:
The continuous burning of an oil lamp requires small,
regulated and uniform supply of oil from the main mass of
oil to the wick, for which it has to rise against gravity to
the height where the flame is burning. this is facilitated
by the structure of the wick which has many small channels
in between the cotton fibers. these small channels have very
small diameter and according to the relation
,
the oil will rise to sufficient height up to the region of
the flame.
Ans: The absorption of water requires that water travel from the
wet places or substances to the cotton, which requires the
path. cotton is porous in nature because of which it has a
lot of small channels in it. Thus their radius is small and
according to the relation
,
water can travel to different regions as much as and where
required. This removes water from the bodies and take them
to cotton.
165. The fibers of a painting brush spread when dipped in a
liquid and cling together when taken out. Explain.
Ans:
The spreading of the brush in water is due to the upthrust
force. The fibers are weak enough and so the upthrust raises
them from their 'clung' state. However, when they are taken
out, most water particles drops from them. The remaining
water however act as bridges between the two fibers. Since
water has high surface tension, their surface is tight
enough. So they hold two fibers near enough. When this
phenomena spreads across all the fibers, all the fibers stay
near each other and so they appear clung to each other.
166. .
Ans:
167. Ploughing of the field helps in retaining the moisture of
the soil. Why?
Ans:
When the soil is not ploughed or is unattended, they become
tight, i.e. the soil particles are held close together with
a lot of thin channels in between. These channels will act
as capillary tubes and the liquid rises according to the
relation
.
Due to closeness of the particles, the radius of such
channels will be very small. The liquids will then rise all
the way to the surface and get evaporated away. In order to
prevent that the channels have to be wither widened or cut
off, the best way being ploughing.
168. Oil spreads on cold water; however, it may remain a
drop in hot water. Explain why?
Ans:
Water has high value of surface tension, the value is even
more in cold water, since surface tension increases with
reduction in temperature. when oil is placed on it, it will
feel a strong tight surface, and so spreads to achieve
uniformity. however, if water is hot, its surface tension is
less, that means the surface is now weak and soft. so when
oil is placed on it, the surface get depressed. Oil then can
not spread from that position, and remains as a drop or
globule.
169. Water remains cool in an earthen pot in summer. Explain
how?
Ans:
An earthen pot is made of soil particles tightened by baking
in kilns. So the particles are held enough close together
with a lot of thin channels in between. These channels will
act as capillary tubes and the liquid rises according to the
relation
.
Due to closeness of the particles, the radius of such
channels will be very small. The liquids will then rise all
the way to the surface and get evaporated away. In order to
prevent that the channels have to be wither widened or cut
off, the best way being 'Ploughing'.
170.
171. Sand appears drier than clay, why?
Ans:
Sand has, by nature large grains of particles. So the gaps
between their particles will also be wide. However, clay has
very fine particles and the gaps between them will also be
less. The gaps in sand will act as wide channels whereas
those in clay will act as narrow ones. Then according to the
relation
,
water will climb from deeper grounds to sufficient height in
clay, making them appear wet. In case of sand, this is not
possible and it will appear dry.
172. A steel needle may float on water but sinks when a small
amount of detergent is added to water. Why?
Ans: Water has high value of surface tension compared
to any other substance. So when steel needle is placed
slowly the surface of water holds it. However, if detergent
is added, their molecules come in between water molecules
and so the surface tension decreases. When the steel needle
is now placed, water surface can no longer hold it and it
sinks.
173. Hot soup tastes better than cold one. why?
Ans: The whole phenomena is based on how well soup (hot or
cold) spreads over the taste buds of the tongue. When
temperature is low, the surface tension of water is high, so
water molecules are held back, which prevents their reach to
the taste buds. This makes the taste incomplete. However,
increase of temperature reduces the surface tension. This
makes the water (soup) spread better, increasing their
access to the taste buts. So the taste is good and complete.
174. A beaker containing glycerin and another
containing water are stirred vigorously and placed on a
table, Which one comes to rest first?
Ans: As evident, glycerine has high coefficient of
viscosity compared to water. So Glycerine would come to rest
first.
175. Why do the clouds seem floating in the sky?
Ans: Clouds are actually not floating, like every other
substances, they are also falling. But the way they fall is
different. Cloud is basically made of tiny water drops plus
dust particles associated with them. However, because of
their small size, the terminal velocity attained by them is
also very low according to the relation
.
Such velocity is so less that for an observer down on the
earth, they just seem like floating.
176. Why does a flag flutter on strong wind?
Ans: Fluttering of flags means rising of the flag and
its subsequent flapping according to the motion of air. This
is due to the pressure difference created by the movement of
air over and below the flag. In general, velocity of air is
high at grater heights and less close to the ground
(remember velocity gradient).
Then according to the relation
(Bernoulli's Theorem),
,
higher velocity leads to low pressure. This gives low
pressure at height and high below, creating a resultant
upward force and pressure. So the flag is lifted from its
drifted position and start to flutter.
178.
179.
