the safe place.
15.7 Earthquakes
You just learnt about thunderstorm and lightning. In Class VII you learnt about cyclones. These natural phenomena can cause large scale destruction of human life and property. Fortunately, these phenomena can be predicted to some extent. The weather department can warn about a thunderstorm developing in some area.
