Short Question Answers
1. Why do clocks not keep correct time in summer and winter? Why is the invar used in making clock pendulum?
Ans: we know that T = 2π√(l/g). In summer, the length of the pendulum increases due to thermal expansion, and hence time period increases. The clock thus runs slower. The case is reversed in winter. Invar (an alloy of nickel and steel) has an extremely small expansion. So the pendulum clock is provided with invar pendulum to give the correct time.
2. Two metallic rods of the same material but of different lengths are heated. A smaller rod has a circular area of cross-section but a large rod has a rectangular cross-section. Will their linear expansivity be the same or different? Give justification for your answer.
Ans: The coefficient of linear expansivity of a material is given as
α = ∆l / l1(θ2-θ1) where (θ2 ̶ θ1) is change in temperature
The ratio ∆l/l1 is same in each case for a material. Hence, a smaller rod with a circular area of cross-section and a bigger rod of the rectangular cross-section have the same linear expansivity.
3. Two identical rectangular strips of copper and the steel are pivoted to form bimetallic strips. What will happen on heating?
Ans: The coefficient of linear expansion of copper is more than steel and hence on heating, the bimetallic strip will bend in such a way that copper strips remain outside, i.e. convex side.
4. Explain why a beaker filled with water at 4◦c overflows if the temperature is decreased or increased.
Ans: The reason is the anomalous expansion of water. The maximum density of water occurs at 4◦c or cooled below 4◦c. Because of an increase in volume, water overflows out of beaker.
5. Define the coefficient of cubical expansion of a solid and hence write an expression for the variation of its density with temperature.
Ans: The coefficient of cubical expansion of a solid is defined as the change in volume of solid per unit volume per degree change in temperature.
6. Does the cubical expansivity of a liquid depend on its original volume? Explain.
Ans: Here, the cubical expansivity of a liquid depends only on the ratio of change in volume which remains constant. i.e. for a particular liquid, the cubical expansivity is always constant and is given by
7. Why does a thick glass tumbler crack when boiling water is poured on it?
Ans: Glass is a bad conductor of heat. When boiling water is poured on a thick glass tumbler, its inner and outer surface undergo unequal expansion as heat doesn’t transmit easily from its inner surface to the outer surface. Because of this, the glass tumbler cracks.
8. How does fish stay alive in a frozen pond in the water?
Ans: In winter the upper layer of water in pond becomes cold. In comparison to the upper layer, the lower layer of water is warm. Because the temperature of lower layers never falls below 4◦c, even though the upper layer freezes into ice. As ice is bad conductor of heat, it does not allow heat to escape out and traps it. Therefore, Fishes stay alive in a frozen ponds in water.
9. Water taps burst in water. Explain why?
Ans: Water has maximum density and minimum volume at 4◦c. When the temperature of the water is 4◦c in winter, the water tap can contain maximum amount of water. If the temperature falls below 4◦c, due to anomalous expansion of water in the tap increases, which exerts large pressure on the wall. So due to large pressure, water taps burst.
10. Why glass windows possible to be cracked in very cold regions?
Ans: In the cold regions, the temperature of the air outside the room will be very low. But the temperature inside the room will be higher. Since the glass is a bad conductor of heat, there is a large difference in the temperature across the glass. As a result, there is an unequal expansion of the outer and inner sides which causes the glass to crack.
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