141. Explain why is steel or iron used widely in constructing bridges?
This phenomenon is related to the yielding stress. .
Ans: The function of a pen is to ensure a uniform flow of ink to the tip of pen for proper writing. In addition to uniformity, it should be extremely slow for fine writing. The major component of ink is water which wets other substance with its high adhesive capacity and surface tension.
When a split is made, it resembles a capillary tube. Since the split has a small separation, it resembles small ‘r’. So liquid slides along the tube away from the mass to other locations according to the equation . This will carry the ink to enough extent, i.e. up to the tip (large ‘h’ due to small ‘r’).
Ans: The readings in a mercury barometer is the height of mercury the atmospheric pressure can hold up. Mercury is exerting a downward force (equal to its weight) whereas atmospheric pressure tries to counter it (hold it up). But at the surface, the phenomena of surface tension acts extra force on the mercury mass. Mercury is a substance which has high cohesive force, i.e. it wants to cling to itself. So there is a net downward force on the surface at the very top, which slightly reduces the height recorded. The reduction in height due to surface tension is given by the relation . So the overall reading will be slightly reduced. (The reduction will however be very small since the density of mercury is very high).
Ans: When heated, the glass tube melts slowly, changing into liquid state. The surface then is acted by the surface tension force (a characteristic of liquids) which makes the molecules be limited and as packed as possible. This makes it take as minimum volume possible. This is possible only if the surface becomes spherical, i.e. round.
Ans: The process is impractical for two reasons. One is that there is the need for a very long tube if water is used (10.3 m for water instead of 0.76 m in case of mercury). The other is the action of surface tension. In case of mercury the net downward force due to surface tension is downwards which reduces the reading slightly (because of high density). In water, however, there is a net upward force. The increase in this height due to surface tension is more because of less density of water. So the reading will be very high than the actual readings.
A pin is usually made of metals which definitely has a
higher density than water. So theoretically, it should sink.
However, when it is slowly placed on water surface, the
surface first depresses slightly, causing it to extend and
become tight slightly. So it comes under tension and, which
develops s tendency to return to initial state. This causes
a resultant upward force. Since water has high value of
surface tension than others, the resultant force is also
high and so it can overcome the effect of gravity which
holds the pin up. However, if it is placed abruptly, the
surface breaks and in no way that it can hold up the pin.
Surface tension is due to mutual attraction of molecules, specially the molecules of the surface in overall by the inner molecules. when the temperature rises, the molecules show a higher degree of motion because of increased kinetic energy. This takes the molecules farther away from each other, causing an increase in the distances in between them. So, the attractive forces are slightly reduced. the effect of this is the reduction of attraction inside as well as at the surface. So the surface tension decreases.
Ans: Water is a substance with very high surface tension, i.e. its surface will act as a tough layer for any other substance placed on it. When camphor is placed on it, camphor spreads on it rapidly (with a very very high velocity compared to others). The phenomena of spreading is random and not uniform. When they spread, they give opposite force on the camphor piece. In addition, the faces of camphor is very randomly oriented, So when they spread and wherever they spread, the camphor will move in opposite direction, causing very very random motion.
150. How does a 'towel' work?
Towel is made of cotton or any other fabric, which consists of very thin fibers interwoven on each other. This causes the formation of a lot of thin channels in between them which act as the capillary tubes. So when a towel is placed over wet surface, water climbs into these channels according to the relation
Ans: Chalk consists of a lot of fine pores in it which are interconnected with each other to form thin channels. So when it is immersed in water, water travels through the channels according to the relation
Ans: When water drops are smaller, the surface tension forces along the surface, produce overall resultant forces towards the center. This presses and packs the molecules as inward as possible. This results in their assumption of a spherical shape. At that time, since the mass involved is more, the force of gravity (i.e. the downward force) is less.
However, when the drop is large, the surface tension has to hold more amount of molecules, which is itself tougher than in case of small drops. Besides, the mass is now large, so the force of gravity will be high. The molecules, instead of being directed towards the center also receive a downward force. so some molecules slide downwards, giving the whole drop a flatter shape.
Ans: The phenomena of washing clothes is related to the access of water to dirt and other unwanted substances and carry them away. This requires that water does not block itself and others because of its high surface tension. This requires that the surface tension of water be reduced, which can be achieved by adding some foreign substances as detergents and soap. Besides, the increase of temperature also decreases the surface tension, so water can reach every region and clean them.
This phenomena has much to do with the dominance of the
adhesive or the cohesive forces. When water is placed on the
glass surface, the adhesive forces dominate over the
cohesive and so glass will try to attract as much water as
possible. So water spreads easily. on the other hand,
mercury has a high value of cohesive force compared to the
adhesive force, and so tries to avoid glass as much as
possible. Due to which it takes up a round shape in the form
Water is one of the substance having greatest value of
surface tension, for which the attraction between their
molecules play the main role. When any foreign substance is
added, they come in between water molecules, which decreases
the overall attractive force between them. This reduction is
seen at the surface also and so the surface tension also
decreases. Addition of detergents and soaps in water takes
advantage of this phenomenon.
Ans: No, and never. The phenomenon is cleared by the following treatment.
These relations show that the radius of curvature of the meniscus depends on the height available for it to expand. Less the height available, more is the radius of curvature of the meniscus. The maximum value 'R' can achieve is infinite, when the height is 0, i.e. the glass tube is taken to the level of the liquid. Below that there is no question of overflowing.
Ans: No, the phenomenon is more physical rather than chemical. Water has high surface tension whereas petroleum has less. So when they are poured on water they spread easily and completely on water. This will make them form a uniform but thin layer over water. When insects come to land on them or the eggs fall on the surface, they get drowned because such layers can not hold them (the reason is less surface tension). Once they are drowned, the oxygen supply is cut off and this kills them.
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