Question

Give scientific reasons.

(a) An ordinary glass bottle cracks when boiling water is poured into it, but a borosil glass bottle does not.
(b) The telephone wires which sag in summer become straight in winter.
(c) Dew drops form on the grass in winter.
(d) In winter, why does an iron pillar feel colder than a wooden pole ?

Answer 1

(a) Normal glass is an insulator which means it is a poor conductor of heat. When boiling water is poured into a glass tumbler, the surface of the glass which is in contact with the hot water heats up and expands as per its coefficient of thermal expansion. But the outer layer of the glass remains cold and does not expand (since glass is a poor conductor and does not transfer heat quickly). This difference in thermal expansion causes thermal stress which eventually leads to cracking.
Borosil glasses have very low coefficients of thermal expansion which makes them resistant to thermal shock, more than any other common glass. So, Borosil glass does not crack when hot liquid is poured in it, but normal glass cracks.

(b) In summers, the surrounding temperature is very high. Due to this high temperature, the metallic telephone wires are heated up and thus expands in length. Due to this, they sag down.
In winters, the surrounding temperature is very low. Due to this low temperature, the metallic telephone wires are cooled down and thus contracts in length. Due to this, they become straight.

(c) In winters, the temperature of the air near the grass decreases to dew point. Thus, the air becomes saturated with water vapour. As a result, water vapour condenses into tiny water droplets which appears on the surface of the grass.

(d) An iron pillar feels colder than a wooden pole in winter because the thermal conductivity of iron is very high as compared to wood. So, when you touch wooden pole, heat from your finger warms the wooden pole just at that place but isn't conducted away. But for iron pillar, the heat conducts away rapidly from your finger to the whole pillar and your finger then keeps trying to supply more heat until it is as cold as the iron. So, the iron pillar feels colder because it doesn't warm under your finger like the wooden pole.